Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Cylinder head "crush" 1/16" or .0.030-40"???

Most references are stating 0.030 to 0.040" crush factor - but the Gunn manual has a crush factor of 1/16" which is roughly double. What am I shooting for here? Preliminary assembly gives me 1/16" clearance between head and gasket. To get to 030 or so I'll need to make some adjustment somewhere.

The engine is a '72 650.
Thanks
Al

Everything I've got says .030/.040. Perhaps someone has milled your head or cylinder? Double up your base gaskets or get a thicker head gasket. Either will slightly reduce your compression. Not enough to notice.

Base gaskets won't make any difference to crush.
Thicker head gasket will.

http://vintagebikemagazine.com/technical-articles/push-rod-tubes/

Hope this helps.

koan, I was thinking about something else. Duh.

Back in 66 I can find a service bulletin reference to 1/16th"
... but by 1969 it was .030" ~ .040"

I used to be able to email the above to myself. Now I have to "copy to clipboard". I did. Now what????

The 72 shop manual I have makes no mention of head crush.

Cancel Cancel. I figured it out

Hi Al, Anywhere between .015 to 030." is what I shoot for.

.040" is more that I like. It really squishes the white seal. Over time I've seen them split.

If you PM me your email I can email you detailed videos on what I do. Exactly how I measure & the difference between thick & thin white seals.

1/16" = .062" That much crush is just nuts. Don't do it!

I've measured a fair amount of these. So far no failures. I have no magic hands. Just am precise in measuring. That tells me what I need to do.

If you have correct head, tappet blocks, PRTs, If you can't get below .040 crush you need thicker head gasket. The next thickness is .030" thicker than normal head gasket.

If you want tight quench you can't use thicker head gasket. Then you must modify something. That will depend on what you measure for crush. In any case I'd not want more than .040 crush.
Don

It is very interesting to read between the lines of that service bulletin.

It suggests that the original setup was intended to seal at the lower end solely by the internal O-ring, but because that didn’t suffice, the additional bottom washer was included, along with the collar (wedding band).
A case of making it up as you go along.

In the original design, the lower seal was captive in a cup at the bottom of the PRT, the upper seal was captive in the head recess/tube flange.
A clear instance of un-progress.

Admittedly Triumphs were infamous for their pushrod tube leaks, but the rubbers available at the time weren’t what we have now. Plus many of the kids running (ruining?) them were as uninformed as myself.
Put on a decent modern rubber and I’m sure you’ll be ok.

Don, your PM is over the limit and won't allow me to send. [email protected],com

One way or another I need to get to 0.030". Quinten mentioned an early ('66) service bulletin with 1/16" but I'm dealing with all later parts. All the parts I'm using are from a 1972 EXCEPT the cylinder head itself which I think is a 1968 (but not 100% sure of that). This could be the source of my problem with reducing the crush figure.
Al

As mentioned before, it's quite all right to use good quality O-rings if you can't obtain the desired compression with what you have. You can also use suitable fiber rings as spacers or make them yourself from gasket material if you need to. I admit to having used RTV sealer on especially troublesome damaged tubes or seats. All this may have been mentioned in earlier posts, I'm sorry if I'm parroting.

SR

It changed over the years so you have to pick your setup and use the appropriate crush.

70-1497 are O rings, not square section white seals

Just finished Pete Suchawreck's 76 750....the existing .098 white rubber showed signs of tearing at around .030 compression. There were no leaks, no bent head so it went back together with the same new rubbers..
No stock Triumph has anything close to effective head to piston quench so no problem there.....

If you haven't seen it, take a look at THIS Note that if that is a '68 head, the hle in the head (so to speak!) will be deeper, and have a diffeent diameter.The casting date (two digits cast into the inner valve spring area of the head) may give a clue of the actual date of the head, but sometimes the casting is produced before the model year.

Some additional info:

https://www.jrcengineering.com/tech...triumph-tappet-blocks-and-pushrod-tubes/

Last three builds for me have been .030-.040 (500,650 and 750 engines) and Ive not experienced any leaks

Hi Al, Sorry I was over limit. Should be good to go now.
Don

Hi, Going .030” thicker head gasket will lower hot cranking compression by about 20#.

