Moparts Tech Archive
Big Block
Main Cap walk and its effects
OK I know it is a way of life with a BB mopar and you just have to deal with it. My buddy pulled his 499 RB apart today and it had major cap walk. It has ARP studs and stock caps. 2 years of running it and around 500 passes. The #4 bearing on the cap was really pounded and in bad shape. The bearing was egged shaped when pulled, but everything was tight, and the main journals were round. Could the cap walk cause this much damage? Also the #6 rod bearing had no tension on the cap side of the rod. I assume this is from junk getting into the oil passage from the #4 main and getting into the rod? The crank is going to have to be turned .010 to get the #4 main back into shape. How much cap walk is acceptable before major damage happens? By the looks of the main bearing it would not have lasted all but a few more passes until diaster struck.
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No possible way to stop it, but alum caps will help. He should be downing that motor every season to have a look, 500 laps is WAY too many before a check. Be sure and inspect #4 main web very closely, I would also mag the crank.
Monte Smith
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Monte, I know there is no way to stop it, but how long until it breaks the main webs? The cap had about 75% markings. The #4 main was really bad. The bearing was wore down to the steal in a few places but the crank wasn't riding on it. Does this sound typical with alot of cap walk? He doesn't want to spend the money on alum main caps for this block. It is already .055 over and if he has to do it over it will be with a low deck. So it's not worth doing to him.
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My last block was a Mega-Block and it walked badly with the stock caps. Not as bad as a stock block, but still more than I would liked to have seen. If I would have kept that block, I was going to have some billet alum caps made for it. As discussed here in great length before, I see no way to control cap walk on a big block without a major rework. And if you were going to that much expense and trouble, you would be better off buying an INDY or KB alum block. The caps on those blocks are doweled in addition to cross-bolted.
Monte Smith
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guessing the caps would start to show signs of fretting in the 500-550 hp range (race car/transbrake stuff).of course this depends on a lot of variables,if it's not pounded on often like a drag car probably wouldn't show up much.i think the generally accepted limit of stock caps is in the 600 hp range. stuff like monte's playing with (heavy hosing) pounds the snot outta the bottom end,as he said i'm sure they look pretty ugly.i doubt the cross bolt setup is of much relief as the forces aren't in the direction that the cross bolt setup would help.
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