[blacklab]

Hi Guys, I'm in limbo at the minute, with everything out getting machined etc.
Hopefully the crank will be ready within the fortnight, drive side journal has been hard chromed and machined, and he's now onto crank journals and balancing.
My query is on the barrel and head. My barrels have been done, bored to 40 thou. The machinist had to skim 2 thou off the top of the barrel. My head just went in and he has said that it had blown out in the centre and pitted and will need to be skimmed as well. He can't give me any details at the minute, until he gets on to it, but he's hoping to keep it to a minimum, hopefully 5-6 thou..
Now I have dropped the piston Comp down already, from, if I can remember, 8.6-1 to 8.0-1.
I understand that to compensate for any such milling, a thicker head gasket can be used to compensate but this also drops compression, which I'm trying to get away without doing. As far as skimming the head down, if it wasn't damaged, as it is and just bent, I wouldn't have this done, but I don't have an option, as he has said, it won't seal properly.
What would be the minimum amount of total skimming one could get away with, before a thicker head gasket would need to be used ??? I may be looking at around 8 thou total, between the barrel face and the head, hoping that's all,, I had heard or read up here and on various formats, 5-6 thou total shouldn't make a difference from standard.
This may sound silly, I'm still learning LOL, Can the 2 thou off the barrel face, be made up with a thicker base gasket ? or have I lost the plot LOL....
I guess hearing from those that have had to use the thicker head gasket, what amount was skimmed, or the amount that would definitely need a thicker head gasket....
Interested in people thoughts/experience here..

It's a 76 t140V...

Thanks in advance
Col

 

[StuartMac]

Hi Col,

head
blown out in the centre and pitted
will need to be skimmed
5-6 thou.

Imho, simply, don't ...

The CR is not the main issue ... The head has to sandwich the pushrod tubes and seals between it and the tappet guide blocks; where are 7~8 thou. shorter PRT and pushrods coming from?

What would be the minimum amount of total skimming one could get away with,

What exactly is this "blow"; a small 'trench' in the gasket surface? If so, this machinist effectively wants to remove a relatively-vast amount of perfectly-good metal from all over the rest the head's gasket surface just to clear a small imperfection in one place on the head? If yes, why not simply weld and machine the small imperfection to fix that?

Hth.

Regards,

 

[speedrattle]

if it was seriously eroded between the cylinders rather than bent, you don't have much to lose, maybe. i would suggest using a head gasket thicker than stock by the total metal taken off the barrels and the head. replacing metal skimmed off with a correspondingly thicker head gasket restores your compression to what it was before.

but are you absolutely sure that the head is eroded in the middle and not just bent? bending is common, and can as commonly be un-bent. any metal taken off the head/barrel interface has to be compensated for by reducing the thickness of the sealing rings on the fixed-length pushrods tubes and pushrod themselves, and if the head is actually bent, skimming the head will leave the rocker box sealing surfaces and possibly valve seats still curved.

be sure that the skimming is absolutely necessary. i've done it, but it made for much more work for me later.

added

stuart is right. first thing see whether you can add a little metal to repair the damage rather than removing a lot of good metal everywhere else

 

[DU75389]

I agree with Stu. And there is an analogy in the craft of woodworking, which I have learned as a DIY type hobbyist that also happens to own a 100 year old home. Whether it is structural or wood trim finish, you must always add before you subtract.

Example: if I have some decorative wood trim that is not structurally compromised, and the goal is to even out a surface, I will choose a wood filler before I reach for the sandpaper. Once the material of the good surface is gone, its gone for good. Thats also partly out of respect of the majestic giant Sequoia redwood tree that gave its life to build my home, which I hope to pass down to some future homeowner as a legacy act.

 

[Rancidpegwoman]

Hi Col
When the bore size went from the T120 650cc 71mm to the T140 750cc 76mm, 5mm of ‘land’ area between the bores was lost. If you overbore 40 thou another 1mm is additionally lost. Triumph found they has problems with the head gaskets blowing or weeping hot gasses between the bores. If the hot gasses were weeping, they burnt a track in the cylinder head. This sounds like the problem you have. It did not help that they made the support washers for the inner head studs too small in diameter, and the point load pressed the washers into the head. When this happened the clamping pressure of the studs backed off.
To combat this Triumph on the later bikes developed a composite materials gasket with a stainless steel protector between the bores. This solved a lot of problems.
The dealers soon worked out that if they used the same large diameter washers as on the outer head bolts, these no longer pressed into the head and clamping pressure was maintained.
Once your head is repaired, it might be worth your while to switch to the composite gasket. You will need to follow an enhanced retightening procedure though.
additionally if the head is indented under the inner stud washers, these can either be point machined flat, or, depending on severity built up with weld and machined flat, then the large diameter washers (82-2184) can be used to prevent re-occurrence.

