[Singleton]

4.375 stroke Cranks, 3”main, 2.20” rods, 4340 steel, $399, free shipping, (I wasn’t taxed either).
Racing Parts Maximum (RPM).
https://rpminternationalinc.com/cran...80-stroke.html

[Jack Gifford]

Will be interesting to hear of experiences with this Chinese crank.

[Singleton]

Fine for a street car. No better or worse than forged eagle or scat., but it’s no Bryant or Crower.
I’ve been running an RPM 4.5” crank for six years at 625 hp level at 6000. Machining was good out of the box.

[65madgoat]

SAE Certified 4340 forged material is used for superior strength with these RPM cranks I just read.

Then I see for increased strength these cranks are given a second heat treatment after rough finishing --- and then Nitride-hardened eliminating any deformation after leaving the factory.

[Mr Twister]

What piston compression height would this use with a 6.7 rod. I'm asking because I don't know how to calculate it. And, would the pin be in the ring land area of the piston?

[Singleton]

Originally Posted by Mr Twister

What piston compression height would this use with a 6.7 rod. I'm asking because I don't know how to calculate it. And, would the pin be in the ring land area of the piston?

CH would be in the 1.32-1.35 range depending on deck height of block. So no, pin would not be in the oil ring

[johnta1]

Calculator to figure out the specs.


Calculate Pin Ht

[Gary H]

Just order one. Thanks for the heads up!

[Mr Twister]

I searched and it doesn't seem like a very popular stroke. I don't now why, appears to make a nice 474 (undersquare) stock block street engine, or even a 525 based on an iA2 (dead square) engine. I'm curious if anyone is willing to post their combo using that stroke. I'm interested in getting one based on the price for my street car.

[srmmmm]

I put the 4.25" crank in a 350 block for 413 cu in. Definitely under square! With 6X heads and a Comp XE276HR cam, it turned out to be I think a good combination. 442 LbFt at 4300rpm and 453HP at 5600 rpm

[FMile Pontiacs]

I took a look at the web site it list for 599.00?

[65madgoat]

Guess they sold enough at $399 and raised price up!

[Mr Twister]

I got one last night prior to the price increase. I saw on Butlers site 1 of their forged cranks part number started with "rpm" so I surmized they MAY be reselling these. Plus RPM is listed on the page with an icon under popular brands". $399 is a super deal.

[65madgoat]

A great price but a somewhat odd stroke. Good if you can make it truly work.

[Singleton]

Stroke is not a problem, just requires custom pistons, which cost slightly more than shelf pistons. An .030" 400 will come out to 473 cid.

[Mr Twister]

Right now I'm thinking a 4.250 bore and 4.375 stroke for 496 cubic inches. NA Street engine for one of my Firebirds. Aftermarket block.

[hurryinhoosier62]

Originally Posted by Mr Twister

What piston compression height would this use with a 6.7 rod. I'm asking because I don't know how to calculate it. And, would the pin be in the ring land area of the piston?

As long as you know your block deck height it is fairly simple. Let’s assume your block has a 10.240 block deck height; subtract the rod center to center length and 1/2 stroke length. The remainder is what your compression height would be at a zero deck height. Racetec can put the pin boss where ever you want it in.

[hurryinhoosier62]

Originally Posted by srmmmm

I put the 4.25" crank in a 350 block for 413 cu in. Definitely under square! With 6X heads and a Comp XE276HR cam, it turned out to be I think a good combination. 442 LbFt at 4300rpm and 453HP at 5600 rpm

That would be a killer street engine in a lightweight early Firebird!