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1G 86.5mm bore?

dwb

Proven Member
238
103
Sep 9, 2021
Broomfield, Colorado
This is a 6 bolt 4g63, never been machined, 270k miles, stockish rebuild.
As I understand with bore sizes (please correct me if I'm wrong), it's 85mm from factory and usually increments in .5mm (0.02in) when machined. I've seen piston sizes up to 1mm (0.04in) over size for stock replacement.
My machinist said my bore size is going to be 86.5mm. I'm hoping this is a typo but if not, what does this mean in comparison to 85.5mm? Different compression? What about 86.5mm piston choices for 1g or 2g oem equivalent? Is there headgasket changes needed?

Thank you!
 

DSMPT

DSM Wiseman
2,130
1,697
Jun 12, 2014
Japan / Mexico, Arizona
How badly did you damage the cylinders? Sounds like it's 85.50mm instead of 86.50mm. And usually the increment is in .25mm (0.010"), so don't cut more than you need if it's not necessary, you are just wasting the margin for the future and making less durable.
What about 86.5mm piston choices for 1g or 2g oem equivalent?
Generally speaking, you won't find a 86.50mm bore OEM style replacement piston kit. Usually up to 1.0mm over.
Your option would be :
Forged piston kit.
Sleeve the cylinders.
Find another block.
what does this mean in comparison to 85.5mm? Different compression?
The compression ratio would slightly be higher. And the cylinder wall thickness would be thinner. If you would boost a lot, 86.50mm bore is not recommended unless you sleeve or fill the block. With 86.00mm bore, you would start to crack the cylinder wall more often, but it doesn't mean you can't not run 86.50mm. You can but it's just you would have more possibility to crack the cylinders in the future.
Is there headgasket changes needed?
If 86.50mm, yes you would need a aftermarket head gasket in 86.50mm or 87.00mm.
 

DSMPT

DSM Wiseman
2,130
1,697
Jun 12, 2014
Japan / Mexico, Arizona
If so, maybe just hone and re-ring or a standard 85.00mm piston kit would be fine.
In case if you would need to cut the cylinders and 0.25mm is sufficient, first look for a 0.25mm oversized piston kit because maybe it's hard to find a + 0.25mm kit in the market nowadays. If you would be able to find one, then go with 0.25mm (.010"), if you wouldn't be able to find it, go with + 0.5mm (.020") kit.
Anyways before buying a piston kit, you have to make sure why your machinist said it needs to be cut by 1.5mm. Maybe he has a reason to say so.
 

CrackedDSM

10+ Year Contributor
2,023
686
Dec 17, 2009
Pensacola, Florida
How badly did you damage the cylinders? Sounds like it's 85.50mm instead of 86.50mm. And usually the increment is in .25mm (0.010"), so don't cut more than you need if it's not necessary, you are just wasting the margin for the future and making less durable.

Generally speaking, you won't find a 86.50mm bore OEM style replacement piston kit. Usually up to 1.0mm over.
Your option would be :
Forged piston kit.
Sleeve the cylinders.
Find another block.

The compression ratio would slightly be higher. And the cylinder wall thickness would be thinner. If you would boost a lot, 86.50mm bore is not recommended unless you sleeve or fill the block. With 86.00mm bore, you would start to crack the cylinder wall more often, but it doesn't mean you can't not run 86.50mm. You can but it's just you would have more possibility to crack the cylinders in the future.

If 86.50mm, yes you would need a aftermarket head gasket in 86.50mm or 87.00mm.

Not exactly true. The OEM composite gasket has a bore of 87mm on it. As does the oem 4G64 DOHC composite. My built 4G64 was bored over by my machinist and thats the gasket I used. Didn’t hang over the cylinders at all.
 

DSMPT

DSM Wiseman
2,130
1,697
Jun 12, 2014
Japan / Mexico, Arizona
Not exactly true. The OEM composite gasket has a bore of 87mm on it. As does the oem 4G64 DOHC composite. My built 4G64 was bored over by my machinist and thats the gasket I used. Didn’t hang over the cylinders at all.
Sure the gasket bore is larger than cylinder bore is normal, and "can" run. What I am talking about is recommendation. Max OEM 4g63 piston size is 86.00mm, the OEM gasket is designed to cover that. So it says the OEM 4g63 composite head gasket would safely work for sure up to 86.00mm. It's just a theory and if you don't care about that, then you can use. This was told by a Mitsubishi motors mechanic long time ago, and I would definitely follow it.
 

slugsgomoo

Proven Member
423
15
Nov 10, 2002
Seattle, Washington
Glad it's just a standard .5mm overbore, should simplify things. All that said...

Way back in the day I had a 9:1 Wiseco/Eagle 6 bolt that was .060/86.5mm, and it ran great. A friend bought it, had a 2-3-2 shift, and spit the rockers and the motor was still good to go after they and the belt were replaced (no bent valves luckily)... I never ran it over ~22psi on an evo3 16g (it was 17 years ago...), but I loved that little motor. I might not go 86.5 if I were trying to make big stinky power, but for a typical street car it should not be an issue.
 

CrackedDSM

10+ Year Contributor
2,023
686
Dec 17, 2009
Pensacola, Florida
Sure the gasket bore is larger than cylinder bore is normal, and "can" run. What I am talking about is recommendation. Max OEM 4g63 piston size is 86.00mm, the OEM gasket is designed to cover that. So it says the OEM 4g63 composite head gasket would safely work for sure up to 86.00mm. It's just a theory and if you don't care about that, then you can use. This was told by a Mitsubishi motors mechanic long time ago, and I would definitely follow it.

You can follow what you like. I tend to trust people that have actually built the 4G64/4G63 platform and pushed it beyond the limits of reasonability and their input. 86.5mm 4G’s have done 600whp reliably.

No offense meant or taken, just offering a different opinion. There’s a lot that Mitsubishi says not to do, that we do anyway with great and long reliable results. Like turn the boost up on 1Gs by removing the restrictor plug in the BCS, or putting on a bigger turbo on a stock engine.

Hope I’m a little clearer. But it’s 100% personal preference here. Absolutely cannot fault anyone for wanting to stick with 0.020 or 0.040 over.
 

DSMPT

DSM Wiseman
2,130
1,697
Jun 12, 2014
Japan / Mexico, Arizona
You can follow what you like. I tend to trust people that have actually built the 4G64/4G63 platform and pushed it beyond the limits of reasonability and their input. 86.5mm 4G’s have done 600whp reliably.
That's exactly what I said. I meant the person who told that is who had been working for this engine series since 80s who was one of the pioneer people in japan and who had its own shop after left mitsubishi. What you saying is "can" run and I am clearly saying yes you can. Please read my first post again. Here obviously I am not talking about if someone has made a good power with 86.50mm or not. Sure there are people who did, but also there are many people who cracked cylinders with 86.00+mm cylinders. I was talking about in general more sure/safer options. Again, follow what you like.
 
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