4-bolt main cap conversion

Good morning, I would like to know if it takes more machine work to fit an aftermarket set of billet steel main caps (Pro-Gram) than a set of GM stock main caps on a 1971 454 block. The guy at the machine shop suggested the GM stock caps. Since then I find out they have been discontinued by GM. On ebay the bid went over a hundred $$ so I quit. If I gotta pay that sum of money for used caps I prefer spending the money on Pro-Gram caps. So I was curious if there is more work involved fitting the Pro-Gram caps.


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It doesn't really matter which ones you buy, they'll need the same amount of machine work to install...

Anytime you change main caps there will be machine work. This is a critical fit in several places 1) the cap has to register into the block fitting properly which may require machine work to the block's registers and or the cap this is a side to side fit; 2) the cap must be vertical to the crank center line; 3) it must align to the block bearing bulkhead in the fore-aft direction, this being especially critical on the thrust main especially on engines where thrust is taken on the #3 main; 4) if using a 4 bolt cap where the OEM was a 2 bolt, additional bolt holes need to be properly positioned, drilled and tapped. Once all this is accomplished the bearing bore has to be machined to center the mains with each other (align bore/hone) so that the crank journals are aligned with each other and the crank is aligned to the block.

Now, it could be that some of this just happens to work out without machine work, so at the very least these dimensions need to be carefully checked but I wouldn't bet on this kind of outcome.

This effort would be the same whether the caps were GM or aftermarket.

Bogie

Machining steel is slightly more difficult than machining cast iron but with correct new tool bits it isn't any more difficult.
Personally, I would probably fit new main caps and take some measurements.
If a lot of material needed to be removed, bolt pairs together and flycut closer to 'finished' diameter. (but, I actually trained as a precision machinist )
Carbide bits are different for cast iron and steel and tend to dull rather quick if you using the wrong ones
Maybe you need a new machinist?

I had Milodon splayed billet caps machined and installed on my SBC, like Bogie says, it was more than a simple drill, tap and bolt on. Besides machining the block registers for proper fit and alignment, the outer flanges on their splayed caps required .003" clearance before torquing, that clearance wasn't there out of the box. You might get lucky, but it's not likely. Straight caps probably do not require that clearance.

And no, I'm not taking a measurement in the picture, the gage is only there for the picture.

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''Life's tough, Pilgrim, and it's even tougher if you're stupid.''---John Wayne

Midnight Sun Street Rod Association

There is a big difference between replacing used production 4 bolt caps vs. billet aftermarket caps. Most times when you replace with production caps you only have to align hone the block. With billet aftermarket caps you have to align bore first and then align hone, so it is a more expensive swap in the end. $100 bucks for a good set of production BBC 4 bolt main caps is a bargain, *********************

better question is why you want 4 bolt mains, and if you are going to be making enough power to need them, get the splayed 4 bolt caps, yes they are more than the useless g.m. 4 bolt caps,, I say useless because of the power being made today, over what was engineered into the stock 4 bolt back in the day. 454's can make 575hp on 2 bolt mains and arp studs..
all bets are off if you spray it..
find out what the cost of adding splayed caps to your block.. and all themachine work.. , you might be better off looking at a dart or world block.

you didn't list what power the build will be, trans brake or stick. etc

but the cost of adding the caps, might not be worth it.. hell you might not need them..

YES! X2 with CNC dude. You can get away, most of the time with an align hone on a factory cap. You can also solve the main seal problem by only repalcing the middle three. When running an external balance crank, it's the #'s 2 and 4 that get beat up if any do. You can also "poor boy" it by drilling/tapping your own holes for the middle three using the cap as a template and a drill bushing for the outer holes. Then the machine shop only has to do the align hone. It won't be quite as fancy as if you let the shop do it all, but I've done it on a couple of S/G motors that went low nines and were very reliable...on a budget. I have an extra set of caps if you're interested.