Regards
Peg.

 

[blacklab]

Spent an hour and a half with the head guy this morning, removing Valves and inspecting and measuring.
Of course any skimming in this area ideally needs to be avoided for sure.
Now that I have gone over it with him, it is a bit clearer. The pitted area will be welded and machined, with a loss off around a thou to the surface face, which he said to expect on. After measuring the head, it has already been skimmed in it's lifetime, around 4 thou. So if indeed I loose 1 thou during his process, 4 thou already has been removed, plus 2 thou off the barrel face, at best I will have a 7 thou discrepancy, 6 thou if I do nothing about the pitting !!.
He had 6 Bonneville heads to be done on his shelf, only one had not been previously skimmed, he said that was the normal % he see's, he said the 80's and 90's seemed to be the era a head skim was the norm or the thing to do.
Nothing I can do about what I have now, in the perfect world everything would be standard, but it's not and I just have to make this work, or buy a new head.
As a footnote, my head also has the usual bend, of which he intends to straighten it himself first job, which he basically said that is a common occurrence, unfortunately, some machine shops simply skim the head, rather than do what he does...

 

[blacklab]

Thanks for that Peg, That was an interesting read.
Luckily I had no indentations under the inner head washers, I had read about the use of larger washers and was planning to use them. In fact, everything else looks good on the head, just the fact it's already had around 4 thou removed.
I was intending on going for a copper head gasket, but after reading this, a composite gasket may well be the path to take here.
Nothing has been straight forward with this rebuild, don't know why I expected anything would be different with the head...😀..

Col

 

[TR7RVMan]

Hi Blacklab, I have personal experience with this on my '73 Tiger. My head gasket blew across front center stud & eroded a groove. To clean up my head they had to remove .009". It still worked out ok with crush for PRT seals.

To lower compression I used thicker head gasket. I went from .050" thick head gasket to .080" thick gasket. This lowered compression 20#. Substantially reduced ping, but didn't seem to lower power. The thicker head gasket doesn't leak any more or less than normal .050". To be clear this is solid copper head gaskets I'm talking about.

Now with thicker gasket I have less crush. Around .020" works perfectly still no leaks in over 20K miles now.

Just as your shop says if you have eroded head or dips in head you must skim head. Of course you could build it up with welding, then skim it. That is simply not necessary.

If head is bent it can be straightened. But erosion & dips must be skimmed. A not flat cyl top must be skimmed.

You can put thicker base gasket & that raises cyl. It also lowers compression.

Skimming & head gasket thickness changes rocker geometry also. However unless way off, it doesn't really matter.

My head was skimmed at about 5k miles. I put thick head gasket in about 12k miles. Motor has a tad over 36k miles now. Original pistons, valves etc. PRT were resealed with thicker head gasket. Still not leaking. So there you go.

We see here a few owners that have ongoing head gasket seeps/leaks. Bike is ridable, but always seeping oil at head gasket, which is condensed from combustion seeping out. Not blown gasket where compression is leaking out, just seeping. The root cause is bent head or more often dips in head surface. The only cure is flattening. That means skimming.

The head must be very carefully measured in many areas with straight edge. Not only for bend or warp, but dips or high spots. I cannot explain how these dips get there, but they do. Again not bent from too much PRT or the like. Often they will be around outer 4 bolts. You must have flat surfaces to seal properly.

I'm not a fan of thick base gaskets, especially the thick paper ones. My feeling is & I may be wrong, but they allow cyl to work or wobble on cyl flange a tiny amount leading to deterioration of base gasket.

I've had some experience with this skimmed situation. Real life. I would use normal base gasket. Get normal thick gasket or use your old gasket if it's still nice & flat. Clean it. Trial assemble head with PRT & measure crush. I go for .020-.030" or so. Even .013-.015 is enough crush. I don't like over crushing. I feel it's hard on head & can bend it.

Spend the $ on .080" thick gasket, trial assemble & measure crush.

