Thread: Insert bearings
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Old 02-01-2017, 11:53 AM   #18
Dave in MN
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Join Date: May 2010
Location: Jordan, MN
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Default Re: Insert bearings

Originally Posted by CT Jack View Post
I have to ask Dave from MN a question. Is it possible the machinist preparing the block for the inserts didn't install the proper dimensions for a good fit for the OD of the inserts?Taking this possibility into consideration along with a tight bearing fit initially and also maybe the engine wasn't dynamically balanced properly could have all contributed to the failure. If this did happen it had nothing to do with the inserts.
All the factors you mention could contribute to a failure.
The main point I was trying to make is that the clearances for insert bearings are a narrow range and very much less forgiving than Babbitt bearings.

When line boring the Model A block for inserts, we find that the impurities in the cast iron are very abrasive to the tooling. Another complication to the process is that the front, center and some of the rear caps are steel. This requires tooling that will work well with both materials. We have found that if you are not using a tool that can stand up to the rigors of these materials, the bit will wear and the resulting bore is significantly tapered. Always tight near the end of the cuts. We also note spots in the cast iron and caps that are hard and deflect the boring bar slightly. These small areas create hot spots in the bearing fit due to reduced clearances as the insert is distorted over these "bumps". We have found the most durable tooling is manufactured by: Micro 100 Tool Corporation The tooling we use: M100 series AL-6 Bit. (5/16") C5 Grade. The tooling is available in sizing to fit most boring bars.

When starting to assemble an engine, the first thing I do is carefully measure the line boring of the block. I have cylindrical ring gauges that are equal to the maximum block housing bore suggested by the bearing manufacturer. This gauge allows me to quickly calibrate my bore gauge and accurately measure the housing bores. I often find that a bore may be slightly cone shaped due to bit/tooling wear on the last pass of the line boring machine. I also detect the occasional small spot that deflected the boring bar. I have Sunnen portable hones set up for "A" and "B" line bores and I adjust/straighten the bores to the upper limit with very coarse stones. This honing step along with specifying a mid-range journal sizing on the crankshaft results in good bearing fit with little final honing of the insert shells necessary. Even with this extra work and tight grinding spec, about 50% of the assemblies require a light honing of the rear main inserts. I usually open them up about .0003" and they are perfect.

When assembling, if one is tight, I could ask the crank grinder to take a few tenths off the journal diameters but that takes time and running. I purchased a Sunnen precision honing machine and built a fixture that holds the inserts, with proper crush, to be able to adjust the final clearances. With the honing machine in my shop, the process takes just under 10 minutes per journal to complete the fitting. It is usually only the rear main that needs this adjustment.

I have not had a bearing failure since I increased the clearance to .002" from .00175" on the rear main. As I stated earlier, I set the clearance between .002" and .0022" for the rear main. If the front and center Plastigage out to .00175", I do not adjust them.

Side note: Inserts, when installed correctly, offer excellent service. I have over 90,000 miles on my inserted engine in my Phaeton. I have never had the pan off the engine since assembling it! I do not "baby" this engine and it has always made it home.

JMO: If you are not able to complete this final adjustment to the clearances, you are probably better off using Babbitt instead of inserts.
Good Day Guys!

Last edited by Dave in MN; 02-03-2017 at 09:43 AM.
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