Custom HG for 86mm Bore Stroker?

[Overlord475]

All you stroker builders.

What are you doing for your HG for 86mm bored pistons?

The stock HG is 85.2mm bore so anything above that has to be custom.

Is there a company already producing oversize bore M20 HG or is this truly a custom order part?

Help please

[nando]

no, you do not need a custom head gasket for an 86mm bore. use the stock one.

[Overlord475]

no, you do not need a custom head gasket for an 86mm bore. use the stock one.

Stock HG bore is 85.2mm. So won't work with a 86mm piston.....

[nando]

yes it will. what do you think I'm using in my engine? ;)

MM uses a stock gasket with modified coolant passage holes, but the bore diameter is still stock.

[Overlord475]

Ha, well not sure how it works but I'll take your word for it! lol

Planning on going with a set of IE forged pistons and he said something about the HG not working with anything over a 85mm bore. Whatever...

Ha

[nando]

if I were you I'd look into JE forged pistons, as they have the stock dome shape for the 885 head.

I could see it not working on a totally flat top piston that comes over the deck, but M20 pistons using the 885 head should be domed, so they won't touch the gasket even if the bore is slightly bigger.

you could always give a call to MM and see what they say

[Overlord475]

if I were you I'd look into JE forged pistons, as they have the stock dome shape for the 885 head.

Do you have the specs that I need to tell JE to make the pistons by?

[acolella76]

Do you have the specs that I need to tell JE to make the pistons by?

You could probably go straight to JE and tell them what crank, rods, and bore you are using and they should be able to help you out. That or Elephant Motorsports is a distributor for JE and they would probably know.

[Overlord475]

I guess I could place the pin position to where the crown is flush with the deck height which would allow the angled lip of the piston head to clear the HG. I would just be worried about rod elongation.

I'm using S50 (135mm) rods... anyone know the max elongation of these?

[nando]

call MM, ask them why it works.

[Philo]

call MM, ask them why it works.

I'm thinking that it's due to the tapered head on the pistons like you've stated. I might have lunch with the guys from MM today, I'll be sure to bring up the topic during conversation.

[digger]

the MM pistons are chamfered thats why there is no interference

[Philo]

Indeed.

[Overlord475]

Yup, talked to the guys over at MM today and they've told me that due to the chamfered crown shape of their pistons the stock 85.25mm bore HG will work fine. Seeing as stock specs b25 pistons also have dome shaped crowns they should work as well when set flush with the deck of the block.

I am using an M52 cranks and S50 135mm rods, a combo that MM does not have a piston for so I've decided to go with a set of JE forged pistons from Top-End Performance.

I'm going to go with 86mm stock b25 shaped pistons which should work with a stock HG. Though some people on the forums say that 85mm bore is the limit for an M20 the guys at MM say that's a load of bull. They've done sonic testing on tons of BMW blocks and have concluded that BMW has used the same cylinder wall thickness in our M20's as they have on M/S5x engines and many more larger bored out blocks. They said with a custom bore HG an M20 could be bored all the way out to 87mm!!!! Wow...

I'm thinking of going with a 10:1 CR but where I live the highest octane pump gas available is 91 (no ethanol). I used to put 91 in my E34 M5 which had a ~9.7:1 CR and it worked just fine.

I think it should be ok... Thoughts?

[Philo]

Yeah, if you are running a 10:1 ratio, 91 octane should be fine. That's what i'm at, and it's also what MM recommends for their 10:1 ratio engines. 11:1 is where you really need to make the jump. Just make sure your chip is mapped accordingly.

BTW, I hope you were nice to Jim's wife when she picked up the phone. She makes an awesome breakfast.