I always 100% of time buy extra PRT orings. I use these for trial assembly Get only viton o-rings. Deburr bore in head & smooth it with scotch brite pad or the like. Deburr tappet block OD where o-ring slides down & seals, & smooth with scotch brite. Micro scratches on o-rings is common cause of leaks. If needed you can trim lower white square seal a little thinner. You know the later motors just used the 2 o-rings & don't leak any different. Care in preparing surfaces cannot be overstated. Fianl assembly I use new oring. Never install the trial fit rings. A thin smear of silcone sealant on rings is not a bad plan. Suzuki Bond works well. Be ready to go so it doesn't cure before assembly. Work quickly. Make sure chrome on PRT is not cracked or peeling or it will leak. Smooth an roughness on PRT oring surfaces with scotch brite as well. Including in inside bottom groove.

Service bulletin #324 states .030-.040" crush. I feel .040 is too much. That's just me. I've observed that much will over a few years smash & split the lower white square seal. PRT will still not leak if o-rings were prepared well though.

I have a fair amount of experience with skimmed heads. You just have to figure it out as you go.

Finally rocker geometry. If the contact face of adjuster is not going off edge of valve stem whatsoever I run it. So on my bike I have factory type thin base gasket. Thick head gasket is extra .030 minus skim .009. So that's .021" higher for head. It has worked good. When head was .009" lower it worked good. I don't know what your valve stem height well be. That's part of it also. Mine was factory fitment from new. I've never measured it. Here again you have to see what you find on assembly. If you have to take things apart & make changes, just how it goes sometimes.

Regarding lowering compression. Don't worry about it. Trust me, you'll not feel a difference. I've been recommending 7.4 "unleaded" pistons in T140. I just road tested one over 200 miles of different roads from steep hill to level freeways. I'd wager you can't feel the difference. This bike pulls very strongly, it really does. Think about what gives best PRT crush. Acceptable rocker geometry.

On an aside, I'm not a fan of reusing head gaskets. I'm wondering if this can give the head dips?? Just thinking out loud. My head stayed perfectly flat after skimming. I've observed others that stayed perfectly flat. Then you get the leakers with dips. I know some had reused head gasket after annealing.

The deal with washers under the four 3/8 head nuts, is originally the factory installed a small washer the size of nut shank. This could indent into head effectively making head not pinch as tight. A larger washer that was used under outer head bolts could be used in place of the small ones. That gave better support. If... the head is dimpled from small washer pressing into it. That should be faced off. I don't know how much difference it really makes if you didn't face it & used larger washer. This is not a hardware store washer, but high tensile. So get it by part # from outer bolts. My bike still has original small washers. Head gasket still ok. I'll use larger washers during up coming overhaul. I've always kept up on head retorque since it blew back then. I didn't really check it before. Dealer was supposed to have done it at 500 mile service. I'm sure they didn't now.

On paper you think you can just remove a few thousands' on head. In real life it takes a bit more to clean it up. The dip must be removed fully to uncompromised metal. If your machinist is speaking of .004-.005" he is very good. Indeed he can't know until he sees good metal. I could be a little more off is better & use thicker gasket.

You have a lot on your plate to think about. Don't stress though. It will work itself out during trial assembly. Trust me on this.

Be certain to replace tappet block orings. Use only viton rings. Very important. They will leak if not. Again deburr the bore in cly base & outside of tappet block. Smooth with scotch brite pad as well. I lightly grease the bore, ring, block. Never had one leak yet.

This how I test PRT crush. Put new orings on tube. Remember these will not be used later! I keep them special for just trial fitting.

Set white square on tappet block. Rubber rings in tubes lightly greased so they slide easily. Set tubes onto tappet blocks. Set head gasket onto cyl top. Set head down & engage tubes Give it gentle wiggle to be sure tubes are seated.

Balance head on tubes. I install 4 outer head bolts ever so gently such they balance head parallel with head gasket yet don't compress orings at all. I just use them to keep head balanced. You'll see what I mean.

Then take feeler gauge & measure the gap between head & top of head gasket. The head gasket should be flat, not bent or warped. You just start sticking the feeler in until you find what fits. I test the area between the front center & outer head bolts so to speak. Test both front & rear. If it's not even front to back, swap PRT front to rear. Whatever way PRT gives more even measurement leave them. For some reason it can vary even .010" or more. Just do best you can. Record measurements.

Decide what you think is best between normal or .080" gasket from your measurement.

The "wedding band" that covers white square ring may or may not be press fit on PRT. I've seen some that fit really loose as some PRT were incorrectly made to skinny at bottom. These will 99% of time pop up & go crooked. Won't necessarily leak, but looks bad. If possible I like to very gently drive wedding band onto bottom of PRT. I find it keeps it even & looks good. Just a tad on like 1/64". Or... lay it around white o-ring. I don't like this because can over compress upper PRT ring sometimes & cause leaks. Personal choice.

