Hi all. Apologies for originally placing this post in the wrong place!
Hopefully someone can help me on this. I’m just building up my 1978 Triumph TR7 engine and I’m struggling on the top end a bit.
Brand new cylinder head is completely rebuilt. Barrels are brand new as are tappet guide blocks, cam followers, alloy stepped type pushrod tubes, SRM pushrods, seals and collars. Rockers boxes all rebuilt and complete.
I fitted the pushrods tubes with the red and black o-rings (red inside the bottom groove and black around external top projection), translucent flat ‘cushion’ washer and ‘wedding band’ retainer.
All new head bolts, nuts, washers etc. New head gasket.
Firstly, it was a real problem stopping one of the cushion washers popping out despite the ‘wedding band’. I’ve heard lots about ‘squish’ so I may have to revisit that again. Eventually I got it to work when I smeared the parts with grease.
I torqued the cylinder head down exactly as the workshop manual.
I then fitted the rocker boxes using the specified torque settings.
Everything turned over OK, but I noticed that a couple of the valves weren’t closing properly. When I tapped the end of the rocker arm the valve would snap back into it’s seat. I thought the valve(s) might be sticking, but I tried again and this time, using a screwdriver, just teased the rocker arm up to it’s fully open height and the valve moved right back up into its seat.
Which means, I think, that either the rocker arm is binding and/or the cam follower. However, I checked that all parts moved freely before assembly.
Could this be due to the squish being wrong on pushrod tubes or are the rocker box bolts (which screw into the cylinder head hex bolts inside the rocker boxes) somehow distorting the rocker box itself. I’m sure I haven’t missed out any washers or spacers (unless something is supposed to go between the top of the hex nut and inside of the rocker box) and I was careful with the use of the torque wrench.
I’ve never encountered this before and it’s very frustrating! :|
First,just slacken the rocker cover bolts a little,this might show if the rocker cover is distorting.Another problem could be valve springs binding on each other.Possibly things are a little tight being refurbished.Pushrods can also bind on the rocker cover gasket.Just have a close look at what happens.
Hi Crispy, As I think you know this is very serious. In operation any faults in valve train will almost certainly result in bent valves & damage on the road. Things will not wear in.
I don't know how far valve was stuck open. That can be key to diagnosis. A tiny bit could be debris on a valve seat. The head should have been cleaned & leak tested with solvent before assembly, but that's too late now.
Here's a bit of info that might help you determine the cause.
Go over valve adjustment again & see what you have. Record any changes. Get back to us on that.
When the valve was stuck open was the rocker arm free to wiggle? Like it had free play? If yes the valve itself was sticking down.
If rocker was tight & pushing valve down then the rocker arm, pushrod, or tappets were stuck.
I have no experience with SRM pushrods so I don't know how they compare to Triumph.
On all the 750s I have experience with they have dowel pins that align rocker gaskets & rocker boxes. The push rods are very close to gaskets but don't actually touch. I just looked at that earlier this week on my bike during valve adjustment.
It can be possible to have pushrod socket not properly seated in tappet end. Then it will snap into place often with a click. This will result in a very loose valve adjustment. Visually when adjusting valves the rocker screw looks like it's too far out, but that may not be noticed. Now the rocker adjustment will be very very loose on that valve as the push rod dropped down onto tappet.
Who installed tappet blocks? If tappet blocks are way off alignment or cam is not centered left/right the tappets can catch on adjacent cam lobe & do odd things. Seems like tappet block/cam alignment would not cause intermittent problem.
Tappets fit very freely in the blocks. I've not seen any binding on the ones I've been involved with. Not much there to bind.
There is not much for rocker arms to get stuck on. Are the washers in correctly & arms move freely? There should not be a hint of binding or stiffness.
Back to the valves, if the rocker was very loose the valve is hanging up. I've had Triumph heads apart & never seen springs bind & hold valve open, but I don't know on yours. My hunch is dirt in guide or slightly bent valve stem. If you determined valve was hanging open take head apart & find out why. As I stated prior a little dirt in valve seat will hold valve open the size of the dirt particle. Tapping on valve can free that, then things are fine. Again, back to how rocker arm felt to know what to do next. Not an easy puzzle!
Pushrod tube crush is something you need to have correct, but I doubt it's relating to the valve sticking. Be sure to check your head for flatness if you have to remove it again. Then go back over PRT crush on assembly.
Don
lots of good advise - you do need to find the cause - only way is to strip the top end and get the valves out of the head - check everything from the tappet block up and slowly build it back up - hopefully having found an easily sorted problem
if the engine is still fully built - do you get a sticking valve every time you turn it over ? if so have you tried backing off the rocker box bolts slightly in case they are distorting under torque down
what torque setting did you use for the rocker box bolts ? ( i never torque the rocker boxes just take them down by feel)
had another thought - with the rocker boxes off and the head still on - rig up some way to use a lever to press down on the valves (make sure the pistons are down the bores a little) - if they don't stick then the problem may not be the head and you may not have to remove it
Thanks all for taking the time to give such in depth advice. Much appreciated! I will strip it all apart and check everything. I think what I might do is temporarily put it back together without the pushrod tubes so I can see how everything moves together. Will report back on any progress.
What gap did you have between the head and head gasket, before tightening the head bolts?
Did you remove the plugs, kick it over and check the oil return?
If there's no lubrication up there, problems can happen.
Valve sticking in new guides does happen. There's a minimum clearance that is not easy to measure.
Although I think you're saying the stickiness is further back in the mechanism.
Pushrod end cups displaced off the rocker or cam follower ball ends is something of a syndrome in home assemblies of Triumphs.
