Triumph Rat Motorcycle Forums banner

21 - 31 of 31 Posts

·
Premium Member
Joined
·
4,721 Posts
I read the Britbike forum and didn't really see any cons on skimming the head, just the alternate to bend it back to normal.
.005 seems like such a small amount of warpage. Wouldn't the head gasket take that discrepancy out?
You can easily flex the head 0.010" when you tighten the head bolts.The amount of torque on the bolt to cause that would be minimal. You could probably bend it 1/2 that in your hands with finger pressure. The head will then sit flat against the gasket and seal OK.

I've seen heads with 0.010" bow that still sealed OK.

Don't let them mill the head. Get that in writing, that they will replace it if they mill it. Measure the overall thickness, so you and they have a record of it. Have them put a tag on it that says "NOT TO BE MILLED OR RE-SURFACED". They will probably still mill it, as others have found out the hard way.
 

·
Registered
Joined
·
6,527 Posts
You have two experts, John Healy and Mr. Pete, saying don't mill the head. I know John has an extensive racing engine background. Mr. Pete comments in the past would indicate he's built a few track oriented machines in his day as well. Milling the head is an easy way to make more power, yet both say don't do it. That pretty much makes any argument in favor of milling heads pretty weak, IMHO.

I have a '66 '650 that had the head milled before I got it. I ended up having to put a steel shim under the cylinder base with a gasket on each side and use an oversize (thick) head gasket to get the proper piston to valve clearance. I had no choice.

To answer your question, I've never measured one, but I'm doubtful that a copper head gasket can be made to compress 0.005" with head bolt pressure. And even if you could, that would mean that you still have zero pressure at the point where it is 0.005 warped.

I have my old head gasket out in the shop. It should be 0.0625 (if I am not mistake) if it were new. We'll see what it measures now.

regards,
Rob

regards,
Rob
 

·
Registered
Joined
·
4,174 Posts
Here's what seems reasonable to think: A T120/T140 head with an out-of-flatness of .005" or so ought to be of little consequence given that Triumph alloy heads are inherently easily bent (ie: if they can be bent out of shape so easily, they'll bend back to shape just as easily, given the right treatment). As long as the PRT crush is corrected, tightening the head bolts down should move the face enough to take up that kind of difference.
 

·
Registered
Joined
·
2,923 Posts
Generally speaking the stock copper head head gasket is .050" The next oversize is .080 for t120 & t140. There is also .125 oversize for t120 9 bolt head. There are a few thickness of spacers for under the cly base, which in that case you need 2 gaskets. The extra gasket becomes part of the thickness of course.
Depending on what happened to the head skimming may be required. For instance a blown gasket at the center bolt can cause erosion of the head forming a shallow groove. Which is what happed in my case, even though the head was nearly flat.
Regarding the difference of using a lower spacer or thicker head gasket can get slightly complicated. The milling of the head or changing head gasket thickness changes push rod tube seal crush, which can make crush better or worse depending on what you want to achieve. While the cyl. base spacer has the effect of the pistons not going as high in the bore. If head is skimmed go with thicker head gasket. Either condition can effect rocker arm geometry in an extreme case. If thicknesses get too far off the valve adjuster screw will go partly off the end of the valve stem.
In my personal case I had to skim head .010" to remove erosion grooving. That had minor effect of crush, but I felt it did indeed over compress the lower white seals. I just left it & it did not leak.
Later down the road, I had a tappet block o-ring leak so I removed cyl. to reseal. To help compensate for modern USA low octane fuel I wanted to lower compression ratio.
So I changed head gasket from .050 to .080". Since the head had .010" off the effective change was .070 from what new would've been. In my case the rocker adjuster moved about .007-.010 on stem, but still not nearly ready to go off edge. I've covered about 1500 miles since to good results.
Regarding push rod seal crush I'm a little light at .015-.020" crush. You can easily test crush on the bench with just cyl. & head & tubes. Leave wedding bands off when testing crush. I used a very light smear of silicon sealant on all 3 O-rings just to be sure no leaks. They are dust dry, not a trace of leakage.
So far no oil from head gasket, which seepage is quite common at. I used Mercedes Benz plastic sealant on head gasket.
Prior I'd used k&w copper coat sealant. It always seeped oil (actually blow by gas I suppose, since the is no real oil supply to combustion chambers), even though compression was good & I kept head torqued.
In any case with the cyl. off reseal the tappet blocks. They are source of leakage that is very hard to diagnose as the wedding band covers the area. Smooth out any roughness on cly. bore for tappet block, & the outside of block as well. Plus smooth the push rod tube bores the head. Most important the o-ring doesn't get any micro tears from rough surfaces.
Checking between old & new head gaskets of the .050 type, the old is about .002" thinner. I bought both new .050 & .080 because I was sure what I wanted at first. Can't say if it crushed that or the production tolerance was thinner from new.
Don
 

