M20B25 bored .030 over

[Akcakc]

A local machine shop has a bare M20B25 block for sale, listed as bored 0.030”.

I’m trying to figure out if that’s a thing. I’m only seeing 0.010” and 0.020” (.25mm and .50mm) oversized pistons for sale when I look around for rebuild parts.

Am I missing something?

[It's Soda Not Pop]

you can buy 85 mm pistons for the m20. You can go up 2 mm over I believe before you run into issues.

[AWDBOB]

I would personally never buy a block that has been bored without pistons. Every good machinist I've ever used has required the pistons being used to be in his hands before agreeing to bore the block.

[TobyB]

They probably tried to clean it up at 0.020", failed, and kept going until it was clean, in hopes that maybe they could recoup some money on the work.
Once the block's set up in the borer, resetting the head and running another cut's a minimal time investment.

If you want to buy custom pistons, they can be made to match the block.
Or you could get 2mm overs and have them bore the block 'one last time'.

Me, I'd keep shopping. M20 blocks are durable. and still plentiful, around here.

t

[ForcedFirebird]

Run away.

[digger]

Nothing inherently wrong with a 0.030 over block if you plan on +1-2mm slugs but you'd also want to understand why they bored it 0.030 over and make sure that there isn't any other issues going on and knowing what other works were performed.

[Akcakc]

Thanks for the advice.

The shop is reputable so far as I know people they've done great work for in the past. They claim the owner asked for a .030 bore not realizing that it wasn't a standard oversize for M20's, saw the cost of custom pistons and walked away without paying the bill.

It's also a M20B27 block originally, which, googling around, seems like it just means it has a different crank and piston rods than a M20B25, so that might not make a difference.

Sounds like, if I want to move forward with it, I need to get my hands on 85mm pistons and have them bore it "one more time" with those pistons to measure against.

[zaq123]

Thanks for the advice.

The shop is reputable so far as I know people they've done great work for in the past. They claim the owner asked for a .030 bore not realizing that it wasn't a standard oversize for M20's, saw the cost of custom pistons and walked away without paying the bill.

It's also a M20B27 block originally, which, googling around, seems like it just means it has a different crank and piston rods than a M20B25, so that might not make a difference.

Sounds like, if I want to move forward with it, I need to get my hands on 85mm pistons and have them bore it "one more time" with those pistons to measure against.

if you are set to do the stroker (this or any other block), I'd buy quality!!! pistons (IE/Mahle)......... then go to this shop and have them show you the exact bore measurement so you can make up your mind.

IE/Mahle Spec below. They don't give you a tolerance but you can rely on OEM specs to figure out the tolerance for the bore :

OEM Spec P to C clearance - (0.01-0.04mm) 0.0004-0.0016" new. ;(0.12mm) 0.0047" limit for wear.

IE/Mahle : -85mm Bore (+1mm bore over stock)**Never have your block machined prior to receiving your pistons. Any good machinist will insist that they have the pistons in hand prior to boring the block.
-Piston-to-bore Clearance: .002 inches over the coating, measured at 7mm from bottom of skirts
-84mm Crankshaft Stroke (M52 crank for example)

-135mm Connecting Rods (M20, M52, S52, or length-specified H-beams) -10:1 Compression (This is 91octane friendly with proper tuning).