The Stovebolt Site Forums The Shop Area Engine Shop Align boring

I've heard you can align bore a 'bolt block in such a way that you can eliminate the shims under the main caps. How much material needs to be removed? Since the mains are diferrent sizes how does this process work?

Any align bore job is done by centering the boring bar up in the main bearing bore, then moving it slightly toward the block, no more than .002". Then the caps are milled or ground about .010" to reduce the diameter of the bearing bore. Torque the main caps back on. Then, a cutter is attached to the boring bar, and adjusted to the specified bearing outside diameter. It's run through the reduced-size main bore, and cuts it back to original specification diameter. The cutter takes .008" out of the cap, and .002" out of the block. Repeat as necessary for the remaining block/cap pairs.

This moves the crankshaft slightly closer to the cam than normal, which will either tighten the tooth clearance a bit on a gear-drive cam like a stovebolt, or slack the timing chain a little on a chain-drive cam. There's a slightly shorter rebuilder's timing chain available for some engines to take up the slack on a line-bored engine. If a gear-drive engine gets too close for cam gear clearance, it's junk.
Jerry

Thanks Jerry. Do you think the risks outweigh the benefits?

Line boring is to straighten out a warped block and is not always successful. It used to be one of the things engine rebuilders would claim they did and tried to convince people they couldn't live without it. But If you can get your mains to plastigauge with shims, why risk screwing up your block. Chevs ran millions on top of millions of miles with factory shims.

With the advances in precision tolerances for bearings in the past 50 years, I'd strongly reccomend a line bore, done by a very skilled machinist, and no shims. It would be important for the shop to uderstand why you want it done, and the desired results when they're finished. I managed the rod reconditioning department for a big machine shop in Nashville for a while, and the crankshaft/engine block guy there was very good. I wouldn't hesitate to have him take on a job like that, but he's one of a very few machinists I'd trust that far.
Jerry

Jerry, if a machine shop takes on a job like this but ruins the block, do they have to get you another or are you just out of luck?

That's something to discuss before the job is begun, not after the fact. I installed a cylinder sleeve in a 1937 Lincoln V-12 block a while back, which is a lot more scarce than any stovebolt engine. The possibility of failure never entered my mind, or the customer's. Just do it right the first time, because that's the only opportunity there is!
Jerry

Heres what I'd like to know. The shop manual says to align bore the block with the bearings installed and four shims under each cap, them after remove enough shims to achieve proper fit. If you align bore the bearings to the correct size, why the shims? And why do the rods need shims? Do replacement main bearings not need boring? If I convert to insert rod bearings, will I still need shims?

Some of the older "semi-finished" main bearings might have required line boring, but I don't think I've ever seen a set like that. The precision-fit mains I've used for the past 40 years or so are simply installed in a round, proper-sized block & cap assembly and the clearance takes care of itself. Shims are just a way of doing a final clearance adjustment, something that I don't believe should be necessary if the bearing fits right in the first place. Rods that have been machined for insert bearings should not need shims for the same reason- - - -do it right the first time, and the clearance won't need tweaking.
Jerry

Edit: Babbited rods used shims as a way to compensate for rod bearing wear- - - -as the clearance increased due to wear, a shim could be removed to tighten things up temporarily. Adjusting the rod and main bearing clearance was part of a re-ring and valve job around the shop where I grew up back in the 50's.
Jerry

This is the first I've read about line boreing. the shop didn't do this to my block. 1962 235. I replaced all bearings. I acheived the proper clearences without shims. The kit came with them, but I didn't use them. If anything I had too much clearance on the rod bearings. I had to go to the max torque to get it in band. Is this normal to see? or do you normally put the shims in? First time building an engine so I just want to make sure before going on too far.

Rods have a nasty habit of getting egg-shaped, so the clearance will be more top to bottom than it will if you measure the bearing diameter at a 45 degree angle to the vertical. I always check the rods for out-of-round and correct diameter, and have them reconditioned if necessary. That's really the only way to assure your bearings will fit properly. At least they're not Babbit rods!
Jerry

If I understand this correctly, the mains on the old babbit engines were also a poured bearing that had to be hand fitted? I always thought that was only the rod bearings. I have never actually worked on one of the "babbit beaters" so I don't know. The old time mechanics must have been real craftsman.

Some of the very early sixes had poured main bearings, but by the time the 216 came along they had switched to insert-type mains. The shims were used to fine-tune the oil clearance on the mains and allow a way to compensate for minor wear problems. "Scraping" a Babbit bearing is quite an experience, where minor tight spots are trimmed away with a sharp tool until a layer of Prussian Blue on the bearing surface ends up as a smooth smear with no bright spots that indicate more material needs to be removed. Once the bearing is perfectly matched to the crank, then shims are added to establish the needed clearance. VERY labor-intensive!
Jerry

Your 62 didn't ever have shims, 57 was the last year IIRC. Shims were used simply because machine tolerances were not as precise and they were necessary. Another example is that cars had soft tops in the center of the roof until 36 or 37 simply because they didn't have the capability of a one piece stamping. As technology increased machine fit tolerances became possible and can be applied to old engines.

As I understood line boring, it was done to the block casting at the factory during manufacturing. This process insured the block was prepared to accept the insert bearings. I don't know what was required for babbitt bearings. It was also required for the cam bearing journals. These were installed with a long tool that insured their alignment as well. In the engines I have dealt with, this was never required to be done.

If you don't have enough clearance, you might need oversize bearings.

Are you checking with plasti-gauge?

According to the shop manuals that I have, they used shims at least to 1960 (I don't have nay past '60).

According to the GM manuals, 1948 was the first year that the 216s and 235s had "precision" main bearings. They used shims from the factory. From about 1934 to 1947 they used insert main bearings that had to be line reamed in order to prepare them for use. A very reliable source over at VCCA stated that GM brought out replacement main bearings for these motors that didn't have to be reamed. They all used shims during this period.