Oversize pistons headgasket?

[Maximumtune30]

Im rebuilding my m20 with 85mm pistons, can i use the standard bore size headgasket or do i need a bigger one?

[2mAn]

ForcedFirebird will give you the best answer

[digger]

people usually use the stock gasket, the bore of the stock one is slighty larger than 85mm. IIRC the OE one was about 85.5

[ForcedFirebird]

IIRC the OE one was about 85.5

Yup, exactly. Either way, I have built many of engines over the years that had a gasket slightly smaller than the bore. So long as the pistons don't hit the gasket at TDC, I'm not afraid to run it.

[nando]

I don't think you need one either especially not for 85mm. I have run my M20 stroker with 86mm pistons for 15 years / 100,000+ miles and the stock gasket without an issue. There can be issues with the quench area and larger bores, or shrowding with oversized valves, but that's more for when you're really pushing the limits of power from the engine.

[digger]

I don't think you need one either especially not for 85mm. I have run my M20 stroker with 86mm pistons for 15 years / 100,000+ miles and the stock gasket without an issue. There can be issues with the quench area and larger bores, or shrowding with oversized valves, but that's more for when you're really pushing the limits of power from the engine.

i'm using a stock gasket as well but you cant use 86mm pistons with OE gasket on anything except the MM design though as pistons dont really come out of the hole and/or there is a whopping chamfer which none of the other manufacturers do

[ForcedFirebird]

i'm using a stock gasket as well but you cant use 86mm pistons with OE gasket on anything except the MM design though as pistons dont really come out of the hole and/or there is a whopping chamfer which none of the other manufacturers do

Yeah, but what would be the interference considering angular squish? I know when I take .002-.003" off a valve face, it only affects the stem height a few tenths of a thou.........but.........that's my practical knowledge. :/

[digger]

Yeah, but what would be the interference considering angular squish? I know when I take .002-.003" off a valve face, it only affects the stem height a few tenths of a thou.........but.........that's my practical knowledge. :/

if i understand what you are asking, the MM pistons do not interface with the OE squish as the angles are all wrong, so it is not a good design at all. if you use a 86mm piston with 85.5 bore OE gasket you aren't going to be able to setup squish correct (gasket is 1.75 and squish needs to be 1mm so it needs to protrude above deck) unless you do some crazy nonsense notching of the corner of the piston.

if you are getting custom pistons then machining the block and head to a quality suitable for a MLS gasket to be reliable is straight forward its a couple hundred extra but the grand scheme not much

[nando]

Maybe it's not perfect, but meh. I've beat the hell out of it for 15 years / 100,000 miles, so it seems pretty good to me. :p

[digger]

It’s not an issue with durability it’s an issue with the quality of combustion due to the shape of the piston dome being nowhere near optimised for the combustion chamber. Even with a larger gasket that won’t change with those particular pistons but the gasket doesn’t allow the use of a more ideal shape on something larger than 85mm

[nando]

well, it runs just fine. Maybe I gave up 2whp or something? Who knows. I'm not saying you're wrong, and I get that it can impact combustion quality. I'm just not sure - based on my long term experience - that it actually matters in practice.

Regardless, the OP doesn't have an 86mm bore, so it's completely moot. He definitely doesn't need an oversized gasket.

[digger]

you only need to look at the importance of squish and mixture motion on high performance engines especilly 2V/cyl. i'm not sure how running a car for 100k miles shows that it's not important for performance or that it is theory only. i mean engines will run under a very wide range of sub otimal conditions especially when only of mild level of performance. There are numerous threads on flat tops (pistons with no squish) vs the OE dish dome including FF's dyno thread it probably doesnt need to be mentined/debated in detail again.

the OP is not running 86mm but others who read this should take note that just becuase MM pistons can use a stock OE gasket doesnt mean their 86mm pistons will.