M50 3.0 stroker rod/piston options

[E34Dave]

So I have a m54b30 crankshaft that I want to utilize in my M50TUB25. I do have the rods and pistons that came out of the m54, but since the pistons need new rings and rods need new bearings anyway, I figured I might as well just forge the bottom end.

My questions:
Should I use m54 forged rods/pistons or m50 rods/pistons
Should I reuse my cometic .140 and keep my cr low (8.5:1 iirc) or go with a cutring and higher compression?
If I weren't to go forged, how much power would I be able to see on the m54 rods/pistons before I run into issues?

Thanks for your time!

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[Goathumper]

I am doing the exact same thing right now. The crank is the same stroke as the s52 and the rods are all the same length (except for non-vanos m50) in the m/s 50/52 series but the pistons will need to be for that stroke. I ordered 85mm pistons from TRM that were designed for this stroke but in hindsight, I wish I would have just ordered some s52 spec 87mm pistons since I will be doing the bore work anyways and they are much more common and can be cheaper. Then just use a cutring gasket instead of the spacer or MLS. Use the piston selection to dictate the new CR.

[someguy2800]

Take the rings and bearings out of the M50, put them on the M54 stuff and shove em in there.

[E34Dave]

Take the rings and bearings out of the M50, put them on the M54 stuff and shove em in there.

Would I run into compression ratio issues seeing as I have a .140 gasket?

Also I've heard from my local bmw guy that every time he's done that he's ended up with cracked rings. Should I go get the cylinders honed? And while I got the rings out, should I gap them?

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[E34Dave]

I am doing the exact same thing right now. The crank is the same stroke as the s52 and the rods are all the same length (except for non-vanos m50) in the m/s 50/52 series but the pistons will need to be for that stroke. I ordered 85mm pistons from TRM that were designed for this stroke but in hindsight, I wish I would have just ordered some s52 spec 87mm pistons since I will be doing the bore work anyways and they are much more common and can be cheaper. Then just use a cutring gasket instead of the spacer or MLS. Use the piston selection to dictate the new CR.

I appreciate it helps a lot

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[M1st]

For NA application 8.5:1 compression is too low.

[E34Dave]

For NA application 8.5:1 compression is too low.

I wouldn't wanna forge it and have low compression if I was n/a

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[M1st]

If you have the chance definitely go with forged rods and pistons then you’ll have less worries about turning up the boost.

[E34Dave]

If you have the chance definitely go with forged rods and pistons then you’ll have less worries about turning up the boost.

Definitely! My purpose of the thread was to figure out what pistons and rods to get when using a m54 crank in a m50 block, so hopefully someone knows

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[Goathumper]

So do you plan on having the block machined? Or will you just use a dingle ball style hone at home? This will dictate the piston choice I think.

If you will be machining the block, order whatever pistons you can find for an 89.6mm crank stroke at whatever CR you choose, then bore to the spec of that piston. Rods are all the same length, 135mm, and pick whatever your budget allows (or use the stock rods)

[someguy2800]

Would I run into compression ratio issues seeing as I have a .140 gasket?

Also I've heard from my local bmw guy that every time he's done that he's ended up with cracked rings. Should I go get the cylinders honed? And while I got the rings out, should I gap them?

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Stock M54 rotating assembly with a .140" head gasket would be about 8.8:1 compression ratio which is fine for pump gas.

Most of the time when rings or ring lands break it is because they did not have enough end gap and they butted together when it got hot. Gap them to like .019" top, .021" 2nd. I have run 800 whp on used stock pistons and rings, there is nothing wrong with them if the tune is on point and you have enough octane. Stock M54 rotating assembly will make 500 whp all day. Don't need to have the block honed but you can dingle ball hone it at home if you want to. That is all I do when I build mine.

[E34Dave]

Stock M54 rotating assembly with a .140" head gasket would be about 8.8:1 compression ratio which is fine for pump gas.

Most of the time when rings or ring lands break it is because they did not have enough end gap and they butted together when it got hot. Gap them to like .019" top, .021" 2nd. I have run 800 whp on used stock pistons and rings, there is nothing wrong with them if the tune is on point and you have enough octane. Stock M54 rotating assembly will make 500 whp all day. Don't need to have the block honed but you can dingle ball hone it at home if you want to. That is all I do when I build mine.

You're a legend, thanks! I planned on 450hp anyway, so that makes my heart happy.

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[Robocop]

only .019 on top?
good to know i went like .023 on my m30.
stock was .017 or maybe .019 i think.

[someguy2800]

only .019 on top?
good to know i went like .023 on my m30.
stock was .017 or maybe .019 i think.

Should be enough for a mild build like this. Stock they are like .012"

[Greenday694]

Stock M54 rotating assembly with a .140" head gasket would be about 8.8:1 compression ratio which is fine for pump gas.

Most of the time when rings or ring lands break it is because they did not have enough end gap and they butted together when it got hot. Gap them to like .019" top, .021" 2nd. I have run 800 whp on used stock pistons and rings, there is nothing wrong with them if the tune is on point and you have enough octane. Stock M54 rotating assembly will make 500 whp all day. Don't need to have the block honed but you can dingle ball hone it at home if you want to. That is all I do when I build mine.

I completely agree. I built my 3 liter m-50 2 years ago. Stock block honed cylinder to break the glaze. Check wear and out of roundness . Used low mileage M54 B30 crank rods and pistons in good condition. Grant rings, gapped at 20 and 22 on the 2nd. ACL Tri metal bearings standard size. Arp main studs and Rod bolts and head studs. Cometic 140 head gasket. And have been running 16 lb of Boost from air-to-air intercooled Eaton m122 H supercharger. Over 10,000 hard miles with some track days. Still running very strong.
Aftermarket forged pistons are great for High horsepower racing engines.
The downside is, forged pistons expand quite a bit more as they heat up. So you have to start with a slightly larger bore size then you would with cast Pistons. The problem comes in on cold startup. Until the Pistons get some heat in them and they expand, they slapped around in the cylinders. After enough cold start, they end up damaging the Piston skirts and the cylinders after prolonged slapping. Coated forged pistons help mitigate some of that.
If the car is to be daily driven with some track days or spirited driving, and you're not going for crazy horsepower numbers, then the stock M54 B30 internals will work just fine.

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