The Stovebolt Site Forums The Shop Area Engine Shop "Line boring" a stovebolt block

Or, to be more accurate- - - -the term is "align boring"- - - - -making the main bearing bores exactly the right diameter, and getting all four of them in a perfectly straight line, is vitally important to the life of the engine. When the block was cast way back in the era of dungeons and dragons and chastity belts, it was common practice to leave the rough castings out in the weather for several months to let them "season"- - - -undergo around a year of temperature changes to stress relieve the castings. Then the blocks and heads were finish machined before being assembled into our favorite chunks of scrap iron. Over several decades of heating, cooling, and pulling heavy loads, sometimes that stress relieving process continues, and the bearing bores get out of alignment. Uneven main bearing wear and/or failure can be the result of the misalignment. Within reasonable limits, the main bearing holes can be reduced in size by grinding a few thousandths off the caps and reboring, although finding a machinist willing to attempt the job can be roughly equivalent to finding a chicken with lips! I have a portable line bore machine that bolts to the oil pan rails and can be set up to rebore all four different-sized holes at once. I'm in the process of fabricating a special cutter that can be used to narrow the width of the #3 main bearing web in the block so a later model 235 crankshaft can be installed in the 216 and early 235 blocks, thereby eliminating the need for Babbit lined connecting rods and cast iron pistons on the early stovebolts.
Jerry

One of my friends, a few years older than me, inherited his dad's shop that was started in the 40's, and called me to see if I wanted some of the stuff before he scrapped it. Well, I got a portable align boring machine,( very lightly used with the manual) and 2 in the block crank grinders, and some other "stuff". Another friend recently sold his engine machine shop in Houston, and is now doing block work at his shop at home. He may be the guy to talk to about any questions about align boring, I am pretty sure he has seen a lot. Good luck on your machining!

Well, all this talk poses an obvious question! All undersized bearings I have seen or heard about are for undersizing the crank. Do they make bearings that are both oversized on the OD and ID, or do you have to look at other engines and size them up yourself?

Good question Phil, I was trying to understand that as well. Hopefully Mr. Jerry can fill in the blanks.

No, the same bearings are use after the boring job, as the hole in the block is first "undersized" by grinding or machining a few thousandths off the main bearing caps. Since most errors happen from top to bottom of the bore, the same way connecting rods wear egg-shaped, reducing the top to bottom distance allows a rotating cutter to be set up to make the hole round, and the original size again. First, the cutter bar is aligned in the block up and down and side to side with a "last word" dial indicator, then the bar centerline is moved .002" closer to the block. This allows a skim cut to be made in the block without seriously changing the crankshaft to camshaft spacing. A .010" cut is made to the mating surface of all the main bearing caps, and a cutter is set to trim the hole back to its original diameter. The block gets a .002" cut, and the bearing cap gets cut .008". All the holes end up the original diameter, and perfectly in line. This restores the proper alignment and oil clearance to the main bearings. In the case of a stovebolt six, and other GM inline engines such as Pontiac, Buick, Oldsmobile, and others, all the bearing diameters are different, bigger from front to rear, so each hole has to have a different cutter setting. That's why today's so-called "machinists" run and hide when someone mentions line boring a stovebolt engine!

The old air cooled Volkswagen engines actually had oversize OD main bearings available, since the main bores were cast into the two halves of the crankcase, and there was no way to shrink the hole before reboring it. Those bearings had an oversize OD, and an undersize ID to handle a reground crankshaft.
Jerry

I understand! Thanks for the explanation! That’s pretty slick and good to know!

The first thing we did with a race engine was a line bore job, since all the other dimensions index off the crankshaft centerline. Block decking, cylinder boring, and other dimensions begin with an accurate crankshaft centerline. It was not uncommon to have dozens of hours invested in block preparation before all the other high dollar parts started getting bolted on. I was always on the lookout for race-prepped blocks that my competitors called "worn out" when they developed some cylinder wear. I'd buy them for pennies on the dollar, or sometimes get them free just to haul them away. Then I'd use those blocks to train my students to install cylinder sleeves. For the cost of eight sleeves and some "free" labor, I'd have a new standard bore block worth a couple of thousand dollars or more. Eventually, some of my competition figured out they were getting beat with their own junk!
Jerry

texczech What brand are your new machine shop toys ??

Phak1 On race engines when they blueprint your mains,the bores are the same size .o.d ,in alignment,but the crankshaft is raised up into the block. The pistons can then stick up above the head gasket decks. This also affects timing gears or chains,may require thicker head gaskets,and may tighten valve to piston clearance on like hi-lift roller cams. Hopefully they can just clear the top of the main saddles by .001 or .002 thousands. Lot of tedious careful work to keep the whole block blueprinted. How do I know,best bore equipment is sunnen !!

