will a bobbed crank for 2.8l stroker balance out?

[redlightpete]

You are confused. The bob weight is a calculated percentage of the weight of the rod and piston. The bob weight does not change to compensate for crank mods.

On a 90 degree V engine (single pin) a balance factor of 50% is typical. If the counterweight of the crank is not heavy enough to counter the required bob weight, holes will be drilled in the counterweight and heavy metal is inserted.


None of this applies to an inline 6 crank as previously mentioned.

Sorry for the additional confusion regarding bobweights.

[LJ851]

2. Is it a problem that the counterweights are smaller? No. It might be an issue in a vee configuration or a single, and then you might add "bob" weights to compensate.

You are confused. The bob weight is a calculated percentage of the weight of the rod and piston. The bob weight does not change to compensate for crank mods.

On a 90 degree V engine (single pin) a balance factor of 50% is typical. If the counterweight of the crank is not heavy enough to counter the required bob weight, holes will be drilled in the counterweight and heavy metal is inserted.


None of this applies to an inline 6 crank as previously mentioned.

[digger]

2. Is it a problem that the counterweights are smaller? No. It might be an issue in a vee configuration or a single, and then you might add "bob" weights to compensate.

yeah its not like there are no counterweight which would be a problem, they are still substantial even after machining

[redlightpete]

I think two different questions are getting confused here:
1. Do you need to get the crank balanced after machining the counterweights to fit? Yes
2. Is it a problem that the counterweights are smaller? No. It might be an issue in a vee configuration or a single, and then you might add "bob" weights to compensate.

[MR 325]

Yes, the counterweights need to be cut down.

Any competent machine shop (with a balancing machine) will be able to balance it without issue!

[digger]

it might be an issue if you were going to rev it extremely high, but there would be bigger issues doing that anyway. i would not worry its been done before.....its worth remembering that elsewhere in the world there are M52B28 cranks that fit without machining. presumably this means the mass of CW is approximately the same as what you will end up at?

[tinkerputzer]

Have you seen bulletride's thread? He did the same engine without issue.

[kickinindian]

It will not be a problem. You are not removing that much material.

Get the crank balanced after it is machined and all will be ok. You are thinking about this too much.

Thank you! that's what I needed to hear how much should I tell them to take off?

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[LJ851]

It will not be a problem. You are not removing that much material.

Get the crank balanced after it is machined and all will be ok. You are thinking about this too much.

[kickinindian]

Gotcha does anyone else have this setup and had it running for a while

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[redlightpete]

redlightpete have u run this? i want this to be a long term 150k runner I'm not racing it

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The short block is built, but it's still sitting in my shed. Hopefully I'll get it installed later this year.

[kickinindian]

redlightpete have u run this? i want this to be a long term 150k runner I'm not racing it

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[redlightpete]

Typically you need to cut down the counterweights to clear the pistons with this setup. After that, the crank needs to be rebalanced. It cost me about $75 for cutting the counterweights and $160 for balancing.

[kickinindian]

so it's technically impossible to get the crank to balance back out after the counterweights have been shaved for piston clearance? and if so would it make the engine less reliable and should I just not do it?

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[LJ851]

This. 9 times out of 10 it's pointless.

10 times out 10.


An inline 6 cylinder engine crankshaft is balanced only to itself, there is no balance factor so no bob weights are used.