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

[digger]

mine was bare crank but it had the sleeve for front seal on, but that is all so no bolts washers, sprocket or toothed ring.

[Madhatter]

The weight is with the crank bolt, the dampener hub, twin chain sprocket and the rear trigger wheel still bolted in place though, so I reckon weights are the same with all that removed. Counterweights are the same, some vary, seems like the first 2 cylinders are the full 73mm

[digger]

I wonder how close a 89.6mm crank would be. When i first started my build with a b25 block and 89.6mm crank i think i ran into some similar issues. That was quite a while ago and i scrapped that block for other reasons. Back then i was a bit hesitant to do any clearancing but now i wouldn't think twice about it. It doesn't seem like there are any big show stoppers on either blocks aside from that awful plug on the back of the early b27 blocks.

i have a B25, B27 and B28 crank but no spare 89.6;) there wont be any show stoppers.

from what the FEA says you dont need much meat for the oil gallery since its close to the bore only in a small area. the area sees no combustion pressure as the rings dont go down that far and the oil pressure is not very high.

[F34R]

I have a spare m30 block, I would like to check out the thickness for that. Maybe in the spring I can add info to this thread. Great read/info.

[tinkerputzer]

I wonder how close a 89.6mm crank would be. When i first started my build with a b25 block and 89.6mm crank i think i ran into some similar issues. That was quite a while ago and i scrapped that block for other reasons. Back then i was a bit hesitant to do any clearancing but now i wouldn't think twice about it. It doesn't seem like there are any big show stoppers on either blocks aside from that awful plug on the back of the early b27 blocks.

[digger]

I wish someone could play forensics to B20 block...does it have the same options for overbore as B25?

b20 block will go out to 84mm easily they do this in the UK often. any bigger i dont know

[jozzze2000]

I wish someone could play forensics to B20 block...does it have the same options for overbore as B25?

[Madhatter]

ill drag my 2.8L crank out tonight and measure it for you.

[digger]

so i found where the B27 block creates a problem not found on the B25

there is more meat around the bearing housing for the intermediate shaft and the big counterweight cranks will foul. this is likely biggest issue with the 84mm crank as these have a larger counterweight radius. you could grind a few off the block though or have the CW machined like some do for other reasons

EDIT:
i got 67.3mm for the radius of 81mm eta and 22.5kg
i got 73.0mm for the radius of the 84mm M52B28 crank and 21.4kg. note that not all counterweights are the full 73.0mm

these masses are different to what MM say can anyone confirm numbers? my scales might be off

B27


B25


hack a bit away


and in she goes (84mm big CW crank)


another problem she wont spin


here is why


no problem to clearance that though

[digger]

So the b25 would be better suited for boost?

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i would think its much of a muchness.

[digger]

there are some internal difference which might have a minor effect on fitting the big counterweight 84mm stroke crank. nothing some grinding wouldn't fix

[spoolinspoon]

So the b25 would be better suited for boost?

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[tinkerputzer]

Way to go mate. This puts some things to rest.

[LJ851]

Nice work digger.

[digger]

A 90* piece cut from the B27 block. cyl 1 exhaust side. 5.5mm is the thinnest area half way up.



view showing wall thickness at front of bore near where water pump sits. this are is thinnest 4.8mm half way up. is probably the least important areas except for turbo guys



the intake side quarter cut off sits ontop of the block. alot of meat here 6.3mm



a completed cut through cylinder 1



so the oil gallery is much the same position.