Well shoot! 6-Bolt vs 4-Bolt Crank/Flywheel

Crowem87

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Looks like my lack of research is going to bite me. I bought a used running GMC to 70 engine to swap in my CCW. However the new engine has six bolt crank connection, and obviously my CCW original motor has a four bolt connection. Is there adapter that can be used? Am I host? Help!


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NDT

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Redrill your 4 bolt flywheel or go search for the extremely hard to find 6 hole flywheel.
 

NDT

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Maybe weld up the old holes? We do it on subsea risers that have to withstand a lot more than a flywheel. Maybe redrill the crank?
 

nattieleather

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Take your flywheel to a engine machine shop and have them do the conversion. This type of work is something done in the hot rod world all the time. Generally it's a four to eight bolt conversion, but a good machine shop should be able to convert four to six.
 

Bulldogger

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I have no direct experience with the CCKW platform but I second the suggestions to redrill. A good machinist won't blink an eye at welding, heat treating, and redrilling something like that
It won't necessarily be cheap, unless you know one, so be prepared to ask around and speak to a few before finding one who will do it as part of the cause of keeping antique green iron in service. That guy won't gouge or complain about being a one-off job or so on.
Totally do-able, it's just moving metal.
Bulldogger
 

Crowem87

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Hey guys - can u use a flexplate in there instead of the flywheel being a Manuel transmission??


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Crowem87

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No. Clutch has to have bolting surface and friction surface.
Got it ok. I think I am going to use the smaller flywheel from the 50s truck if I can get my hands on it and have a machine shop transfer the ring gear over


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NDT

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I needed a 6 bolt hole 270/302 GMC flywheel back in the 80's and it was hard to find then.
 

Crowem87

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I needed a 6 bolt hole 270/302 GMC flywheel back in the 80's and it was hard to find then.
The guy I got my donor motor from has the flywheel for the engine. Aside from counting the teeth is there any way I can determine this from the flywheel casting? They are both 14” flywheels


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NDT

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If the CCKW pressure plate bolts to it and the recess for the pilot bushing is the same, that may work fine.
 

Dr Deuce

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If you have a 6 bolt flat flywheel, and it has been ground for the bigger clutch, you can use a 1974 Chevrolet Corvette clutch kit, all parts. I have a 1956 GMC civy 302 in one of mine and that is what fits perfectly! I asked on another forum about having 4 bolt flat flywheels made. I had found someone to make them if I could get 50 of them. People said the stoopidest things: 'I want to keep it original' (no one can see it). The "GMC" (CCKW) clutch is an odd design and has a tendency to fail. In failure mode, many times the diaphragm will twist and not be able to disengage. Great in a city.... All clutches, especially the CCKW original clutch should be adjusted so that the clutch completely disengages about 3/4" before the pedal hits the floor. The free play is just an indication of how worn the disc is. As the disc wears, free play will go from a lot to none.
 

CMPPhil

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Hi Crowem87

I was hoping Dr Duce would chime in with information. If anyone is going to know the answer on a CCKW question it is him. At one time or another he has owned and worked on almost every variation of CCKW ever built.

Cheers Phil
 

Crowem87

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I figured I would revise this and post what I ended up doing... I had the original 4-bolt engine completely rebuilt. Ended up going with a 50s pulley so it does not have the hand crank pulley assembly anymore. I’m ok wit that and excited with the build!



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