So, after building 4 bolt main caps for my basket case 331, i find the main bores way over size from factory specs. Turns out someone previously bored the mains to 273-340 LA platform size. I was worried at first, not knowing why the mains were so far from spec, but now Im glad. Easy to find bearings for the sbm. So, Im curious.... is this common practice?
I don't know anything about Hemis, but are you referring to the block was align bored larger for LA parts? I'm just curious.
Yes. Mains bored to 340 size. Both engines have a 2.500” crankshaft main journal, but the 340 has a larger OD bearing shell. Of course, I already have the factory spec main bearings.
No, not common practice. The cost of the needed machine work is substantially more than the cost of 331 bearings unless there is a very demanding need for a particular style of bearing that is not available for the 331 such as the old Clevite 77 or such...but, anything is doable if you throw enough money at it..... If the block has been bored to the LA size was the thrust thickness on the web also machined for the LA thickness? .
yeah, Bob's comment @ the time was it would save money if you were a racer & tearing it down regularly, but not worth the $ if a typical "Once & done" hot rodder.
Been thinking about this subject. I have a couple good blocks without main caps. Trying to make up my mind which way to go.
The thrust hadn’t occurred to me. I assume that since the block had them, it can work, I will definitely check into that. I made the caps thrust register to match the block.
Anytime you Aline bore a block and reuse the stock bearings, you move the crank closer to the deck. If this block received major damage in its previous life, oversized main bearings would be a way of salvaging the block and returning the crank back to its original position. Maybe this is the reason, for your oversized bearings. Bones
Well, a very good machine shop will offset the alignment bore so that when the new caps are mated the crank is in the same place. Roy Chambers (RC Automotive) did my race engines, including the one in my avatar, like that. He did a shitload of nascar cranks and block prep.
Not sure I understand why a line bore would move the crank. Anyone? (Truing up the individual bores to a common CL)
To Aline bore a block, you must grind some material off of the main caps to make the bore smaller, then you bore the block taking some material from the block and the main cap. Usually it is not a lot of material just a few thousands to make everything square to the block and round. If for some reason the block is damaged quite a bit. You could bore the main journals for oversized outer dimension main bearings and you will keep the crank in the same place. To offset grind a crank bore successfully would depend on where and how much damage you are trying to correct. Bones
The Pontiac rod bearings require rods with the tang on the other side of the rod. The bearings are for a 389-400 Pontiac. Hot heads sells the correct h beam rods. For the cost to refurbish a set of old rods and buy expensive bearings for them your money ahead to buy the hh rods and much cheaper Pontiac bearings.
When I had my rods rebuilt for my 331 I had them notched to accept AMC bearings. Much cheaper at the time. Sent from my iPhone using H.A.M.B.
So, i just checked the thrust bearing sizes. The bearing to crank dimensions are the same from the 331 to the 340. The block to bearing dimensions are different. My block has already been cut to fit the narrower 340 bearing. So, to be clear, the crankshaft main bearing surface dimensions (both O.D. and thrust) are the same between the 331 and the 340. The difference is the bearing shell is thicker in both dimensions, so both the bore and the thrust of the block need to be resized to fit a 340 bearing into the low deck early hemi. As stated previously, this is a nice way to repair a damaged block, or a way to open up choices of bearing style/ manufacturer.
I have a large stock of early Hemi parts, including complete main cap sets. Call , text or email me if you need anything. [email protected] or 928-412-4049
If you alignbore for new caps or different caps and the machinist does noT have his bore tool to just touch the block taking the material off the cap; the timing gears and chain will be a little “sloppy”. Cloyes did make sets for this... just something to think about...