Loose 340 main caps??

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swinger340

swinger340

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I have a 340 block that 3 of the main caps fit on real snug and propper, but 2 of them fit loosely. Not loose enough to rock but go on and off w ease. #s 1 3 5 tight #s 2 and 4 loose. Can these be tightened or is this realy not an issue? This engine ran great when pulled. Just freshening it up. Engine has ARP main studs and mains were line bored on last build. Once tourqed is there a chance of main caps walking? I guess they didnt seem to before.
 
6pakattack said:
Did you try Plastigauge to see what kind of clearances your bearings had ??
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No. Couple scratches in crank so i had it turned .010/.010 New bearings will be intalled of course, but i think that the # 2 and 4 main caps fit kinda loosely onto the block. The others take a little effort to push them into place. These 2 just drop on and can be pulled rite back off w 2 fingers. Is it OK for them to slide off and on that easy? Most of my 340s fit tighter.
 
Time for some billet main caps and line bored. Rather be safe then sorry.
 
1970Duster said:
Time for some billet main caps and line bored. Rather be safe then sorry.
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Thats kinda what i was thinking. This is a #s matching 340 for my fathers 70 340 4 speed duster. He dont want to spend much money and thinks it will be fine because it was running good when pulled, but personaly id like to see them fit tighter. I know the main caps on my 340 are a much tighter fit.
 
You can use a chisel and re-stake the caps. Doesn't take much to do it. Couple of taps on the raised web area on the block.
 
hi, he's right, can restake main caps carefully. to swap to billet caps will cost you a lot of money. caps plus realign bore main saddles. I've restaked main caps before, it works good, never had an issue with bearings, just food for thought.
 
yup, for what it's goign to do (#s engine, not too much abuse) I'd just stake them. I use a punch at 3 of 4 points just adjacent (within 1/8") to the parting line, with the cap torqued in place. After they are staked, pull them and the block should be line honed just to make sure they didn't negatively affect the bore. Loose caps will move. The hardware alone is not desinged to completely hold the cap with no interference.
 
i had a loose no.2 cap ....so i staked it carefully...then used a bore gauge to then 'play with it' till it was snapping in better and straight.

proper line honing will make sure it's perfect though.
 
1wild&crazyguy said:
i had a loose no.2 cap ....so i staked it carefully...then used a bore gauge to then 'play with it' till it was snapping in better and straight.

proper line honing will make sure it's perfect though.
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If you're going to line hone the block you might as well upgrade the caps so they are stronger.
 
In this case, I don't think the ends justify the means of adding new caps. You have the cost of the caps, the cost of fitting them, then an align bore, and align hone, just so the mild 340 can go to cruise nights and buzz down the highway.
 
moper said:
In this case, I don't think the ends justify the means of adding new caps. You have the cost of the caps, the cost of fitting them, then an align bore, and align hone, just so the mild 340 can go to cruise nights and buzz down the highway.
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I did, don't want any excuses for my motor to fail
 
1970Duster said:
I did, don't want any excuses for my motor to fail
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the only way really be sure...would be to leave it on the engine stand as well..lol

do what u want...but thats way overkill imo till ur into really big power...and even then...id be looking at possibly a race block for 2k as opposed to spending 1000+ on a stock block.

on the cheap side...it cost me 600 to have regular prep done for studs n such, then add caps and a line bore onto of that...and now ur over a 1000$...and u still have stock cyl wall thickness, stock valley, and stock main webs.

i know there is a grey area where u do the caps on a stock block...but...really man.

just my opinion.
 
Im going to re-stake this one. It is a #s matching engine that has a turned .010/.010 stock steel crank, stock rods(resized w/ ARP bolts), .020 TRW 2316s,(all balanced).284/.484 purple shaft, stock intake, stock carb, original J heads w new stainless valves (gasket matched w/ bowl blending) Just going to be a car show cruiser driven by my 59 yr old father (no races or high revs for this old girl). If i were doing it id probably set the #s engine asside and build a stroker for it. Pops dont care about going fast.
 
hi, we run 340 stockers and use stock main caps with good bolts. we have ran same caps in a 7000 rpm 340 for 20 years. the motor was freshened about every 3 years, with 300 runs on it. in all those years never had a cap break. the stock main caps are quite strong!!!!nuff said!!
 
Knurl the sides of the caps. Then check the line bore. May need a correction hone
 
As Mr Hemi says, its very common and a very simple and quick fix to knurl the sides of the caps. Peening will work, but knurling is by far the best. Most often you will want to do a quick touch up hone to the main bores, but this will last you a good long time. You do not need to go to new main cas and such, simply not needed at all. Your machine shop should have the proper equipment to do main cap knurling. If he doesnt, take it somewhere else.
 
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