SEALING PUSHROD TUBES

[Matty]

My Comet exhaust valve pushrod tube always seems to leak a little. Over the years I have often replaced the lower seals which are OK for a time and then leak again
The pushrod tubes are the correct diameter - years ago I had aluminium ones which were 25 thou small which leaked very badly - but these were replaced by ones of the correct diameter.
Any ideas please on how I can get a good seal - have tried silicone instant gasket and some other sealants with little success. The trouble is it means taking the head off to replace these lower seals which is a real pain.!!

Matty

 

[Vincent Brake]

upon overhaul I smoothen the Alu casing, as it can be very course machined. then I use 10 sec glue to hold them down.

i fyou have no further leaks, i guess the breather system works, or not overloaded.

have fun

 

[greg brillus]

Matty use an "O" ring, the original type seals work ok on some engines and not on others, plus they harden up and lose there tension on the tube. Sometimes you can unscrew the tube nut, remove the pushrod, and squeeze an o ring over the top of the tube, but worst case you can cut the o ring and rejoin it with some special fast drying glue, then massage down into the case, I also use a bit of sealant as well. They don't seem to leak after this. I think 3 mm o ring is what you want, cheers......Greg.

 

[Pushrod Twin]

It surprises me that more dont leak. When compiling my crank case inspection sheets I gleaned from 2 sources, KTB & 40 Yrs on, that the crankcase pushrod bores should be 15/16", or .9375" nominal with a tolerance of .935" to .940" for the the bigger one, & 49/64", or .7656 for the lower one. (No tolerance found but the +/- .0025" could be applied safely) On my die cast cases the holes measured any where from +.004" to +.011" over size and one has cold short porosities visibly through it. Just the oversize holes will ensure that the seals are struggling to do their jobs.
How did you find the "correct diameter" for your pushrod tube ends? My stainless ones are so out of round its a guess as to what the intended dimension might have been!

 

[Matty]

Thanks for the replies.

Have measured my push rod tube outer diameters and they are 0.746" - 0.750" diameter where the lower seal goes, which looks OK.

Also the bores in the crankcase where the seals fit is 0.934" which also looks OK.

Fitted a new seal (with a bit of careful juggling) into one of the bores as tryout and had to work the pushrod tube in (the end has a taper) and the fit was very tight so I would be surprised if it leaked.

Will put a smear of a special black silicon sealant on the seal and pushrod tube when I finally assemble the cylinder and head and hope for the best.

If this does not work I will try the O ring trick next time.

I'm pretty sure my breather is working OK and properly timed - I can feel the puffs at the breather pipe with the engine running and there are no other leaks or drips from the bike. The oil at the pushrod tube is not really a leak - just a bit of oilyness in the dirt and perhaps a slight trickle on a long run.

Thanks again

Matty

 

[Robert Watson]

I had one that would not seal and of course it was the rear one which I couldn't even externally get at to seal until the next time the head was off so the next time the heads were off I made some small alloy rings with internal 1/16 o ring grooves and installed in the lower tube end recesses with locktite. Used a Viton o ring and they haven't leaked in years. Each one made for each recess/tube. I think just a thou or two clear on the OD for the locktite and about .010 for the tube with the proper groove for the ring.

 

[Pushrod Twin]

Thanks for the replies.
Also the bores in the crankcase where the seals fit is 0.934" which also looks OK.

That's heartening, undersize, so the factory could machine to the recommended tolerances, almost! Maybe by the time the die cast cases started the machines were that much more clapped out?