Stroking/boring M20B25/B27: Sectioned blocks show critical dimensions. PICS!

[LJ851]

digger, this is for you. ;)

The question arose about how much material can safely be removed from ceiling of the crankcase at the bottom of the bores where the rod bolt shoulders get close/contact when stroking a M20.


Unused B25 block + Sawzall = answers.

















The casting is fairly rough in the crankcase so act accordingly. Pictured is the smallest measurement i could get on each side. I would guess there is at least a solid 5mm of meat on each side.


Other potential points of interest.






Thank you for watching.

The Sawzall is still warmed up if there are an other questions before this block gets recycled.

[digger]

hey thanks just what i was after ! there doesn't appear to be much meat for an overbore?

probably is wise to stick to 86mm bore leaving 1.5mm in that area. MM told me they think you can do 87mm and it gives you room for one overbore but seems risky

[LJ851]

I was surprised how close the main oil galley was to the cylinder.

[tinkerputzer]

Thanks for doing this. This will be great reference for the future.

[TobyB]

I was surprised how close the main oil galley was to the cylinder.

yeah, it sure is tight, innit?

t

[MR 325]

That is very cool! Thanks for doing this.

[whodwho]

Very nice! Any chance of a fairly accurate measurement from the deck to crank centerline?

[LJ851]

Very nice! Any chance of a fairly accurate measurement from the deck to crank centerline?

On this block, which appears original, it is pretty damn close to 206 mm even from the centerline of the crank to deck. I would estimate my accuracy at .2 mm or tighter.

[whodwho]

On this block, which appears original, it is pretty damn close to 206 mm even from the centerline of the crank to deck. I would estimate my accuracy at .2 mm or tighter.

Thank you, that is what I had concluded but there are inet references of 206.7 for the block height but that is the stack height and can make a .5 difference in compression.

Thanks again those pics will be a great reference

[digger]

the deck height on mine is 206.2 and stack height 206.7 so the piston sits 0.5mm above deck giving a nominal squish clearance with 1.75mm gasket of 1.75-0.5 = 1.25mm 0.05" which is a number thrown around at being upper end of whats considered good

that is my take on it atleast

[whodwho]

I was ok with your take on it, it was all those saying it was 206.7 that I have always tried to correct ;)

[digger]

I was ok with your take on it, it was all those saying it was 206.7 that I have always tried to correct ;)

i thought the internet rumor was that the ETA blocks were 206.2 from C/L to deck and thus different to the B25?

does anyone have the mahle piston chart? i cant find a clear version. i thought i had it on one of my hard drives but cant find it.

[whodwho]

That is one of them

See if this is what you were looking for

http://www.scribd.com/doc/12801628/Mahle-Pistons-M20

http://web.archive.org/web/200505261...Mahle_m20.html

[bmwstudent]

A sawzall? Holy cow, you are a baddass. How many blades did that take?

[LJ851]

A sawzall? Holy cow, you are a baddass. How many blades did that take?

Believe it or not, one blade for whole thing. I used a milwaukee blade and was pretty impressed.