Topic: +1mm pistons with the stock head gasket - CB750

[Kickstart]

Has anyone else used +1mm pistons with the stock head gasket and noticed the gasket bore ID edges slightly overlap/overhang the cylinders (on some of the cylinders).

I know the stock gasket is suppose to be good up to a +1mm bore, but I can feel a slight overlap with my fingers on two of the cylinders when trying to line-up the head gasket on the cylinders.

I'm using an OEM head gasket.  If I position it over the 1&2 cylinders so that I don't feel any overhang, I notice a slight overhang on one side of the 3&4 cylinders (and vice versa if I line up the gasket perfectly over the 3&4 cylinders) .  It's hard to see, but I can feel it with my finger.

I'm concerned this will cause my piston to contact the gasket... especially since I decked my cylinder and my pistons sit proud of the deck by approximately 0.010" at TDC.

Is it OK to sand/grind that metal ring on the head gasket to provide some extra clearance, or is that asking for trouble?

Thanks,
Chris

[lucky]

No you are not crazy. You ARE doing the right thing.

You are right. IF you can feel the gasket protruding and you check it with a straight edge,
then the problem has to be rectified or you know what is going to happen.
And remember that as the gasket crushes it will spread.
The question is really .....When the piston is at TDC how close does it get to that gasket?
The piston may never reach that gasket.

Call Cycle-X and see if they have a better gasket for you.
I would get a better gasket.

[Kickstart]

I believe one or more of my pistons will contact it, as the edges of my pistons sit slightly proud of the deck at TDC (about 0.010" as best as I can measure)... because I had the deck milled a little - a mistake in hindsight, but that's where's I'm at right now

On a couple test assemblies I've done (I did a few to measure the valve clearance at different cam timings) I did notice some resistance when turning the crank through TDC for the 2&3 pistons - which made me go back and take a closer look at the head gasket when I took it back apart.  However, I wasn't able to notice any impressions on the gasket's metal part around the cylinder (so maybe it just slightly touched?)

So, in summary:

• Pistons do sit slightly proud of the deck at TDC (not just the center, but the edges of the piston)
• The head gasket appears to overhang some of the cylinders (I'm not sure how to measure this, but I can feel an overhang with my finger... so I can get the gasket lined up with some of the cylinders but not all of them
• The overhang is slight, and the piston doesn't stick up that much, so it's hard to feel or see any indications of contact during the test assembly or tear-down
• I'm using a band new gasket - but it's already been compressed as I've used it for my test assemblies

Can, or should, I grind/sand the metal (copper?) ID of the openings on the head gasket?
It looks like that the metal piece is one folded piece, so sanding/grinding might result in two edges of metal and exposing the area between the folded metal... which may cause a sealing problem?

Does anyone make a "slightly" larger head gasket?  I don't want to go all the way up to a 836 gasket.

Has anyone else experienced this with +1mm pistons?

Thanks!

[bwaller]

If you're using a stock thickness gasket (0.043") then you have 0.033" clearance to the head and less when the gasket is compressed slightly. You've checked valve/piston but have you checked with clay or solder piston/head clearance. Shouldn't be an issue but always best to check. There is always some rod stretch at high rpm.

You cannot grind the metal fire ring on the composite gasket or you'll make two pieces out of it and lose the seal then blow the gasket. I'd suggest an MLS (multi layer steel) gasket. Contact our Mike Rieck here, I think he can help with one. If your pistons are proud of the deck for sure you need a gasket with a slightly larger bore.

Good luck.

[Kickstart]

Thanks bwaller.  I assumed the stock headgasket thickness gave me plenty of clearance (even with the pistons sitting proud) - 0.033" inches as you've  mentioned.

What is the recommended piston to head clearance these engines should be built to?

I'll contact Mreick about getting an MLS gasket.  Are you saying the MLS ones can be grinded/cut?

Thanks

[bwaller]

No I wouldn't grind any off those either because they are coated with a thin layer of Viton to seal. Any steel grit would embed in that layer me thinks. You'll need a gasket with a larger bore and I'm not sure what sizes are available. Eons ago I made my own big bore head gaskets using the larger diameter fire rings from a 450 gasket. It was painstaking.

