M47 crankshaft question

[fresh_TD]

Normally people will shave the M47 crank nose when building an M42 stroker to fit the crank sprocket. Can the crank sprocket be milled to fit over the M47 crank nose instead? any effects by going this route? thanks in advance.

[AndrewBird]

I don't know anything about it, but I would assume they modify the crank because the key way would also have to be deepened, which is a lot hard on an inside bore than it is on an outside bore.

[ccsdo5]

You can probably do that but you'll have to broach a new key in the sprocket, like Andrew said. I've broached a couple keys for some stuff at work on just an arbor press and it was a pain in the ass. Also would wouldn't you need to do the same for the crank pulley?

[Fsnow]

There used to be a ton of info on metric mechanics site about this crank but it seems they took it all down. Is the diameter of the crank different or is the key just in the wrong location? Maybe there isnt enough material to modify the sprocket? There must be a reason that everyone does it that way but I don't know what it is

[fresh_TD]

Here's a couple of pics I took yesterday of both cranks side by dide. M42 left, M47 right. The M47 crank nose is a little bigger. I talked to a guy who did a stroker build, and said he had to mill off 0.6mm off the M47 nose to fit the M42 crank sproket.




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[Fsnow]

Boring the sprocket to fit would be easy but as said cutting existing key deeper or cutting a new keyway in a hardened sprocket is going to be a pain with a broach or shaper/slotter. So I asume the difference in location of the key don't matter? If you were really brave you could just mill .6mm off the top of the key instead... sketchy .

[stonea]

Sub'd for responses. I'd love to know more about this for future ideas.

[AndrewBird]

If you were really brave you could just mill .6mm off the top of the key instead... sketchy .

If it is infact .6mm off the diameter, all you would need to remove is .3mm off the key, which is only .012", not much at all. This would work.

[fresh_TD]

If it is infact .6mm off the diameter, all you would need to remove is .3mm off the key, which is only .012", not much at all. This would work.

It's more the just the key way..the nose of the m47 crank is bigger then the nose on the m42 crank

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[AndrewBird]

It's more the just the key way..the nose of the m47 crank is bigger then the nose on the m42 crank

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Right. So modify the sprocket to have a larger hole, then shave the key down so it still fits. No need to modify the crank.

[fresh_TD]

Right. So modify the sprocket to have a larger hole, then shave the key down so it still fits. No need to modify the crank.

That's the plan..wanting to see if anyone has gone this route before.

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[Fsnow]

If it is infact .6mm off the diameter, all you would need to remove is .3mm off the key, which is only .012", not much at all. This would work.

Oh yeah, woops. If it is that simple why in the hell would anyone machine the crank? I'd say go for it then

[fresh_TD]

Oh yeah, woops. If it is that simple why in the hell would anyone machine the crank? I'd say go for it then

People have always machined the nose of the crank, because that's what everyone else before them did. It would be easier and cost less to machine the sprocket.

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[Fsnow]

Also cheaper and easier to source if the machining gets screwed up, I know which one I would rather machine. Always take the cheap/lazy/safe route if there aren't any downsides, I don't see any here yet. Keep up the good work dude

[AndrewBird]

An automotive machine shop could easily modify the crank, but might not have a mill to do the sprocket. Hence why people might have gone the crank route.