Overbore head gasket on std head

[randy]

Guys, Gals, GURUS, New Moderators all,

What in your expert opinion(s) would be the ramifications of installing an overbore (74mm) head gasket on a standard bore (71.46mm) cylinder & head?

My personal opinion is that the 2.54mm additional diameter wouldn't make any difference, however I'm soliciting an EXPERT opinion here!

Prom, please note that I have included the proper metric notation, so please don't rip up this post!

[DiverRay]

Randy,
I don't think you will have any problem with the head gasket. You WILL end up with a little space for the carbon to build up, but when you do next pull the head, it will just be a little more work. I think the difference in compression shouldn't be much. Just be cautious at first, as the air in the chamber may not "flow" as designed with that extra "gap" between the head and barrels.
Just my bit....
Ray

[John]

Randy

I wouldn't do it. I would worry about combustion pressure in the gap between the head and the cylinder. There may be an opportunity for a leak to develop between the cylinders. Overtime the carbon build up in there may act to pre-ignite the fuel, and could cause the head to run hotter than normal. Just my $.02

Would be willing to swap that big bore gasket for a stock one?

[TopCatGr58]

Hey Randy,

Do you already have the BB Gasket, or are you looking at getting one because they are cheaper than buying a whole gasket set, or an OEM one??

I remember doing a search some time back and found a place that was selling the Cometic brand/style gaskets that fit the OEM size, not just the BB size, but I didn't save the link!

T.C.

[MAXIMAN]

Randy

I got an extra one. PM Me.

[randy]

T.C.;

Got big plans this winter, cam chain, valve lap, stem seals, etc.
I was looking for just the head gasket because the one in my full set has a nick in it in a bad spot, don't want to chance it. Saw an overbore one on Ebay, thought I'd better run the idea up the flagpole before jumping in.

Thanks for the input, guys. I was also concerned about carbon in the gap, John and DiverRay, but not so concerned about compression or leaking.

[prometheus578]

Randy,
When I was down at Kawasaki earlier this month, had a seminar on rebuilding and such, especially head/cylinder surface preparation. The gasket needs to be the same diameter as the cylinder. Combustion pressure seeping past the gasket as John mentioned, or the chance of the pressure wanting to expand the head from the cylinder.
If I may pass along a little of what I learned:
Cleaning the mating surfaces: Best technique is gentle use of a razor blade. Do not use one of those "Scotch-brite" pads chucked into a die grinder. Inadvertantly, you'll bevel the edge of the cylinder.(which would be like the gap you were worried about, only not as extreme)
Not pertinant to our machines, but for those who rebuild their kid's high speed dirt bikes, that bevel cuts away the nickle plating on the cylinders that overlaps onto the mating surface, and compression can get behind it and peel the coating away from the walls.
The cylinder wall, gasket and head all must be the same diameter with no gaps.

[randy]

Prom,

Thanks for the insight and explanation. I considered using the gasket because of my experiences with auto head gaskets. What I've notice over the years is that the auto gaskets, even felpro are not made to such close tolerances as the bike gaskets. last auto head gasket i changed, on my 6.0L jeep, there was plenty of extra room (a couple mm's) around the cylinder wall/head joint.

i'm guessing this has a lot to do with water cooled engines being more heat and vibration tolerant vs. air coooled.