Re the clearance issue. A lot of sohc4 stock engines I have checked are 0.016"- 0.020" below deck, plus a 0.043" gasket gives you the clearance Honda engineers thought was needed. Another example, the first season I built a squish clearance of +-0.032" in my race engine. When I tore it down after the last race all four pistons had kissed the head, this with a copper head gasket that doesn't compress. Mind you these are low silicon forged pistons that really do "grow" as well as stock rods/bolts. This year with Carillo rods, same pistons and even tighter squish no problem. To me this equates to rod stretch.

Bottom line with 1mm oversize....sorry but I think your clearance is a tad tight. Address the head gasket somehow and consider a thicker viton coated steel base gasket. I know Cometic makes several thickness options for the 550 so the same should be true for the 750. You can even stack two gaskets if you have to.

[Kickstart]

Well, I can get the back the 0.010" I had milled of the head by getting a thicker base gasket.

But that doesn't solve the problem with the head gasket overhanging the cylinder slightly.  I really don't want to go to a 811-836 cc head gasket.  That would be around 3mm larger than I need - not sure how that would effect compression.... maybe it's insignificant.

[bwaller]

Copper is an option, order with a 61mm bore and size it to fit. They aren't the best option on the street as they will eventually weep a little. I use Gasgacinch with copper gaskets and I like it.

[bjbuchanan]

Use an MLS gasket, superior sealing. You can use a lightly oversize gasket, you see this more commonly with car engines with the cheapie stroker kits. They use one gasket for 'similiar' bore sizes.

[Dream750]

Cometic offers custom made gaskets, but I’ve never had one made up for a CB750.

Just a thought ...

http://www.cometic.com/custom-gaskets.aspx

[Kickstart]

Maybe another gasket (possibly the MLS one) is the way to go.

It appears the bores/openings in my stock gasket are large enough (just), but the alignment of the 1&2 bores to the 3&4 bores (in the gasket) is slightly off.  Maybe another gasket will fit better.

The stock gasket should be good for the +1mm pistons.  Maybe the gasket I have is just off slightly... or my cylinder bore job!

[bytio]

It may be a silly suggestion, but is the gasket the correct way round, try flipping it upside down. I have1.00 oversize pistons, and the gasket is genuine Honda, one way it's perfect and no over hang but upside down, a slight non alignment plus overhang!

[Kickstart]

I'm pretty sure it's on the right way.  I think I recall it can only go one way and have all the bolt/threads/cam guide opens line up correctly.  I'll double check once I get the engine back apart (I currently have it assembled hollow for cleaning and soda blasting).

So you're using +1mm pistons as well?  I do have another OEM headgasket that I'll also try (it's still in the packaging).  I bought another one as I kept on using my first one for mock-up assemblies and figured I should probably get a new one for the final build.

I also have a really old (but not used) Vesrah head gasket.  I might lay that on there as well to see if it's slightly larger than the stock one.

[bytio]

Yes +1.00 here is a pic just before putting barrels on ( you can see 1.00 on side of piston )

[Kickstart]

I laid my other OEM head gasket, and my old Vesrah head gasket, on top of the cylinders for comparison... and sure enough they lined up better than the first OEM head gasket I was using.

So I guess the first OEM head gasket is off on it's tolerances... odd. I'll try to take a picture later to show the difference (sorry, I should have snapped one before).

That said, the other gaskets "just" fit.  I think I might use some copper head gasket spray to make sure it doesn't move during assembly.  Going +1mm is tricky... I think on the next build I do I'll either go 811, 836, or if I stay stock I'll just go +0.5mm to give myself a little more leeway with the head gasket.

[bryanj]

Big thing is you need to get those piston level with the metal block by either machining the pistons or extra thick base gasket, if you go the thick gasket route you may get oil seepage where the oil feed "O" rings are

[Kickstart]

Hmmm...  I didn't think about those O-rings.  Good point.

If I use the MLS base gasket, I might not need the O-rings anyway (not sure about this though). 
I could also order o-rings that are thicker (Might have to break out the calculus book to figure out how much thicker the o-ring cross section would need to be to fill the extra volume.)

Or, maybe I'll just stick with the standard base gasket - or is 0.030" not enough piston to head clearance when using stock rods?

Also, does anyone know what's required to achieve a "50 RA" surface for the MLS gaskets?  Is that more high-tech then using sanding a block with some wet/dry paper?