Siezure of the rockers on their shaft should have been obvious before assembly. The oil you assembled it with would keep them lubricated for something like ten minutes' running.
I had a problem with machined aluminium push rod tubes when I rebuilt my T120R. I found the the machining on the internal diameter at the bottom of the tube was the problem. It had not been bored clearance far enough up the tube and when the push rod reached full lift, the larger diameter of the steel end cap of the push rod fouled and jammed in the unmachined I/D of the tube. I had to take it all apart again and get the tubes bored out deeper.
I thank everyone for their help on this. In the end Pharisee put me on the right track. It was the pushrod end fouling the inner bottom part of the pushrod tube. See attached pics.
I decided to fit the head, rocker boxes and pushrods without the pushrod tubes and everything worked fine and the valves closed fully shut.
I then fitted the whole lot again with the alloy pushrod tubes, but without the bottom squish seal. Back to original problem of a couple of the valves sticking slightly open. Incidentally, before I bolted the head down I measured the gap between the cylinder head and gasket at about 70 thou.
Furthermore, I noticed that on tightening the head (gently by hand) the pushrod tubes pressed down right flush onto the flange of the tappet guide block. The top of the pushrod tube was firmly up in the cylinder head, so there appeared to be no gap at all for the lower squish seal. I've attached a couple of photos of before and after the head was bolted down to show this.
I then found an old original set of chromed pushrod tubes and fitted these, still minus the squish seal.
The valve train operated perfectly and I had a small gap at the base for the seal this time.
On really close examination of the internal lower part of the alloy pushrod tubes I noticed a couple of slight indentations (see photo), similar to what Pharisee had said about his.
Not only that, but when I put the old and new pushrod tubes side by side there is a noticeable difference (about 30-40 thou) in the height between the top lip for the black ‘o’ ring and the base for the squish seal, the alloy one being the slightly higher of the two.
So I guess my only options are:
1. Ream out the bottom internal surface of the alloy pushrod tubes. However, I would presumably have to take 30-40 thou off the base of the tubes because of the height discrepancy?
2. Use the old originals (looking a bit tired though)
3. Buy new alloy tubes (probably the same as what I have so same problem).
3. Buy new chromed steel tubes from LF Harris (would these be any better?). Although I thought that's where the alloy ones came from!
Any final thoughts appreciated, but I think the problem is recognised if not quite rectified!
Once again, many thanks for all your previous help and advice on this subject.
There seem to be a lot of problems with SRM parts and quality.One of the reasons i seek out genuine used parts when i can or parts that have had a long established reputation.One of my mates bought a BSA magnetic drain plug plate by SRM then had to cut it to make it fit.Me,i just threaded the original square plate and fitted a large bolt with a rare earth magnet attached.Total cost about 50p
Hi Crispy, Thank you to Pharisee! You got to the root problem so you can move on with confidence.
On my '73 TR7RV the length of my original prt were slightly different, but close enough to set the crush fine. I ended up with only .015" crush but no leaks at all.
However my intake prt was too skinny at the bottom so wedding band always slipped upwards & white seal split & came almost out. Yet did not hardly leak at all.
I thought the wedding band was too large, but turns out the tube was actually too skinny. Turns out this is common on early 750s. My hunch is that is one of the reasons the aftermarket aluminum tubes were made in the 1st place.
Mechanically & leak resistance the steel tubes are fine. From what I have observed in the last 3 years the main cause of prt leaks on these models is poor surface finish on head & tappet block surface o-ring sets on. Also many leaks are actually the tappet block ring. Again surface finish on block & cyl. contribute greatly to the leaks.
I personally like the gleam of chrome tubes, so passed on aluminum ones.
I ended up taking my head, prt, cyl with tappet blocks installed to Rabers. Michael allowed me to experiment with different tubes.
All the new ones he had were correct length & correct diameter so wedding bands fit tight as they should.
He had a box of used ones. Looking it over they were rusty or dented, or... many looked perfect, but were the skinny ones that don't work.
Thank God good local shops still exist! They are getting rare though as internet sales take so much business. The cost may be higher, but you can see what you get. That is worth a lot.
After pricing them out I decided to just make a custom wedding band from aluminum since I was low on money, but had spare time. Otherwise I'd had to buy a new one. I have access to lathe so fabrication was possible by me.
Looking at your photos I'm not so sure you could remove enough metal from the aluminum one at the bottom to get the crush you want. It also appears the drain holes on top end are too high & might pool oil.
Since your trial fitment of the old steel tubes seemed good, I think you may be better off to get new steel ones. If possible call the seller & have then fit wedding bands to make sure they will fit ok.
Here in the USA I attend swap meets. Junk used parts are easy to find. Good used parts have gotten quite rare. Not too many good 750 twin parts around these days. They didn't make that many & many bikes are still being stored or on the road.
Quality new parts can be hard to get. New old stock is nearly non existent.
So if you can spend a day driving to a real shop & take your motor and/or parts to see what really fits properly.
Good luck on your searching. Hope you can find a good set.
Don
Hi Crispy, On a side note, I used to always put white seal on tappet block, set wedding band around it & put on prt, then head.
On the last few I've done, I gently tapped wedding band onto prt. Just a little ways. Less than 1/16" Then installed. I think it makes the O-rings settle in better.
Also micro tears in seal ring from fitment or poor surface finish can cause leaks. I have a set of 4 "test" O-rings I put on tubes while checking crush. Then fit new rings for final assembly.
I personally always use viton rings top & bottom on the PRT. That is the 4 round ones. Have very good results with no leaks.
Don
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