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I have an '81 320 E21 with the m20b20 motor, I also have a crankshaft and pistons from an m20b23 (also from an E21, if that even matters). Plan was to rebuild the engine using the b23 internals to avoid the legal stuff involved in swapping an engine, however my mechanic is telling me the pistons sit too tall in my block.
I figured I could fix this with b23 rods, however the mechanic is telling me it still won't fit. Is this true? I don't see a reason why this wouldn't work, it's not like one block is taller than the other, so is my mechanic just not telling the truth or am I missing an obvious reason why swapping the internals wouldn't work?

Aas far as I know these bottom end are the same. Same bore, same rods (surprises me but this is according recent ETK), but different crank and pistons. Better as your mechanic what's not supposed to fit. Perhaps you don't have the right crank?

Are you sure he hasn't mixed the pistons up?

standard setups
320 crank(66mm) - 320/323 rods (130mm) - 320 pistons (CH 43.4mm) = total deck height of 206.4mm pokes out of block by 0.2mm
323 crank (76.8mm) - 320/323 rods (130mm) - 323 pistons (CH 37.8mm) = total deck height of 206.2mm and is flush with the top of the block

If they are mixed up
323 crank (76.8mm) - 320/323 rods (130mm) - 320 pistons (CH 43.4mm) total deck height of 211.6mm, so pokes out of the block by 5.6mm

These figures do not include the dome or the valve cutouts.

Regards

Ben

Or mixed up the other way
I they are mixed up
320 crank (66mm) - 320/323 rods (130mm) - 323 pistons (CH 37.8mm) total deck height of 200.8mm, so would be down the bore by 5.4mm

Yeah, correct Ben. As long as crank and pistons stay together, there won't be a problem, since rods are the same. But what about the head? Have they both the "200" head?

Duracel79 wrote: Thu Nov 30, 2023 12:58 pm Are you sure he hasn't mixed the pistons up?

standard setups
320 crank(66mm) - 320/323 rods (130mm) - 320 pistons (CH 43.4mm) = total deck height of 206.4mm pokes out of block by 0.2mm
323 crank (76.8mm) - 320/323 rods (130mm) - 323 pistons (CH 37.8mm) = total deck height of 206.2mm and is flush with the top of the block

If they are mixed up
323 crank (76.8mm) - 320/323 rods (130mm) - 320 pistons (CH 43.4mm) total deck height of 211.6mm, so pokes out of the block by 5.6mm

These figures do not include the dome or the valve cutouts.

Regards

Ben

I doubt they have the pistons mixed up because my 320 pistons were very obviously damaged and the 323 pistons, at least from what the eye can tell, were good. Thank you for these numbers, from what you're telling me it appears the mechanic isn't very honest, or at the very least not well informed.

Jeroen wrote: Thu Nov 30, 2023 8:44 am Aas far as I know these bottom end are the same. Same bore, same rods (surprises me but this is according recent ETK), but different crank and pistons. Better as your mechanic what's not supposed to fit. Perhaps you don't have the right crank?

I definitely should the right crank. I have my car's original M20B20 and bought another M20B23 (oilpan, block, internals, head, valve cover).

uwbuurman wrote: Sat Dec 02, 2023 9:45 am Yeah, correct Ben. As long as crank and pistons stay together, there won't be a problem, since rods are the same. But what about the head? Have they both the "200" head?

I have a head from a 323i as well as the one from my 320, even if they are different I should be able to use whichever I need, right?

Correct. I know a 320/6 with 323i block and internals and carb, but not sure which head.