1972 307 cam selection... HELP! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**MikeB** · 01-20-2021, 11:30 AM

Interesting article from Super Chevy, but they were wrong about GM producing a 307 with a small journal crank. Quite unusual for them.

Here's an excerpt from Hot Rod Magazine. It's the second paragraph.
https://www.hotrod.com/articles/ccrp...-engine-specs/

And here's a Wikipedia article on small blocks. Read the first and last paragraphs under the section entitled "3.875 in. bore family (1955–1973)".
https://en.wikipedia.org/wiki/Chevro...l-block_engine

Finally, go halfway down the page here:
http://chevellestuff.net/qd/crank/crank.htm

Bottom line is I think GM needed a new block for the 307 with more "cast-in" clearance for rods (due to .25" longer stroke). And why cast a brand new block with small journals, when they were moving to medium journal cranks on all other small blocks (302, 327, 350)?

I did find a list of crank castings at Mortec.com that shows only three castings used for the 307, and they were also used for 327s. So, they were the same raw castings but machined differently for balancing with different piston weights.

3911001....307,327...cast....medium journal...3.25" stroke
3911011....307,327...cast....medium journal...3.25" stroke
3941174....307,327...cast....medium journal...3.25" stroke

A couple pictures of my 307 crank rebalanced for 327 (+.030") pistons and SCAT rods:

01-20-2021, 11:30 AM	#27
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,568	Re: 1972 307 cam selection... HELP! Interesting article from Super Chevy, but they were wrong about GM producing a 307 with a small journal crank. Quite unusual for them. Here's an excerpt from Hot Rod Magazine. It's the second paragraph. https://www.hotrod.com/articles/ccrp...-engine-specs/ And here's a Wikipedia article on small blocks. Read the first and last paragraphs under the section entitled "3.875 in. bore family (1955–1973)". https://en.wikipedia.org/wiki/Chevro...l-block_engine Finally, go halfway down the page here: http://chevellestuff.net/qd/crank/crank.htm Bottom line is I think GM needed a new block for the 307 with more "cast-in" clearance for rods (due to .25" longer stroke). And why cast a brand new block with small journals, when they were moving to medium journal cranks on all other small blocks (302, 327, 350)? I did find a list of crank castings at Mortec.com that shows only three castings used for the 307, and they were also used for 327s. So, they were the same raw castings but machined differently for balancing with different piston weights. 3911001....307,327...cast....medium journal...3.25" stroke 3911011....307,327...cast....medium journal...3.25" stroke 3941174....307,327...cast....medium journal...3.25" stroke A couple pictures of my 307 crank rebalanced for 327 (+.030") pistons and SCAT rods: Attached Images __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!



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