FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: KMcCullah on January 08, 2014, 12:04:25 PM
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I'm considering a build using a 4.375 RPM crank. RPM recommends using a 6.8 BBC rod. My concern is with the 6.8 rod that the wrist pin will intersect the oil ring groove. I don't want to deal with that scenario if possible. Anybody ever try this combo before?
I've read about several builds recently where a 6.7 BBC rod was used with the 4.375 crank. My question is did the crank have to be turned down so the bottom of the piston would clear the counterweights?
A blueprint of the crank would be really handy right now.
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I just did a 4.375" and used a set of Scat 6.700" rods. The pistons cleared just fine. If I remember correctly though, and this build was back several months ago, the pistons came with oil ring support rails. For me, it's a non-issue. A lot of guys make a big deal out of it, but there are a gazillion 347 Fords running around with 1.090" pistons, and they're not blowing smoke. I mentioned this on another post, but one of the editors from Modified Mustangs & Fords magazine that I talk to regularly has a 347 with over 100k miles on the clock....doesn't use any oil.
The RPM cranks are pretty nice, especially considering that they're forged. However, a lot of them will need some machining to get the journals in spec. I've cut several down .010/.010 before starting mock-up.
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I use the 6.7 on all of those. No problem.
You will pierce the oil land though - maybe go to a .043-.043-3.0mm ring pack while you're at it...
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I use the 4.375 cranks with 6.700 and even 6.625 rods and never hit the pistons. My pistons do not require the support rail. Not alot of meat right under the oil ring at 12 o'clock of the wrist pin, but never a problem. You need to check the back four rods for cam clearance. Most of the time we have to grind a little on those rods with a 2.2 rod pin.
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CaptCobrajet are you running the 4.375 crank in 428 block ? .. Bud