·
Registered
Joined
·
7,204 Posts
You might laugh here.I have never measured my cylinder head for straightness.I use,and re-use the old copper gaskets.No torque or re-torque of the head and no head gasket leaks ever.No squashed headbolt washers digging into the head.It seems like these heads are damaged by incorrect spannering or heavy handed work.No helicoils,no stripped threads anywhere,clutch never sticks.I ride as fast as i can every time i go out and have fun.
I see some brilliant workmanship by people here but find all these dead accurate measurements not required for these old engines.
Quite happy to run at 6500rpm knowing i assembled every part of the engine just checking a few bearing clearances and torquing big end bolts.
Careful and clean assembly and re-check everything as i go.
Last build was untouched until the crank snapping after 10 years for no particular reason other than snapping the throttle off at 80mph.
 

·
Registered
Joined
·
1,046 Posts
You might laugh here.I have never measured my cylinder head for straightness.I use,and re-use the old copper gaskets.No torque or re-torque of the head and no head gasket leaks ever.No squashed headbolt washers digging into the head.It seems like these heads are damaged by incorrect spannering or heavy handed work.No helicoils,no stripped threads anywhere,clutch never sticks.I ride as fast as i can every time i go out and have fun.
I see some brilliant workmanship by people here but find all these dead accurate measurements not required for these old engines.
Quite happy to run at 6500rpm knowing i assembled every part of the engine just checking a few bearing clearances and torquing big end bolts.
Careful and clean assembly and re-check everything as i go.
Last build was untouched until the crank snapping after 10 years for no particular reason other than snapping the throttle off at 80mph.
Hey, Rambo.... are you sure that's a Triumph you're riding and not a CB750 in disguise ? :) :)
 

·
Registered
Joined
·
7,204 Posts
Maybe the factory workers were having a good day when building.Every part of this 71 bike seems to fit without having to use force.Bearings need just light pressure to press in or out.Enjoying it all now for another 10 years until it might blow up again.In the Summer,i might do a video of a high speed ride across the winding Common.Joy to hear the engine pick up and fly in every gear.I have this engine just right again with no pops,bangs,or flat spots.
I have firmed up the handlebar mounts so the rubbers have no movement at the P clamps.Next outing 26th December on the annual XMAS ride.
 

·
Registered
Joined
·
6,527 Posts
That's good input, TR7. I forgot all about checking my old head gasket.

John, I think it is important to start out with a flat head surface. Otherwise, you'll be using some of your torque to straighten the head in the center, which will reduce the force on the head gasket at that point and set you up for a leak between the jugs, IMHO.

The head bends easily from too much PRT crush. The outer head bolts, which do the damage, are working at a mechanical advantage out there. But it is probably safe to say that when that happens, the sealing force on the head gasket under those bolts is reduced like it would be as described above. But since there is more surface area, there is probably less chance of a leak.

If you struggle getting your head around this, here's an example. Put a leaf spring, convex side up, on a bathroom scale. Push down on the center of the spring until the center just barely touches the scale. Whatever the scale reads is how much force you have on the spring. But since the center is barely touching, you have nearly zero compressive force where the center touches. This would be what the area beneath the warped section experiences. Using that same example, this time put a flat piece of steel on the scale and press down on the center the same number of pounds that it took to make the spring center touch. I'm sure you have already figured out that the area beneath your finger is seeing the full compressive force you are reading on the scale.

regards,
Rob
 

·
Registered
Joined
·
4,174 Posts
Yes, I see the theory Rob and it makes good sense. I also see Rambo's way of doing things and the results speak for themselves (well, apart from the small matter of a broken crank, but I reckon we can put that down to an aberration). Enough torque is enough, too little is too little and too much is too much. If the end result is an emgine that doesn't leak oil from the prts and on which the head gasket seals well, over thousands of miles, whatever the torque wrench might have said if it had been used, is not very important. I'd always use a torque wrench or micrometer on conrod bolts, but I never have fussed about using one anywhere else I can think of just now (I'm sure there have been a handful of other instances, T160 plain main bearing cap nuts for example). In some ways, the provision of torque settings for a lot of fastenings is meant as a safeguard against idiocy, in recognition of the fact that common sense is not actually common!
 

·
Registered
Joined
·
7,204 Posts
Broke at 40,000 miles.This is the only part that has actually destroyed itself and those standard journals were in perfect order.It had been keeping together with no engine faults for 10 years after my last build.Nearly another 2 years since building again and after some PRT leaks and then a new set of rings,its all very reliable now.Useful to keep a file of mileages and wear.Main bearings were changed at 30,000 miles.
 

Attachments

·
Registered
Joined
·
6,527 Posts
In some ways, the provision of torque settings for a lot of fastenings is meant as a safeguard against idiocy, in recognition of the fact that common sense is not actually common!
Ain't dat the truth!!

You know what they say, "Make something idiot-proof, and God will build a better idiot."

regards,
Rob
 
21 - 31 of 31 Posts
Top