I have a good pair of 1960 390 heads that I want to install on my 1950 331 caddy. I did a thread search and messaged some members but still have conflicting answers. Some say that they are a direct bolt on with the 390 pushrods and some say the 331 block has to be bored to 4” like the 390 because of the wider valve center in the 390 heads. That would give me cylinder wall thickness concerns. I’m just looking for a definitive answer of what needs to be done to bolt my 390 heads on my 331. Thanks
We put some 390 heads on a 331 at the shop. It was not bored to 4". If I recall correctly, we had pushrods made, I'd have to check on that. The engine has only been broken in at this point, but there have been no issues.
Thanks for the reply. Has it been bored out at all or is it still the stock 3 13/16” bore? I dry fit my 390 head on one side and the intake valve clears the cylinder wall but the exhaust has interference at a little more than max lift. I plan to bore it .060 so hopefully that will give me enough room
I don't remember off hand, and I don't want to give you incorrect information, so I'll look into it when I'm back at the shop this week.
Ours was bored to .030 over. Not for clearance, just because we were building a fresh engine and that was the next step in bore. We don’t quite remember what we did for push rods. Although looking at some photos, the push rods don’t look new, so maybe we did have and use the 390 pushrods. I suppose someone else will have to chime in on that, or you’ll need to do some measuring. I’d open my 390 and help you with pushrod lengths, but with my current situation I’m unable to. Hopefully someone else here has some they can measure. Hope that helps.
In 1955 Cadillac changed the rocker arm ratio. The same ratio is used on the 390's. That's why a complete 390 head assembly is a direct bolt on for a 55 331. You must also use the 390 cam, pushrods, and lifters to make it all compatible. I fail to see how 390 heads are going to work properly on a 49-54 block. Am I missing something?
You'll have to excuse our improvised run stand, but here is a '49 331 running 390 heads. If the '55 331 has the same rocker ratio as a 390, I'm going to assume (correct me if I'm wrong) that the 365 rocker arm ratio is also the same. We have also successfully run 365 heads on a 331.
You might give these guys a try they do some pretty intense stuff: http://www.modifiedcadillac.org/forum
Sorry I missed this before. It is running the Isky 270. Also, I'm about to put together a '49 331 with 390 heads for my roadster, so I'll document the process.
Any chance of bringing this back to life? The internet still lacks a definitive answer. Can 390 heads work on a 331 and if so, what parts/modifications are necessary?
Yes, it works. Use the 390 heads and rocker assemblies. Every time we have done it, we have had pistons made, and measured and ordered pushrods, so I won't speak to what factory stuff there works. Although the pistons were to bump up compression.
I may be wrong but I was explained to me , that pre ‘63 390 heads will bolt on to an earlier 332 block. I think the 4 inch bore thing you mentioned comes from the following info that was given to me : it was explained that unless you have custom pistons made or custom rods used, you can bolt a ‘59 - ‘62 rotating assembly into a 331 block (IF) : - you bore it out to standard 390 piston bore (4.00”) Is that what you mean by going out to 4” ? - and you need to turn the mains down to standard 331 main size. I haven’t fired it, but I have an early ‘49 331 that’s been taken out to 4.0 and has a 390 crank in it. It’s got the early heads on it though which seems counterproductive.
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