The Stovebolt Site Forums The Shop Area The HiPo Shop Billet steel main bearing cap- - - -

Back in the dark ages when I was learning to build engines, a lot of my experience was gained by watching my father build flathead Ford round trackers. If you think a stovebolt is a primitive design, then the old flatty had to be downright stone age! A V8 engine with only three main bearings- - - -and the valves, intake and exhaust system all in the block! One thing race-modified flatheads with long stroke crankshafts were prone to do was break crankshafts- - - -they would twist themselves in two at the center main bearing. It was a pretty common practice to install a "billet" main bearing cap on the center position- - - -a piece of solid steel plate machined to duplicate the usual cast iron main bearing cap.

On the stroker stovebolt engines I've got on the drawing board, I'm considering doing the same kind of thing for the two center main bearing caps. They will be machined to use Chevy small block 400 mains, and the only place a spacer will be needed will be in the upper half of those two bearings to take up the extra space between the main bearing and the block. Since the front and rear main caps are shaped to accommodate the timing cover and the rear seal, they will need to remain original. The billet caps in the center will make the assembly more rigid, and help the main bearing clearance stay at a minimum- - - -maybe even using four bolts instead of two. I'll probably also use studs instead of bolts to attach the main bearing caps.
Jerry

Sounds like quite a project. I recall changing out a broken center main bearing cap on a F 6 Ford bus a 1000 years ago. No broken cranks though.

Carrying the torque of the four center cylinders on a single main bearing was the flathead's downfall. That became especially critical when another 3/8" or more of stroke length got added to a design that was on the ragged edge of disaster to begin with. A broken crank at racing speeds pretty much hand grenaded the whole engine!
Jerry

Just did some measuring- - - - -a stovebolt block should be able to get 4 bolt steel main bearing caps 6 inches wide, fitted to the two center positions, similar to the 4 bolt main bearing small block V8s. That will make the bottom end of the block a lot more rigid, better able to support that long, twisty crankshaft. A 235/261 crankshaft with 3/8" more stroke will be a torque monster, particularly when the added compression of the higher displacement is factored in. No need to wind one to astronomical RPM that the cylinder head design can't support, or do radical stuff with the camshaft.
Jerry

Milodon makes their main cap conversion caps from ductile iron. They have a little write up on why it’s the better material. I can’t say one way or another but I thought you might like to see their reasoning for using ductile iron as opposed to steel.

https://www.milodon.com/main-caps/main-caps.php

Since a stovebolt main cap does not have the step built into the main bearing area like a small block V8, there's no reason to use an angled outer bolt. I'll be using the original size 9/16-12 inner bolts, and adding a 7/16-14 bolt on each side of the original inners. That will add a lot of rigidity to the #2 and #3 main bearing area, and probably support the crankshaft better than the original cast iron main caps. That's probably overkill, but I wouldn't be surprised to get 125 HP or more from the 216 stroker engine, and probably 200+ from a bored and stroked 261. All while looking totally unmodified from the outside! My dyno will be the proof of the pudding- - - -no guesswork and Bovine Scatology involved!
Jerry

Very interesting idea for strengthening the bottom end. Always amazed at the ingenuity of Bolters.

Jerry, was actually asking about your opinion re ductile iron vs steel main caps, taking into consideration the short little discussion on the Milodon site regarding their use of ductile iron for their caps.

At the power level produced by a stovebolt six, even a modified one, I don't see an advantage to ductile iron over plain steel. Those Milodon ductile iron caps with the angled outboard bolts are for engines that are on the ragged edge of blowing the crankshaft out the bottom of the block. The primary purpose of a wider, taller main cap with multiple bolts is to reduce the flex of the block under high speed- - -high stress situations. The only reason I'm considering doing that is to add a little rigidity to the middle of an engine where I'm going to be running a much smaller diameter main bearing than original- - - -converting 235/261 crankshafts to run 400 small block V8 main bearings from front to rear- - - -all the same diameter. They're MUCH less expensive and more available than the original mains.
Jerry

Do you plan to run studs for additional strength and clamping power?

I'll probably use 9/16-12 Grade 8 bolts on the inner holes, and drill and tap the block for 7/16-14 Grade 8 outer bolts. Studs are OK if the block area where the threads are located is pretty beefy, but that's not the case on a stovebolt engine. It's also necessary for the cap and lower bearing shell to self-align with the crank as the bolts are torqued, since a stovebolt does not use alignment dowels or a stepped block to positively locate the cap.
Jerry

Just curious as to why studs would be different from bolts in terms of allowing the main caps to self locate, as all the holes are in the same places either way? Or do original stovebolt crank bolts have a larger diameter on the unthreaded shank which locates the cap, which a stud would not?

