M20B28 build problems

[TriggerFish]

Hi all,

I know all of this has likely been covered before, but with so different permutations, I start to get a bit lost in model/part numbers! Add in naming differences with the US market on Google, and it's even worse!

Anyway, I'm trying to build an M20B28, and I put the pistons/rods onto the crank for a trial run earlier. The result? Not good! The piston hits the webs on the crank.

These are 130mm rods from an M20B20, the pistons from a low comp. M20B25, and the crank from something. I can't remember though what the crank is from. It was sold on this site as being the right thing - I think it was an M52B28.

These are the casting numbers anyway:





(2242898)





Now, given the above, I thought that it should all go together. Do I need to find a different crank shaft? Is this a double VANOS one? I understand these have bigger webs?

Thanks,
Joe

[reggid]

How much do you need to take off the skirts? Does the crank have the toothed wheel at the back under the rag ?

It's fine to machine some off the skirts as there are several skirt profiles . Just makem them all the same and balanced after you do.

If it still doesn't clear then you might want to find the early b28 crank with smaller radius counterweights. The guys in the U.S. Machine the crank and rebalance as they don't get the smaller counterweight cranks

[gromgsxr]

they are the KS brand pistons, there are mahle brand in some engines that have shorter skirts, they are flat at the bottom.

[dave525]

I think its the pistons.My 2.8 crank has webs all the same size and it rotates fine with no mods. I also have another crank and some webs are larger than others.This wouldnt rotate.Your crank webs look identical so you need to take a bit from the skirts.I cant remember which brand pistons mine are.

[HairyScreech]

Early cranks have smaller webbs, later cranks have some large some small. I think it was a revision to adjust bearing wear, it's not a balance thing. (well it is but local not overall balance).

Long skirt pistons need something like 14mm removed from them, this makes them the same as the shorter pistons. I think the Wiki notes this.

A good set of 0.1g scales will get you inside acceptable balance with no trouble.

[DanThe]

The only way to be sure of the crank is to measure it, 2.8 has 84mm stroke, that crank is definitely an early M52 (single vanos) but maybe its a 2.5 which has 75mm stroke

[TriggerFish]

The only way to be sure of the crank is to measure it, 2.8 has 84mm stroke, that crank is definitely an early M52 (single vanos) but maybe its a 2.5 which has 75mm stroke

I fear it's a 75mm crank I need to measure more accurately as I stopped for dinner, but I was getting a reading of around 39-40mm from the middle of the rod part to the middle of the main journal.

How much do you need to take off the skirts? Does the crank have the toothed wheel at the back under the rag ?

It's fine to machine some off the skirts as there are several skirt profiles . Just makem them all the same and balanced after you do.

Sorry, the rag was an oversight. Under that it's just another journal thing - no cogs/wheels in site.

I've got another bottom end at my lock up which I'm going to whip a piston out of in a bit to see if that's the right size or not. Fingers crossed!

[TriggerFish]

Just checked more accurately (by suspending the crank, measuring the height of the top of a journal when pointing up, measuring the height of the same journal pointing down, subtracting them, and the diameter of a piston journal from each other) the stroke of the crank, and it's a 84mm affair, so that's all good!

I'm talking to magspower about sorting the pistons etc. out