M20b20 - overbore 84mm ?

[MattyX]

Hello ,

It is possible at m20b20 block bore cca 83.7mm ? (original cca 80mm ) I can b25 pistons + 130mm connecting rods and m52 crankshaft in new modified block for b25 bore
thx , sorry for worse translation

[daimlerman]

Yes,you can bore out a 2 litre block from 80mm to 84mm,are you in a country that insists on 'standard' engines?

If you are planning on using 325 type pistons,you really need the 325i head as well,as the combustion chambers are a different shape to the 320i head....you could use the 320i head if you can find some 325e pistons,though....

[MattyX]

I can at B20 block (overbored at cca 84mm) use 325i head. in b20 block will stroked at 2.8...

have overbored b20 block sufficient thickness of the combustion chamber ?

[daimlerman]

Combustion chamber is in the head on these engines,if you are building a 2.8,you are starting with the very best idea,boring out a 2 litre block will give you a perfect bore with no wear ridges at the top and bottom to upset the piston rings when using a longer stroke crank.

Both the 2 litre block and the 2.5 litre block are identical,except for bore size.

Have a read through 'Hairyscreech's' M20 development thread in this section,there's some interesting stuff on improving these engines there.

[MattyX]

And how cylinder wall thickness (mm or inch) is after overbored ?

[MattyX]

I paced it at the water pump and there will be only 3 mm wall thickness after overbored...

[eta]

Well a M20B25 has the wall thickness and that works very well indeed.

[HairyScreech]

3mm should be ok in iron or steel.

[hongkongfuey]

ive seen one that was bored poss not perfectly central, that developed a hole in the cylinder wall i would say if your going through all that use a 2.5 engine, do it once

[kimbo]

Both the 2 litre block and the 2.5 litre block are identical,except for bore size.

Not strictly true. The core used in casting the B20 is different to that used in the B25, so the casting thickness for the cylinder is thicker in the B25.
Having said that, I've overbored a B20 block to 84mm with no probs - wouldn't be comfortable going any further though!
Kim

[Ant]

^^ B20 will just go to 84.5mm, I did a few random bores in an old block and then bandsawed each 2 ways afterwards.

86.5 on B25 casting iirc is the maximum ??

[HairyScreech]

most press in wet liners (as in not pressed into an alu bore but self supporting) are in the 3-5mm range, i wouldn't be comfortable going below 3mm but theres nothing to say thats a true minimum, just outside why i would expect to be safe.

casting surface on the water jacket will have the largest effect on cracking in use, as the smooth honed surface of the bore will be unlikely to be the crack initiation point.

84mm on the b20 block has been done many times and provided you have a block with no core shift the it should be fine.

what thickness were you looking at after boring on those blocks ant? ~3mm?