But if head was skimmed that’s usually about .008-.015”. So the .080 thick head gasket doesn’t lower compression much in that case.

In my experience if your way out on crush, head was skimmed.
That’s what the thicker head gasket was made for.

People make a big deal to not skim head. Reality is seeping head gaskets the surface has dips between bolts. Not bent head at all. Just dips. Actual blown head gasket can leave a “groove”. Where flame passed through. My blown head gasket front center head bolt left a groove. So skimming was a must. The first shop did a rougher finish than is best so I had it reskimmed during recent overhaul. .019 total was removed.
I sanded top of cyl flat on surface plate. Had .0007” dip at front center head bolt.

Regarding head thickness my head is still thicker. A few years back I took my calipers to Rabers used parts loft. Measured thickness of many heads. All over the place. I couldn’t make sense of of it. It’s like Triumph surfaced one side, flipped head over & surfaced the other. Thickness be what it may. I know that’s not true, but it was certainly not consistent.


Very important to remember the head machining for PRT is very different starting in 1969 with the “new” sealing system. Basically forget the earlier system when working on later motors.
It changed again in ‘79.
Whichever system you have all the parts must match that system. Each system must be looked at in its own right.
Don

Kommando's o-ring list was very helpful. I have about 0.022 to 0.025" right now (I think - will be rechcecking this several times before I tighten anything!)
The o-ring inside the tube is 71-1283 round viton o-ring.
Bottom o-ring on the tappet block is 70-3547 black viton square profile (with the outer sleeve of course).
Top o-ring at the cylinder head is a found o-ring, round,black and 0.068" thick. This is about 30 thou less than the other o-rings on kommandos list. If this will work it puts me in the ball park with a bit less than 0.030" crush ( 0.022 to 0.025" as stated above).

So the two rings and the band on the bottom of the tube are essentially normal - and the thinner top o-ring makes up the difference for whatever the problem is with the cylinder head. It may have been skimmed. It may be an earlier head than I thought it was. Or who knows - but it should work now. I'll recheck it for a day or two before I put a wrench to it.

I think the tube seals are not a static situation...there is the slight expansion and contraction from temperature changes.....and possible fretting from vibration. The white seal compression helps holds the tube in place.....I believe any changes to the white rubber thickness and or using a substitute needs to be done carefully...I believe the o rings move about a bit and no sealer should be used on them....Others may have different opinions based on their experiences...
I really never had problems with tube oil leaks and never gave it much thought on assembly in the past....

The 70-3547 is often referred to as a white seal but the ones I have are black Viton rubber as supplied by Coventry Spares. While not the thicker 1/8" rubber I hope it will be substantial enough to hold up.
I want to thank everyone for their input.
Al

I suspect some of the discrepancy of opinion and info comes from the changes of arrangement made through the years.

I think of a pre-unit situation (which probably also applies to the earlier units) which have a thick (conventionally black) seal captive at the bottom, and a fairly thick white seal at the top (captive between the PRT flange and head recess.
Because these seals added up to a fair thickness, a ~60 thou crush was advised (1/16”).
This is fair enough considering the depth of the “spring”.

The later models use thinner seals (as is apparent from the service bulletins, this setup wasn’t originally intended to use lower seals at all, just internal O-rings, in 1969, but it didn’t work, so lower seals and wedding bands were added in short shrift.

This meant that the combined thickness of the upper and lower seals is less on the later setup. It makes sense that thinner rubber spacers require less compression (crush) to achieve the same sealing force on their surfaces.

That is the important engineering factor, that the force of the seal against its surface is sufficient to deal with internal engine pressures, otherwise it will allow oil to escape.

Just thoughts…

That's what I was trying to say in my thread - just filling the void below the bottom of the PRT with sealant won't achieve anything other than just that. The force against the upper O ring would be non existent without the bottom end having something to push against.

You're right, filling with sealant won't do at all.

Hi Koan & Tigernuts, I’m with you 100%.
Don