318 Main Caps On 340

Tech Question


Danny Carrier, Rock Island, TN, 1972 Plymouth Duster 340

I just recently got a bare 1973 340 block with no main caps.Could I use stock 318 caps and have it line bored? Is there a difference onthe forged and cast cranks as far as caps go? Any help would be appreciated.Thanks.



Basically 318 and 340 main caps (but not 360s) are all the same – for decades. Still, swapping can be a real headache. As you know, caps are machined on the block to which they will be married for life. Depending on production tolerances, the “new” caps you find might be offset (left or right) enough so that the pre-align-bore surface grinding required might be excessive — there might not be too much left of the cap at that point. If you’re lucky enough to find several sets of caps, a quick trial fit makes sense. Unless you find a near – identical set of twins, the rear main seal bore (groove) is also not easy to get right at that point (if ever).

Aftermarket caps are machined way undersize to allow for any and all eventualities during the boring process. Since the machining costs would be the same with junk iron or good billet steel caps (Pro Gram, etc.), you might want to look at this as a golden opportunity to build a stronger-than-stock bottom end.


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