Technical Nick's 1950 331 Cadillac with 390 Heads Engine build

Can't you bore into the water jackets and sleeve the thin cylinders to 4" bore?

 

Good idea & we discussed doing that. But I need to increase the bore Radius by 0.109” to get from 3 13/16” to 4.030”. At the thinnest part of my cylinder walls, that would leave 45thou, maybe less, of the original cylinder cast. If the sleeve was 80 thou, I’d be breaking into the water jackets for sure. Im aware that I could partially solid fill the water jackets, or perhaps bore the existing bores out entirely, but that’s all getting a bit drastic & expensive.




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So whilst the 390 heads are being machined, I’ve started de-shrouding & lightly porting the 331 heads. I’m no expert but have read a few books & done this on three 4 banger engines previously. There are some obvious areas to take out to allow air to flow more freely past the intake at low lift & into the combustion chamber. When I’ve reshaped all 8 combustion chambers, I’ll measure the cubic centimetre volume of each with a burette and take out a little more here or there as necessary so that each is within 1 or 2 cc of each other. Next I’ll need to regrind the crank & finish the bottom end to check how far up the bores the new pistons come when assembled. Following that I’ll work out how much to skim the heads to raise the Compression ratio to 10:1, up from 7.5:1 for this 1950 motor. It will benefit from hardened seats on the hotter exhaust side to lessen the effects of any pre-ignition. Hopefully with the modest Iskey 270H Cam there will be sufficient piston to valve clearance available & I’ll loosely assemble the heads on the original 331 block with the new twin Stromberg Offenhauser intake to see how that will fit with skimmed heads.


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Any updates Raygun?


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I have a good set of 1956 365 heads. They have big ports like the later 390 heads but have a narrower valve centerline so they can be used on a 331 without boring it to 4"

 

Hi Jeff, thank you. I’ve just read that the Intake Valve is the same 1.75” on your ‘56 365 Heads as my 1950 331. Do you have the Cylinder Head bolts too as they’re different lengths to the 331 bolt set? And are the exhaust ports the large upright rectangular shape as per 390 heads? Could I ask how much you’d want for your heads & could you send a few pictures please, it may be better than fitting my 331 set. Nick


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After a prolonged pause whilst other things in life dominated, I’m back on with my 331 Caddy Engine build. The crank is now ground, and I’ve had an adapter made for its nose to accept a modern hydraulic damper. That’s away for balancing at the moment

The original 331 block is now bored 30thou oversize, as much as the available cylinder wall thickness would sensibly allow. I’ll be fitting standard replacement pistons & using the original rods in this block now saving the fancy pants 7.1” centre H Beam rods & lightened pistons for another day / 365 or 390 block.
So block cleaned, painted & ready for assembly.


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Glad to see you back at the motor build! Keep at it, and let us know how it progresses


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Hi Raygun, fellow Brit here. I’m looking at adding a little more zip to my 55 331 caddy. I was interested in your headers, they’re the Sanderson ones aren’t they? I didn’t think these would fit the 331 being for the later 63 on series of engines I was told. Do they fit, I’d love a set for mine? I’m also thinking of a cam, how are you progressing with your build, I’d like to hear how that 270 cam performs

 

Me too, as I'm getting ready to build 2 of these myself,
1 to run at the next Monterey races in the Allard.
Cheers,
Kurt