Sounds to me like your machinist has told you the truth.

Wouldn't it be nice if they sold head gaskets in .010" increments!

I've never used a composite head gasket. The few I know that had them went with copper. I never worked on those bikes so I don't know how surfaces etc. were.

I seem to get stuck with bikes that's been not correctly worked on prior. So heads are beat up ect. Leaks bad. Rocker box studs & bolt holes all need helicoiling & of course head gasket surface is horrible. In the end they work very well. I hate leaks!
Don

 

[TR7RVMan]

Hi Peg, You are using composite gaskets on your head? About how thick is the composite gasket when you take it off? Meaning the effective thickness.

I've not been able to find extra thick. Are you aware of any?

What brand do you use/recommend?

I'd not considered composite. But you have me thinking about are they better for 10 bolt head?

Composite with stainless were very popular on cars for many years. The worked well, but... they were not Triumph motorcycles. Cars had much greater sealing surface.

I only know 2 owners from club, with late bikes, but don't know them very well. Both had repeated failures with composite & went to copper. That worried me. But I don't know what the true circumstances were. Flat surfaces? Did they correctly torque & retorque? Only saw them on club rides. I haven't seen them in 1.5 years now, so no follow up.

My motor will go .020 over soon. I think about the gasket pinch.... I'm going with 7.4 cr pistons so can go back to normal head gasket?? If my head doesn't need skimming again?? How is my cyl top?? Every thing I touch turns to crap so I expect the worse. It happens, I got exactly what I expect. It's good. I got lucky!!

Just today I tested compression. Right side has oil crust on piston. Was 152# 20k miles ago. Now 160#. Left piston top is white with tiny pocks like detonating, no obvious carbon... Compression was 150#. Now 166#. Go figure. I'm looking with stick light. I don't have bore scope. I can't see combustion chamber. The left side been biased lean from day one from new. Why? No intake leaks or leaks to head nuts. Can rod stretch?? Or carbon in combustion chamber?? I'll know in a few weeks... Still doesn't use oil & runs really well. Pistons rattle like mad! Guides as well??
Don

 

[blacklab]

Hi Don, thankyou for that detailed reply, very much appreciated....
I guess in reality, it would have been brilliant, for my first rebuild, to find an untouched head, that I could assess and make the right decisions and move forward,,, Alas, the dreaded previous owner, left his calling card everywhere inside...
The head guy is an old school Brit bike bloke, that prefers working on Brit bikes, his passion, rather than the bmw and lancer race engines he had sitting there, in fact when I walked in he was well pleased to see me and tinker with my head.
A quick measurement after removing the valves revealed a previous skim.. The rest of the head apart from the erosion was quite good as mentioned above. From his quick measurements, mine came in at 70.7 consistent at all points, I think standard is 71, so that is well more than the 4 thou I previously mentioned..
He was confident the oversize 80 head gasket would work, but again, he also mentioned as you did, that final set up and careful measuring is key here. I believe, that when the previous owner skimmed the head, proper attention wasn't adhered to with the head gasket and crush, which then just made matters worse and here we go again !!
He will do a good job of straightening and welding the erosion areas, I saw an E head that he had just welded cleaned up and ready to face, this one, had already been skimmed prior and it was already down to the fins in one spot, well worse than mine by a long shot...
What I end up with in the end, is what I end up with, I'll have to work with it and short of buying a new head, It is what it is unfortunately.
I've changed a couple of things with my rebuild, I have the new 3 piece up grade bearing for the timing side, swapped out the exhaust cam for the old 650 type, but I dropped the piston comp, from 8.6-1 down to 7.9-1, so hopefully as you mentioned the further drop in compression using a thicker head gasket, wont be too noticeable.
I am interested in what Peg mentioned, regarding the composite type head gasket, for the reasons he mentioned, which kinda makes sense, but I'll cross that bridge when I get up to it. At the minute, I have to get new Valve springs and some new black diamond valves down to the head guy, so he can start his work...
Thanks for your input, appreciated...

Col

 

[TR7RVMan]

Hi Col, We have to deal with real life. Triumphs are not very durable really.
Easier to find gold nugget than good used head.

Id like to know quality of new LF Harris heads.

Thing is the thick head gaskets do work well. I’d use thick head gasket in a heart beat if rest of head is good. Want some fun? Find a good used head on eBay. You think you have troubles now!!

