Topic: CB750 Valve Cover Gasket Redux

[kvallelunga]

Third time replacing valve cover gasket in as many seasons. Need to fix this for the last time. All gaskets from the local aftermarket bike shop, I think they are reputable. Don't recall the brand name.
First time leak due to gasket shifting, no adhesive or sealers used.
Second time, Permetex 2a, Semi Permanent sealer. Leaked after about 1000 miles. I don't remember the failure mode.
Third time,  Three bond 1194(edit; I checked the tube in the garage, not 1207, either F or G, I don't recall). I used it to seal the cases, it's worked great there for 10k miles. The gasket split, almost like the bond was too good and the uneven head/valve cover expansion/contraction through the heat cycles caused it to split across the cross section.

Here are my thoughts this time.
1-Hylomar Blue sealant, and the same brand gasket.
2-I have some FelPro rubberized sheet 1/32" thick. I would like to make the cross section wider where possible, in some places the aftermarket gasket is only 3/32" but could be as much as 1/4" or wide in many places. Is this material too thick to cause the tach drive gear insufficient mesh? Does any have a thickness of the original gasket or a word of advice?

Kevin.

[mick7504]

It might be an idea to lay a straight edge (steel ruler) across the gasket face on the cover to check it for trueness.

If it's been chromed previously, there is a possibility the the gasket face is not "billiard table" flat.

Are you using locating dowells during installation?

I've used Hylomar previously and also seen it break down after a long period of time so I'm not a big fan of it.

3 Bond is good stuff but it will only be as good as the mating faces allow it to be.

Mick

[kvallelunga]

It's flat, I've checked it on a surface plate, within a couple of thousandths.  Heads good too, I checked it when I rebuilt it. Dowels are in place. The leaks are at random places. Once on the left , now near the center, and once on the right. No it's not chromed, an F so it's only painted black.

[mick7504]

I think that all you can do is to clean any oil residue away with some degreaser or petrol (gas) on a clean piece of rag,
then wipe the faces over again with some methylated spirits to dry them right out.

3 Bond 1194 is a semi drying liquid gasket sealer, and with a new gasket, should hopefully stop any leaks.

Tighten the cover down evenly and firmly but not overtight.

That should sort it out.

Mick

Edit: One other thing to check is that the breather hose isn't blocked or restricted.

[kvallelunga]

Thanks Mick, any one else with similar problems, things they've tried?

[kvallelunga]

Bump. My replacement gasket should be in today, I should get time tomorrow to button the top end up, just wanted to get this in front of fresh eyes if anyone has anything to add before then.

[kvallelunga]

I installed a new gasket from Marshall. It was made in Japan for better worse. About 1/32" or 0.030. I did use the Hylomar sealant. It seems to spread well and is super tacky. I guess I'll see how it works.

[MCRider]

I don't know if this can be verified, but I think you should start tightening from the center out. A little at a time. Think of a rolling pin, you are rolling the aluminum and the gasket out from the center. Also, like the head nuts, tighten in stages: 70% first, then 80% then 90% then finish, always starting from the center and work your way out.

If you did it haphazardly you could build a bubble into it in places.

Or so I think, couldn't hurt.

[kvallelunga]

Thanks MC for bringing this up. I did do that, I think it is important too.