The Stovebolt Site Forums The Shop Area Engine Shop Inner Rear (thrust) Bearing- Issue

I'm not sure how to reference a post from another forum here, so I'll copy and paste the Journal entry I made today below. Has anyone else had this nuance with the inner rear main thrust bearing and shimming? This is a 1955 (cast on Dec 27, 1955) full pressure 235.

Copied text (see the Journal post for pictures:

"Oct 14, 2023 Engine Update - Did the clearances for the main bearings today. Bit of a learning curve with using the plastigauge, but was going smoothly by the end. I had initially measured clearances with the original shims, which were all between .003" and .004". Decided that I would need to reduce the shim thickness to .0015" as I was going for the lower end of the .0005" to .003" range for my 1955 235. Initially was getting clearances averaging around .002 except for the interior rear bearing (thrust bearing), but more on that in a bit. The crank was spinning pretty easily with these clearances, so decided to try no shims. Pictures 1, 2 and 3 show the rear, inner front and front bearing in that order with no shims. Pretty much where I want them.

While I was working through the process, I noticed that when I torqued down the inner rear (thrust) bearing it locked the crank solid. Could not turn it by hand at all with the .0015" shim I had in there. So continued to try different shim combinations until I was at .002" on ether side. At this point I could turn the crank, but it was comfortably snug. I went back and measured clearances with the plastigauge again, first with out shims and then with the .002" shims on either side. Picture 4 is no shims and showing a .002" clearance and picture 5 is with one .002" shim on each side and showing a .003" clearance. I would have assumed that adding .002" shims to both sides would have increased my clearance more than it did. Would also assume that the .003" bearing clearance would see the crank turn easier than it did. However, I iteratively increased shim thickesses, sometimes by as little as .0005" with only that bearing torqued down, and I can't argue with the crank being locked, and then not locked. The side clearance of the thrust bearing is about .011", which is slightly high (I think, I only have the manual for a 1952 216), but shows that it wasn't binding.

Will be going to the engine forum on this one for thoughts."

Your narrative is a little confusing. The thrust bearing on a 235 is the #3 bearing, not the "rear" bearing at the flywheel. The rear bearing has four bolts, while #1,2,and 3 have two bolts each. By reading between the lines (which often leads one to incorrect conclusions) it seems that it's possible the bearing bores are slightly out of alignment.

Personally, I detest Plastigauge. I set bearing clearance by sandwiching a narrow strip of shim stock between the lower bearing shell and the crankshaft, and add or remove shims between the cap and the block until the desired amount of drag on the shaft is achieved. A firm pull on the flywheel which achieves a smooth rotation of the crankshaft is my goal. I use a .0015" strip of shim stock about 1/4" wide as a setting gauge, and adjust shims as necessary. Sometimes it's necessary to "split" shims- - - -one less layer on one side of the cap than the other, usually a difference of .0005" side to side. Once the desired drag is achieved, remove the checking shim and proceed to the next bearing. Do the clearance setting BEFORE fitting the rope oil seal! Obviously, the crankshaft must be round, and with little to no taper wear front to rear to get a good clearance adjustment.

On the Babbit rod 216 and 235 engines, the connecting rod clearance is set the same way- - - -use a .0015" check shim, and adjust the rod to cap shims the same way. Rods can also be scrape-fitted to achieve a perfect fit to the crankshaft journals before setting the clearance, something that should not be attempted with a precision insert bearing. Good luck!
Jerry

Thanks for the reply Jerry,

We are talking about the same bearing as the thrust bearing. I called it the inner rear bearing, which I got from the shop manual I have, but the shop manual called it the intermediate rear, so I remembered it wrong.

I don't have the flywheel on the engine at the moment, so don't know what to look for as far as the drag you're talking about. For that matter, I don't know what drag to look for due to lack of experience. Any extra help on that? Am I at risk of setting clearances wrong with plastigauge? Currently 3 of the bearings have no shims, so not much room to adjust. I'll go back and measure all the journals again, but pretty sure they are .010 under spec and my bearings are .01 over.

And my rod bearings aren't babbits, they're inserts. What's the clearance spec for the rod bearings?

Thanks,

Mark K

For starters, it may not be a 55 motor if you are correct on the date being December 55. Production for 55 stopped at the end of August 55, factory changed over for the 56 model year during the month of September, production for 56 begins October. That would make a December 55 casting date a 56 production motor. For 56, those motors used precision bearings, no shims. You might be using the wrong bearings.

Use the KISS principle- - - -clamp a vise grip on the crankshaft snout to give yourself some leverage or bolt a piece of flat bar to the flywheel bolt circle. Grabbing the crankshaft counterweights and trying to turn it that way will produce some skinned knuckles, and bleeding all over the inside of the engine makes it rust. Clearance on a precision insert rod bearing is determined by how accurately the machinist reground the crankshaft, and whether or not you got the rods reconditioned so they're round and the right diameter. Ideally, I like to see .0015" clearance on the rod bearings, but very few machinists are willing to work to my specifications. That's why I bought my own crankshaft grinder, line bore machine, and a Sunnen rod honing machine. If the clearances are wrong, it's the fault of the guy who brushes my teeth every morning.