It really sounds like you have a good experienced machinist. This will turn out good.
Don

 

[Rancidpegwoman]

Hi Col and Don,
Head gaskets on the late engines are one of those subjects that tend to polarise people. We seem to fall into one camp or the other and then entrench our positions.
I have several reasons why I like to use composite gaskets, but that does not mean that copper are not OK, both can work well but I think they need different treatments.

It is easy to distract yourself from the reason for gasket failure by blaming the material rather than the root cause, this I believe in most cases is lack of clamping pressure from the head bolts
A lack of clamping pressure can be for many reasons:
a) As already mentioned, the inner stud tunnels compressing due to undersize washers.
b) Pushrod tunnel pressure too great due to oversize seals/poor tolerances/surface grinding.
c) warpage to the head .
d) Lack of care initial setup and lack of maintenance of torque.
e) re-use of copper gaskets without annealing or annealed without care so the metal distorted.

These are the reasons I like to use Composite, with some caveats.
1) Triumph themselves used them (I believe post 79) to overcome the problems with copper gaskets blowing. I read the were successful in reducing the warranty claims-but there may have been other factors contributing, CR reduced from 8.6:1 to 7.9:1, electronic ignition giving a even L/R spark timing, resulting in a more even temperature spread across the head and barrels.
2) The majority of engine manufacturers switched away from solid metal gaskets pretty soon after Klinger invented the composite gasket in the 1930’s, a notable exception was the British bike industry. But maybe thhey did not uder pressure to change as they did not have the tricky issue of water cooling passages to deal with.
3) extra re-enforcemet is placed where you need it, increasing the thickness at these points, therefore the compression can be zoned by smart design, concentrating it in the areas where it is most needed.
4) Klingerite is stable thermally, it does not expand much. So a stable barrier is placed between the alloy head and iron barrels very different expansion rates. A copper gasket places a third expansion rate into the mix.
5) Most important to me is that if a composite gasket fails, it tends to fail spectacularly. The engine stops due to a large section of gasket getting blown out instantly. A copper gasket is more resilient and does not blow out instantly, however this might not be the blessing it first seems. A failing seal on a copper gasket will allow small jets of combustion gas to leak out and hang on for a long time, these jets of hot gas can burn the metal away from the head or barrel. The damage is a distinctive groove burnt in the metal. Having a composite gasket that sacrifices itself rather than damage the head/barrel is a comfort. Having said this I personally have not had a gasket fail, either copper or composite.
6) The composite gasket insulates the cylinder head from the barrels, a copper gasket will transfer heat freely.
7) Composite gaskets are non reusable, this is expensive but at least you get a new one every time. You can reuse a copper gasket, but I have seen people refit them without annealing them. I have also seen people try to anneal them poorly and unevenly, distorting them with too much heat or leaving some areas harder than others.
If you need a description of how to anneal a copper gasket properly, the best method I have seen was in a previous post by @StuartMac.

These are my main reasons for preferring a composite gasket, I am definitely not saying copper gaskets are a bad thing though. many bikes have happily run many millions of miles on copper gaskets.

There are simple steps to prevent head gasket failure.
a) Spotless assembly.
b) flatness of surfaces.
c) correct pushrod tube pressure.
d) The cylinder head around the Inner head studs protected from compression by larger washers to spread the load.
e) Care in tightening the cylinder head down.
f) For copper gaskets only, the use of a heatproof sealant (copercote, etc.) to glue the gasket to the head and barrel.
g) Vigilance in a re-tightening regime. Torquing down the head several times at increasing mileage intervals, until the nuts/bolts no longer tighten up.
This can take 3 or four rounds for a copper gasket, however it seems to take a lot longer for a composite gasket maybe taking 5 or 6 rounds of re-torquing. Retightening the head fixings several times is a complete p.i.t.a,but preferable to risking a blown gasket.

This is the regime that works for me, I stop when the bolts/nuts no longer tighten. (Usually 50 miles copper, 500 miles composite)

i) Torque on assembly.
ii) First heat cycle, to check oil return, ignition timing, let cool and re-torque.
iii) 10 miles + carb balance, let cool and re-torque.
iv) 50 miles + oil change if new pistons are fitted or crankshaft rebuilt)+ re-torque
v) 150 miles
vi) 500 miles
vii) 1000 miles oil change + re-torque.

Regards
Peg.

 

[blacklab]

That's a great informative post, thankyou peg...

Col