That's the key. If the bar is center align to the bearing opening (after the caps are milled), it will raise the crank upward by 1/2 the mill distance. Mill .004", move .002", etc. This will close up the crank to cam distance and raise the piston domes the same amount.
The hard part is aligning the bar to barely touch the block saddle, and remove metal only from the cap.

On a "belly button" engine (everybody's got one!) like a small block Chevy, the main bearing bores are all the same size, and a long hone can remove a little material to get them into alignment. There are also timing chains available with a tiny bit shorter length to compensate for the shortened crankshaft to cam dimension if a careless machinist removes too much from the block side. Stovebolts have different size main bores and gear drive cams, so there's a danger of bottoming out the timing gears. I've never seen a "stepped" hone mandrel for those engines. The setup time to get an accurate cut is many times the few minutes involved in actually making the machining cuts.
Jerry

panic The real rub comes in getting a good finish at the parting line,if the bearing has moved enough to scuff the parting line finish then it's tougher to save. You can always throw the caps away and put on steel ones or use good caps from a ventilated block. Last one I dealt with was a merlin 3 block with a blackened no. 2 main bore. Blackened means getting ready to spin a bearing.

Lots of worn out reamers,pullers etc,,, a Heyer distributor stroboscope- distributor machine, Kwik Way portable main bearing line boring machine for US army, winona crank grinder, and UG Willis crankpin grinder 2125293 UG Willis Dallas Texas. This Willis thing fits around the rod journal(I think) and has several carbide cu tters that shave the crank. The Winona crank grinder has a motor that fits in the starter hole ,spins the crank( without the spark plugs, and grinds the crank, I'm not sure if it does mains and rods. Lots of stuff got scrapped before I was called, but I got enough. Also 30 -40 carburetors, some of them the desired one barrels for the 6 cylinders( carters)not the rochesters. Looks like they had a cast iron body.

texczech Thanks for the reply,sounds like some stuff you might get a little good out of,those iron carbs ran OK for me when I burned gasoline but never got much economy out of them. Does the winona grinder use a stone on the end of a long shaft?? Made so the actual grinding is out on the face of the stone. The trouble there is that the center of the stone binds on the shaft (journal). A cup stone I think would be better,however those antique tools are mostly just for fun.

panic You know you have to consider that even with a perfect cut on the main bearing bores any other cuts ,squaring,decking,taking out wind,cam bore work,can release other dimensions you've already worked on.

That’s why old time machinist’s would machine shafts oversized, then leave them out for several weeks/months to let them stress relief before final machining.

Somehow, no matter what I say, someone has to correct me.
If I didn't post at all, would I still be wrong?

According to your wife, Yes.

I'm just following this discussion for my own education. Panic, with the crank distance changing can oversized bearings be used to compensate for this?

Other than air cooled Volkswagen engines, I've never seen a main bearing with an oversized outside diameter. We're getting a lot of pontificating by people who have never actually line bored an engine. Theory gleaned from an internet search is great, but actual hands-on experience is usually better.
Jerry

Jerry, I had my machinist do all the block work on my engine. I don't have the tooling for any of that. My thought was that if you removed material it would need to compensated for somewhere. With line boring do you remove material from the main caps to obtain the the correct bearing OD?

Yes, first the boring shaft is centered in the block bores, then it's CAREFULLY moved .002" closer to the block. The mating surface of the main bearing cap is ground or machined .010" to give the machinist something to cut. With the main caps torqued to the block, a cutter is set up to the original bore size and passed through the bore. It cuts .002" from the block, and .008" from the cap, re-establishing the original diameter and getting all the main bearing bores exactly in line. Moving the crankshaft the thickness of a strand or hair closer to the camshaft usually doesn't seriously affect the cam timing on a chain drive engine, and it doesn't bottom out the gear teeth on a stovebolt cam drive system.
Jerry

As I had all of the machining completed by a shop I was not aware of the actual set up. Thank you.

Having grown more crotchety & suspicious with passing decades, I'd like to believe that if the machinist explains to me the correct (above) procedure, rather than me describing it to him, and he says "Yes", that he will actually do that.
But I don't.

The Machine Shop I use is owned by a good friend, Jim Bailey. He held the 1981 NHRA record for a naturally asperated 230 CID Camaro.

It is increasingly hard to find a shop that will do easy things right, let alone hard things.