Topic: 1973 CB750 Leaky Puck

[chewbacca5000]

I could use a little help with my 1973 CB750.  The engine is out of the bike valve cover is off and I am at TDC 4.  Now I need an easy quick way to remove the cam towers to replace a rubber puck to keep oil form dripping down between 3 and 4.  The oil drips down and runs along the fin making it look like a head oil leak.

Thanks and Happy New Years!

[mrrch]

No quick and easy way, cam needs to come out. Hardest part of the job (removing engine) is done already.

[chewbacca5000]

I got the cam out, and sure enough the 2 center pucks in the center had oil in them. I am still not confident that the there is no head gasket oil leak. Compression on the side of the oil leak is good around 125 PSi.  No loose head bolts.

Looking at the head it looks like there is a little gap between the head and the gasket.  This might just be the placement of the gasket.  Any ideas here?

[ekpent]

Are you planning on replacing the head gasket ? Your almost there anyways,new head gasket and change all the pucks would be a good idea where your at.

[chewbacca5000]

Are you planning on replacing the head gasket ? Your almost there anyways,new head gasket and change all the pucks would be a good idea where your at.

Ekpent,

I think that is the route I am going to go.  I just took the head off and everything looks pretty good. A nice even layer of carbon on the pistons and the head no heavy caking or deposits.  Exhaust valve 3 is badly rusted so that will need attention. Bores 1.4 look nice and glassy.

If I take the block off does that force me into having to replace the rings.  Thanks!

[mrrch]

Probably a good idea to seal the 4 studs for the cam towers, oil can leak past those threads. I had found them loose on the 2 heads I have worked on. I used some Threebond on the threads.

[chewbacca5000]

Probably a good idea to seal the 4 studs for the cam towers, oil can leak past those threads. I had found them loose on the 2 heads I have worked on. I used some Threebond on the threads.

You mean the 4 10mm Phillips head bolts that attach the head to the block?  What about 3 bond under the rubber pucks?  Will this helps with leaks?

[ekpent]

If it were me and I was not going to do anything with the pistons or rings I would leave the barrel alone. May be asking for more trouble breaking rings etc.Do the bores look good.?. Others though will suggest you change out the base cylinder gasket also.

[chewbacca5000]

If it were me and I was not going to do anything with the pistons or rings I would leave the barrel alone. May be asking for more trouble breaking rings etc.Do the bores look good.?. Others though will suggest you change out the base cylinder gasket also.

Bores look good and all rings are free, but will measure the bores to be sure.  That said I can probably get away with new gaskets and a valve job.

[K5owner]

Yea, I'm one the others...   I'd change the base gasket while you're this far.

[seanbarney41]

just more opinions but...I've yet to actually see an undisturbed cb base gasket leaking...but of course, pulling the head could certainly be considered a disturbance...

[mrrch]

There should be 4 6mm studs left on top of the head once the cam towers have been removed.

[Powersnatch]

I just wanted to chime in on this thread. I had the same leak, got into it and found my old pucks hard and  shrunken. New pucks were soft and a perfect fit, not concerned with gasket sealing them in at all. Will know tomorrow if they fixed the leak.

[SKTP]

I did not seal my pucks and I now have "special" oil pants to ride...pulling the motor and sealing those pucks...

I plan on following this thread

http://forums.sohc4.net/index.php?topic=107040.0

[Wobbly]

just more opinions but...I've yet to actually see an undisturbed cb base gasket leaking...but of course, pulling the head could certainly be considered a disturbance...

My experience exactly. I wouldn't touch it.

[evanphi]

I did not seal my pucks and I now have "special" oil pants to ride...pulling the motor and sealing those pucks...

I plan on following this thread

http://forums.sohc4.net/index.php?topic=107040.0

THAT THREAD IS ALL OP NEEDS! I'm going to be following it too! I started with puck leaks, then a head gasket leak. Going to be replacing my head gasket and my pucks when the weather warms up a bit.

[flybox1]

I did not seal my pucks and I now have "special" oil pants to ride...pulling the motor and sealing those pucks...

I plan on following this thread

http://forums.sohc4.net/index.php?topic=107040.0

THAT THREAD IS ALL OP NEEDS! I'm going to be following it too! I started with puck leaks, then a head gasket leak. Going to be replacing my head gasket and my pucks when the weather warms up a bit.

+1
..and, if you have Hondamans book, it'll list the necessary puck thickness range. 
I had to purchase 10 pucks to eventually get 6 that were thick enough (based on my puck cutout depth) for adequate compression.  I still sealed them.   I'm still leak free...

[PeWe]

When head is off, there are things to do and money to spend....
Check the valves, seats and verify guides (stem - guide play) by wiggle the valves when a few mm out. CB750 steel guides seems hard to damage/wear out. Minimum lap all valves and replace all valve guide seals.

- Check if the cam tower studs leak oil. It should be easy to check with finger from the ign plug side. Also check if the threads for these are OK. Maybe need of helicoil that has to be glued, stud mounted with thread sealer with teflon.
A small radius on the head close to the stud indicate coming thread failure. The 2 inner seems to get thread failure easier than the outer.
The other threads for the cam bolts might need to get helicoils too?
My head have all these holes got timeserts.

New rubber pucks that are not too thin, seal them with a non hardening sealer.
The 4 small o-rings for oil channels to cam towers need to be replaced too. Many of them are too thin, no rubber over the head surface meeting the towers. It must be some extra for rubber to seal
-All threads for valve cover OK? More thread repairs...