rod bearing clearance with shells
 

For those of us that have put shells in our engines,what is the proper clearance? Still .001 or ? Thanks

Re: rod bearing clearance with shells
 

.001 per inch of shaft. 1.500 inch shaft, would be .001-50 thousandths minimum, and Plus .000-50 more would be Max.

Re: rod bearing clearance with shells
 

Listen to Kohnke, he knows his stuff. .0015" would be ideal, .002" wouldn't hurt.

Re: rod bearing clearance with shells
 

I agree with both. Kohnke and Jim

Re: rod bearing clearance with shells
 

FWIW: I have completed almost 80 engines with insert bearings. When Antique Engine Rebuilding released their insert bearing designed specifically for the Model A & B engines, I purchased stock and have used the product exclusively since. So the last 70 engines have had insert shells from AER and this is what I have learned. When I used the (early) babbitt lined shells Herm’s clearance spec (.001" per 1" of shaft diameter) worked good...no problems. 30 engines had babbitt lined (AER) steel insert shells. The last 40 have had the aluminum lined (AER) steel insert shells. For the aluminum insert shells you need a bit more clearance at the rear main.
For the AER rods: .00125" to .00175” is perfect. If you go above .00175" you may have an occasional noisy rod.
For the mains: .0015" at the front and center bearings is good but the rear main needs a bit additional clearance...0018" to .002" works well. If you strive to achieve .0015" or slightly tighter, you may melt/weld the rear main shell to the crankshaft. How do I know? I did it twice on my test/experimental engine's rear main. I built up a weighted and balanced crankshaft and then sent it to a shop that completed a Nitriding process of hardening the journals. It had exactly .0015" main bearing oil clearance prior to the process. After the process, the oil clearance was reduced to .0013". Within 10 minutes of running the engine on a test stand the rear main heated up and failed. The external symptom was a rear main leak. I pulled the engine down to discover excessive heat was present on the back side of the insert bearing (rear ½ of the insert). The aluminum surface of the rear main insert was galled and deformed. There was no damage to the crank journal I could detect so I thought it a fluke but when re-assembling, I increased the clearance to .0016" and lengthened the parting line oil reservoir of the shell. I ran it for a couple hours on the stand...everything seemed fine. I installed the engine in a car to further test and the rear main failed within 200 miles. The same oil leak and signs of excessive heat were present. Upon reassembly I increased the clearance at the rear main to .0019" and have not had a problem since. To further test, I assembled a second test engine with a standard crank, AER aluminum lined shells and set all the main bearing clearances at .0015”. I ran the engine hard on a dyno and locked the rear main insert tight to the crankshaft. It ran for about 6 hours before it failed. I pulled the engine down and had to split/remove the bearing shell from the crankshaft with a chisel. I cleaned the rear main journal of the crankshaft with acid to remove any remaining particles of the aluminum lining from the insert shell. I then had my crankshaft grinder polish only the rear main journal to provide .0018” oil clearance. I left the front and center at .0015” oil clearance. I ran the engine for 4 hours on the dyno at full throttle and loaded to pull the rpm down to 2000. I tore the engine down to check the main and rod inserts…everything looked new. I now run between .0018" and .002" oil clearance in all the rear mains with insert bearings. If you are concerned with an oil leak…with these target clearances I have had no detectable oil leaking past the standard slinger and seal. I absolutely will not go below .0018” oil clearance at the rear main with these aluminum lined steel inserts. The rear 1" of these rear main insert bearing shells are devoid of an oil reservoir, I suspect, to minimize oil leaks. I suspect there is not enough oil getting to this area to keep the surfaces cool and separated from each other. The extra clearance solves this problem. The crankshaft could be flexing slightly resulting in reduced clearance between the rear of the journal and the insert. To quickly adjust the rear main clearance, I purchased a used but good Sunnen Precision Honing Machine (large floor model) and new mandrels. The area Sunnen rep. helped me select the correct stones and bronze shoes to hone the insert shells to increase the clearance to the spec I need. I fabricated an insert shell fixture from two rear caps to hold the shells at the same tension or “crush” as when installed in the block. The fixture was bored to match the block housing bore and the inserts are clamped in the fixture during the honing process.
The inserted engines with AER insert bearings are working great. Most of the recent orders I have received are for bench run long blocks that are run under load on my dyno for about 3 hours.
Good Day!
Dave in MN
www.durableperformance.net

Re: rod bearing clearance with shells
 

Thanks for a very clear and through explanation. This kind of technical helps us all. ken

Re: rod bearing clearance with shells
 

Great explanation Thanks

Re: rod bearing clearance with shells
 

Re: rod bearing clearance with shells
 

Dave what you have discovered, is just what I gave for bearing clearance. To be safe, the clearance should always be .001 thousandths per inch of shaft , and plus .000-50 max. That is what to set at, and after 5, to 10 thousand miles, it will be more than that.

Now at those clearances, babbitt is way more forgiving then Aluminum inserts, as the shaft can push the babbitt back with out as much danger of smearing the bearings.

Now your AER rods set at .001-25, is not as good as the .001-75, and the reason is the insert is not forgiving, like babbit.

If you had the same bearing in a pressure system, that would be different, as the oil will be forced in the bearing to give it a chance from smearing.

The problem with bearing clearance, very few people take into consideration that a 1-1/2 inch Pin will almost enlarge to 1-1/2 thousandths larger from heat, and if your clearance is any smaller then .001-50, where is the room for oil going to be. That is why we have a break in period, as the clearances that are to close on any, or all bearings will have to struggle there way free with out smearing any one bearing.

Now the main shaft, at 1- 5/8's , should never be set at any thing under .001-60, and that will push babbitt back. We set mains at .002-00, .and that being babbitt. Inserts in a Model A have to have as much clearance, as babbitt, because they are not as forgiving.