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

[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

[Madhatter]

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

[jozzze2000]

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

[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

[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.

[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.

[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.

[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]

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]

yeah. There would be 1kg weight in the hub, the sprocket, bolt and the trigger wheel. what did MM say they weigh?

[digger]

yeah. There would be 1kg weight in the hub, the sprocket, bolt and the trigger wheel. what did MM say they weigh?

24kg

[LJ851]

Thread title updated to be representative of all content.

[digger]

some more details on crank clearancing

[digger]

84mm crank with stock rod profiled to match an aftermarket Molnar rod

there is about 3mm clearance to the raised lettering on the shaft.

add in a 89.6mm crank instead and you lose 2.8mm clearance (5.6mm stroke but you lose only half in the throw). hence MM sell a machined intermediate shaft which does little more than clean up the surface.

note that clearance problem is in the width at the big end not where i have ground material away. i have a 24V M52/S52 style rod and it is the same width. Aftermarket rods are often wider here for strength and stiffness so take note with a big crank and aftermarket rod

[digger]

here is where the rod gets close to the bore.

note i have slightly chamfered the bottom of the bore but the stock bore comes with a sharp corner.

i also profiled this rod so a stock rod is alot closer and the M52/S52 rod is slightly closer.

if you use the big 89.6mm crank you lose 2.8, plus you lose about 0.5 to 1mm ith a S52 rod compared to what i have shown so if the bore is not chamfered there is a light interference





i will attempt to do more photos with a un modified 135mm stock B25 rod and maybe a 130mm B27 one.

i dont have a M52/S52 stock rod only a "special" one with 21mm pin size as opposed to 22mm and as a consequence i have no junk piston to match