There is no such thing as a .010" OVERSIZE bearing, unless you're working on an air cooled Volkswagen.
Jerry

Hey Dragsix, I had previously posted about finding main bearings. The thread is here:

https://www.stovebolt.com/ubbthread...in-bearing-availability.html#Post1504343

Pretty sure I have the right bearings after that. My engine does not require the locator dowel and when I search Summit Racing for 1956 bearings it gives me the same part number I bought.

However, if they were not supposed to use shims, I still have to use shims on the thrust bearing or it locks the crank. The other three bearings fit within spec with no shims.

960M main bearings are 56-63 and do not use shims. Not one, and not for any purpose. 56 got precision bearings, a neoprene rear main seal, and the 848 head. These changes raised the compression, horsepower, eliminated the shimmed bearings, and an improved rear main seal.

If your crank is locking up when you torque the No.3 main thrust cap, you have a bigger issue. Bent crank, mismachined crank, journal out of wack, main bearing bores way out of wack (not something these motors are known for but does happen), something.

Might be time to have the crank inspected, and if good, machine the crank to specification.

Could the main cap for the thrust bearing have been installed backwards?

So . . .

I did all the clearance inspection, with the thrust bearing issue and was revisiting things this morning when I realized that the new bearings I bought were still in the box. I did all the work with the original main bearings. Oof.

I installed the new bearings and everything measured out between .002 and .0015 clearance. Even the thrust bearing. In the end, I'm all good and the crank turns well.

Dragsix,

My engine did not come with a 848 head. It's a Canadian block, so not sure if that makes a difference. The code stamped on the machined area left of the dipstick is 1744C, which I have not been able to decipher.

I did look at manuals online. The 1955 manual still referenced shims. The only thing I could find for a 56 engine was an addendum to the 55 manual, but did not have a section updating to no shims. If you could point me to the right resource for my block it would be appreciated.

I did have a machine shop recondition and balance the crank. The fit of the new bearings would supposedly confirm the crank is good. Must have been another issue, but now that things work, I'm not going to spend anymore time investigating.

Thanks,

Mark K

It’s a 56 block, not a 55. 55 used shims, 56 did not. It’s in both the pass car manual and the truck manual, both of which were supplements to the 55. If you look at 6.2 in the 56 truck shop manual, the supplement, it’s under main bearings, it’s spelled out.

Dragsix,

I went back and reread the supplement. You are correct. It's very subtle and I missed it by reading too quick. It says, "dropping of the crankshaft in "Main Bearing Replacement" is unnecessary and "Main Bearing Adjustment" is eliminated for 1956 engines."

As an aside, interestingly enough, the 1955 section is titled "Adjust Main Bearings". I do a fair bit of technical writing, and difference in section titles would be considered a faux pas. That said, because I do a fair bit of technical writing, I should have picked up on the directions of the supplement.

Thanks for helping me get it sorted out.

My pleasure. I cant tell you have many times guys will tell me I am wrong about the motor changes in 56. Many, like yourself just missed it in the manual or never read the 56 supplements to begin with. I have been playing with these motors for 47 years so while I may not know everything or even a lot, I know enough, lol. Truth be told, I steer clear of the 55 production year blocks just because the 56 and up were so much better. Frankly, the 56 motor, with its 848 head dated 56 is a really great candidate for a 55 swap in that the 56 head has has the temp sending port the same size as the 55, so its a direct swap, whereas the 57 and up use the larger temp port (requiring adapters). The 5913 head has a combustion chamber volume of like 86.2 cc where the 848 head is 79.1 cc. The benefits of course are, higher compression, better HP, neoprene rear main seal, precision bearings.

I have swapped to an 848 head now, but for whatever reason, the engine I pulled from my grandfather's 1 1/2 ton did not come with the 848. Glad I ended up with the optimum swap in the end! (Although I do need the temp sending port adaptor) Additionally, I inadvertently found out about the neoprene rear main seal, so thanks again. I incorrectly had the old rope seal in my head.

Good on you, Mark! We all learn from our mistakes. It is a real man who freely admits his mistakes so that the rest of us can learn from it.

52Carl,

I have a rule I use at work with the young Engineers under me. If I learn something new in a day, I get to go home. So please prove me wrong in the morning.

I find you're only as good as the amount of things you screwed up.

I'm in the process of designing and building a fixture for my line bore machine to cut all the stovebolt main bearing bores to the same diameter, and install reducer sleeves to make Chevy small block 400 main bearings fit the stoveboolt block. Then the crankshaft will be reground to SBC 400 diameter on all four journals. That will eliminate the search for the scarce and expensive stovebolt main bearings. Some years and sizes of those mains are either unobtanium, or outrageously expensive. The modification will work on 216, 235, and 261 engines, using the same inexpensive and readily available bearings.
Jerry