Thread: I broke a main cap. Please advise

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350 sbc 4bolt Last night i put in crank, instlled caps, started to torque them down, and snap. i broke one of the ears on the 4 bolt cap. must not of seated it right. so now i have to replace it. whats the right way. can i replace one from another block., or do i need to replace as a set, remachine ect. Please advise, ive never done this before. now i feel stupid (LOL) thanks T.

You can replace the cap from another block but the right way is to have the block (and caps) align bored after.

which cap and did you put it on backwards??

Its the 4th cap back, and yes i did put it on backwards. OPPS. Havent built a motor in 25 yrs, guess im learning the hard way. did this starting to plastiguage the mains. the cap in question was blocked by the counter weights, and i missed the arrow. Go ahead slap me, i deserve it. thanks for the help guys T.

Well----if the crank is all you have in the motor , take it out and fit another cap---you can fit one with a dial guage by taking vertical measurement and then diangular to see if the off set is equal--you will need one that is equal side to side and if its bigger top to bottom you can cut it down until you get it to specs =====or

Cut them all and line bore/hone it ( which would be what I would do to fix it, however if I was in the middle of no where and engine was together otherwise and in the vehicle---

Originally Posted by jerry clayton

Well----if the crank is all you have in the motor , take it out and fit another cap---you can fit one with a dial guage by taking vertical measurement and then diangular to see if the off set is equal--you will need one that is equal side to side and if its bigger top to bottom you can cut it down until you get it to specs =====or ---

yep i done that if you have alot of caps you can get lucky GM was never that great on hitting dead center on the caps but i have bump them side by side when i have line bore them. with used steel cap or making a 2 bolt to a 4 bolt with gm used caps i bump them by welding the side face. then side face the cap in the cap cutter till it on center or close . i had a merlin a guy soild me with the wrong main cap # 3 bump it over got it on center but was tall by a 001.5 trim it in the sunnen cap cutter then lap it to .0001 call it good .no line bore

always have it line bored.ialways have it done on any engine i rebuild.

Originally Posted by hotrodstude

always have it line bored.i always have it done on any engine i rebuild.

yep i have as well many times. and made rear caps drill for oil pump dowel pins thrust rear seal etc. but if you can hold it under 0002. why? i done it many ways line bore will not make right or better . if the guy with the line bore can not hold it to a size . even so you may not see very many that can or will hold size under 0001 or even 0002 .just trying to tell him how it can be done... maybe over some heads ?? done it just like jerry said line bore will not help much if the caps do not fit off the bat. a line bore will not make the caps fit side to side and make the rester fit someone still has to do that

OH thats .0002 to .0001 not .002 to 001 you get them that close you do not need a line bore you will be lucky if your bearings hold size thru the set under 0002

A good machinist using good equipment can set up on your block and machine a replacement cap from another block after the cap has had some material removed from the mounting face. Takes a good machinist to do this though, and again, good equipment. You may get lucky mixing and matching, but I would take the machinist route if I intended to keep the engine for any length of time. All is not lost.

The biggest issue when using a cap from a different engine is the lateral offset( reason the op broke his cap by putting it on backwards) With the proper lateral spacing it then becomes a vertical diminsion issue--too big, the cap needs to be cut, too little it needs to be bored---or shimmed.

Also---for the op---the cap can be built up with weld and machined back to fit the recess in the block with a simple flat file!!!

I consider my equipment to be top level and my abilities/experience to match--I probably line bore/hone REPAIR a deal such as this every month or so and as for just the routine line bore/hone operation weekly plus all the billet spayed caps that we install on the engines we sell ( currently we are doing several LT sbc blocks to 383/396 strokers)---
the touchiest part of the process still is cutting the stock caps flat as they are not square so that they fit in a cap grinder straight---always takes some finess

Originally Posted by deuce4papa

A good machinist using good equipment can set up on your block and machine a replacement cap from another block after the cap has had some material removed from the mounting face. Takes a good machinist to do this though, and again, good equipment. You may get lucky mixing and matching, but I would take the machinist route if I intended to keep the engine for any length of time. All is not lost.

yep like jerry said... my equipent is good . i done and fixed things some with more could not do and did think they were machinist ... i deck. bore, sleeve. recon rods. bore rods. valve jobs. turn valve down .lathe.mill.work bore rods fly cut pistons .AND FIT MAIN CAPS

Originally Posted by jerry clayton

the touchiest part of the process still is cutting the stock caps flat as they are not square so that they fit in a cap grinder straight---always takes some finess

yep rods can be like that as well i do a very lite cut take it out look at it and if needed i get out the sunnen shim kit i have the sunnen shim kit for the 750 sunnen cap cutter just for the main caps and rods/ caps . that goodson white stone works very good jerry .... on the cap cutters like we have . i just cut the resters down on a dart big M that i did not want to over cut the main caps

I did 5 today

Originally Posted by jerry clayton

I did 5 today

did 5 what ? 5 caps or five blocks

Just came home from the machine shop, and he said run it. as this motor is only going to be used untill i build me a big inch 400. this ones a 396. the machinist said it index's good and the broken peice wont fall out after its torqued down. just told me dont spray it, and try not to beat it all the time and it should be o.k. this is the third person that told me this. all of witch are people i trust. this should stir things up a little (LOL) this is not the way i do things, but i really dont want to spend alot more money on this motor. the cash isn't as loose as it used to be. i'll try to add pics so you can see it,