9/16 studs are scarce as a chicken with lips, and E$$$$PENSIVE. There's really no need for the extra clamping power of a stud, and even a modified stovebolt engine probably won't be dismantled often enough to justify saving the threads in the block by doing a one time installation of a stud. Our round track engines got refreshed every 1K miles or less, (if they didn't hand grenade first) so wear on block threads was definitely an issue there. I do plan to run studs on the cylinder head, since the extra displacement (30 cubic inches +/- a little) will raise the compression noticeably.
Jerry

Also, why whittle that long, twisty crankshaft all the way down to SBC bearing diameter? Are there no other main bearings available which are closer in size to what the stovebolt came with? I've really enjoyed watching how people are using pistons from various other engines, maybe something like an Isuzu diesel would be closer in diameter? Also maybe wider, aren't SBC mains thinner versus the stovebolt?

The OD of the SBC 400 main bearing is only about .030" smaller than the OD of the front main bearing on a stovebolt, so if one wants a set of mains that are the same diameter front to rear, that's the logical choice. I'll probably bore the front hole in the block a little bigger and custom machine a spacer for that one. A narrower main bearing shell will cut down on rotating friction, while still giving the crank plenty of support. Making the #2 and #3 main caps from billet steel with four bolts instead of 2 bolt cast iron caps will also stabilize the crank and make the block stiffer to minimize the flex on both items.
Jerry

I've also discovered a custom length connecting rod intended for use in an altered stroke Chevy big block that will allow Chevy 265 V8 pistons to be used in a 261 custom buildup. The rod is just enough longer than the 261 item to compensate for the shorter compression height of the 265 piston. It's an H beam rod that's 7.100" inches long, and it's rated for 800 HP or so. A bushing in the top end would adapt the wrist pin size to the pin used in all the small block (and 261) pistons (0.927") A big block connecting rod bearing could be used by regrinding the 235/261 rod journal down. If someone is brave (or foolish) enough to bore a 261 1/8" oversize (0.125") a 283 piston could also be used since the compression height is virtually identical. That would probably be a "race only" application, since a solid copper head gasket with stainless steel wire O rings would need to be fabricated to hold the compression pressure.

A little over 15 years ago, Webrodder did a stroker 261 build with dyno results that was pretty neat. They did quite a few Stovebolt oriented articles and tests back then, including a one piece rear main conversion on the GMC and Stovebolt blocks....

Would you have to buy more than one set of pistons to get 6 that matched? I have it in my head that most V8 pistons have wrist pins located slightly off center….so there’s a RH/LH aspect to them. Perhaps they could just be flipped around to be oriented the same, I suppose.

How does the BBC connecting rod and bearing compare in journal width? Would you have to shim the sides or do any crank matching besides reducing the diameter?

I have one (used) 7.1" BBC H beam rod on order, to do some test fitting with it. I'll probably need to TIG weld a spacer onto both sides of the rod cap, similar to what I do with 292 rods, or use bushings at the top to center the rod in the piston. Either way, the rod will ride the middle of the crankshaft journal. A set of 8 265 or 283 pistons would end up with a couple of extras, but they're inexpensive enough to justify the purchase, IMHO. I also weight match every moving part in my engines to 1/10 gram (A dollar bill weighs almost exactly a gram). Each piston/rod/ring/wrist pin assembly is weight matched as a set. Ditto for matching the weight of both ends of the connecting rods to the same tolerance. Wrist pin offset compensates for crankshaft rotation and rod angularity- - - -not right or left bank on a V8. The only bank-specific pistons are on engines with custom dome shapes- - - -flat tops don't care which cylinder bank they're on. An.060 or .080 oversized 265 piston could be used to increase the displacement a little without of going to the extreme of a .125 overbore.
Jerry

I have new Scat 7.100" BBC rod I bought about 12 years ago for demonstration purposes for my Stovebolt rod swap book I did at that time to show the differences between the GMC rods and the Stovebolt rods and swap possibilities of it....

When I get busy in the machine shop, my "Mad Scientist" gene goes into overdrive! Fortunately, I've got a couple of lathes, a Bridgeport or two, two crankshaft grinders, a couple of Sunnen hones, and a line bore and a line hone to play with! Also a surface grinder and a radial drill press. That 7.1 rod is also a candidate for use in a flathead Ford stroker engine I've got on the drawing board. There are several Japanese Diesel pistons that would be compatible with highly modified antique engines- - - -Ford, GMC, and Chevy.
Jerry

It's a nice looking rod and I'm sure it can find numerous ways to swap into other engines....

Like the old chicken nugget TV ad used to say- - - - -"Parts is parts!" Chunks of metal, either pistons, connecting rods, valves, or bearings, don't care where the part number crunchers say they're supposed to fit- - - -as long as they get the job done and the clearances turn out right, there's nothing wrong with mixing and matching!
Jerry