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slight problem and i need sugjestions

J

JABO

Active member
I have a 400 small block Chevy. when I bought it everything looked fine, but while I was cleaning it up I noticed a small hairline crack between the two middle cylinders on the passenger side. its obvious that this motor got hot (hint: rubber plugs where freeze plugs should be) I'm not sure if it has been bored or not. but it is a small enough crack that I could yank a hair from my head and lay it in the crack to fill it up. maybe the head gasket would seal, maybe not. what about sleaving the two cylinders? what about a machine shop fix? any ideas? I don't have alot of money and i need help
 
hey jabo , you can have it bored out and oversized pistons put in but make sure that the block hasnt been bored out allready, if it has u can have it sleeved or thre machine shop if they r good can sometimes weld and rebore the cylinder, let me know what u find out mike
 
Having it magnafluxed won't do anything but show a crack that is already there and visual. You could have it pinned if you think it may leak. You would need to pressure check it to see if it does. If it doesn't leak, it may start eventually. If it goes into the bore, I'd recommend a sleeve. Depending on the shop, prices will differ.
 
i should be able to put a pick on here tomorrow, but its literally hairline and it does go between the cylinders. I've never heard of pinning, what is that?
 
the crack is on the surface where the head gasket would lay and its no deeper than a piece if hair. i don't think that would work
 
The center siamesed bores in the SB400 are notorious for cracking when it's overheated. There is very little meat in between the cyls and are prone to cracks especially after boring when material is further removed. Can it be fixed...as always, all it takes is money.
 
By the time you fix the crack on the block the right way you could have bought a new short block. 400's are little harder to come by but we have several 350 shortblocks that create about the same amount of power.
 
i have a good 350 block but it has a bad crank and a bad rod, I'm thinking that if i spend the money to fix the 400 i could probably build a 383 with that money and be better off.
 
If you run your motors hard a 350 block with a 400 crank is about the best all around airboat motor! The 350 main web is way stronger and the deck surface is much stronger. Also a smaller bore is better for preventing detonation. To be honest I have yet to tear apart a 400SBC that was on an airboat that wasn't cracked in the deck around the steam holes! If the crack is steam hole to steam hole it's no big deal if it's steam hole to bore then you have issues! There is no good reason to build another 400 using a used block! Either go with a 350 block or go after market! By the time you find a good 400 block and mag it and bore it and check the mains and stud them and line hone it you could own a Motown which has several major cooling, oiling, deck and main web improvements over a stock GM block!
 
I say find a new block. If you have a weak spot on an airboat it will come back to bite you at the worse time in the worse spot.
 
thanks guys, i appreciate the help. I'm gonna go with the 383, but do i use the 400 connecting rods or go with 5.7 and new pistons, remember I'm on a serious budget.
 
This will open a can of worms but I would run the shorter rod if you have anything other then a 2.68 ratio and even then if your going to turn it less then 5,500RPM just use the stock 400 crank and short rod's this will save you a bunch of jack and there are advantages to a shorter rod in an airboat! The 5.7 rod will help with skirt and bore wear however the shorter rod is less prone to detonation and is cheaper! If you can I would spring for a Eagle kit with new 5.7 rods and crank buy the time you turn the stock 400 mains down and re-size your used rods and upgrade the bolt you will spend the same. So you need to go cheap all the way with your stock parts or go with a all new kit which you can get pistons, eagle rods, eagle crank rings and bearings for around $850!
 
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