Main Bearing Clearances - Why Not Check?
 

I am in the process of rebuilding my 3.0 Ltr motor for my RSR clone and am following Wayne's book. I just finished the assembly of the crankshaft and used the plastic gage to measure the rod journal bearing clearances. Everything worked out like expected.

I am now preparing the assembly of the crankshaft into the case and the mating of the case. I am wondering why Wayne does not include a plastic gage check of the main bearing clearances in his assembly process. I have heard that if the crank journals measure to the correct standard size and if the standard size bearings are used, the main journal bearing clearance check is not needed. Why would one measure the rod journal clearances but not the main journal clearances?

The reason you don't check is because of the following reasons.

You already know that the bearing journals of the crankshaft are within specification, and are free of defects. Micropolishing and magnafluxing the crank is highly recommended

You already know that the bearing saddles in the case are within specification, and did not require machining.

Therefore, with standard bearings, everything should fit perfectly, because the two areas where you could introduce errors have already been eliminated.

By all means, if will put your mind at ease, go ahead and check them. Unfortunately, it is a rather time consiming process.

AFJuvat

I am planning to skip the main bearing clearance check for all the reasons you and others have mentioned.

Tom

1/ Your chances of doing an accurate plastigauge check on the mains is slim to none (in fact a lot of engine builders don't use PG because its fairly unreliable) With modern measuring tools you can get just as accurate measurements

2/ If you want to check your mains put the bearing shells in the case and torque up the thru bolts then measure the shell bore with a bore gauge

HTH

Neven

If the bearings are standard and the crank is standard, there shouldn't be any problem (assuming case didn't need boring).

The reason you do it with the rods is because, theoretically, a machinist could resize your rods incorrectly for your bearings/crank and that would not be fun to find out as you rev it past 7k.

Usually case work is not needed on the aluminum cases. But rod work is always highly recommended (or usually mandatory).

Goodluk
Scott

If you have a bore gauge, checking will take less than an hour. Which is nothing compared to the total time it takes to rebuild the engine. It all comes down to wondering if everything is ok vs. knowing everything is ok. Just because something 'should' do something doesn't mean it will. Just an opinion from a non-gambling man.

MHO.
Resizing a connecting rod (big end) is the same as resizing the crankcase saddles (line boring). Both restore the opening to a standard dimension and concentricity. The final dimensions depend on the accuracy of the operator and machine.

That said, one should measure both rod and main bearing oil clearances just to make sure. Whether this is done with Plastigage or with inside/outside mics/bore gauges is up to you, but it should be done so there is no question.

If the incorrect bearing is packaged, will you assume it's correct? One could in most cases, but the drawback can be costly. If plucking bearings out of a crankcase were as easy as changing spark plugs and if there was no chance of engine damage, you'd see more people attempting 911 engine rebuilds. If "good enough" is a skill in rebuilding, everyone would be doing it.

Sherwood

Just to play it safe I assembled my case and measured the main bores to make sure they hadn't been beat out of shape or made too small by lapping my case halfs.

I was happy to discover they were dead on the minimum of spec.

Since my crank is right in the middle of spec as long as the crank spins freely when the case is assembled I will know all is well.