Model A Ford Main Bearings

[Gerry Marsden]

I bought my 1929 Tudor some time back. The engine was rebuilt by a previous owner, but having taken out the engine to deal with a leaking gearbox, decided to strip the engine and check it out. All bottom end parts are fine, crank ground etc. Have had crank, rods, pistons & flywheel balanced. On reassembly, I find the thrust clearance on rear main is 0.008". It should be 0.003" max. Any thoughts/ideas or do I start again? (its Babbit metal not shells)

Gerry Marsden, Lancashire, England

 

[jpage]

Since the thrust "washers" are cast as one piece with the bearings and the clearance was obtained by filing or grinding the "washers", I'd say that you'd have to have them rebabbited and adjusted to the crank. Your description only states that the rear main is loose, so I assume that the other 2 are within specs.  I don't know if that .005 would make a great deal of difference but because the crank only had three bearings it did have a tendency to move end to end due to the stresses of a heavy flywheel pounding it and the rear main is larger to compensate for some of that stress.The center main also takes a lot of stress, especially at higher speeds due to some crankshaft whip. Of course,any damage may not be quickly evident but could present in the long run, especially if you plan to drive the car often. Unfortunately, I don't know of any other way to safely compensate for the difference without a new bearing. I had an "A' for 16 years and now I really miss it! Good luck! You might try the "Ford Barn" site or contact MAFCA or MARC Model A clubs as they have technical help on staff.

Edited March 14, 2018 by jpage (see edit history)

[Gerry Marsden]

 Thanks for your response. All 3 bearings are good diametrically. It is the end to end movement on the rear bearing that is worrying. I'll see if anyone in those clubs has experience of running an engine in this condition before going down the remetalling route.

[JFranklin]

Your thrust clearance is within tolerance. .003 is minimum. I don't have any books handy to give you a max but it will work ok much greater than that.

[Gerry Marsden]

Thanks for that. Model A Mechanics handbook by Les Andrews states 0.00.3" as the MAXIMUM clearance. Is his book incorrect?

Rgds

Gerry

 

[jpage]

The specs. that I find state crankshaft end play to be .002 -.004. so at .008 it's a little loose, at least on that bearing. You'll have to measure the end play with a dial indicator to find out if the end play is within specs. overall,  just measuring one cap will not give an accurate measurement. I would imagine that if the end play is withing specs. that the other 2 brgs. are good and you may not have any trouble in the short term but the rear main takes the brunt of the stress so If you plan on using the car frequently, I would have it repaired.Just my opinion.

[Gerry Marsden]

I'm coming to the view that whilst the engine is out and stripped down, I should repair this bearing. I checked the backlash by inserting feeler gauges between the crank web and the bearing white metal. It is also possible to feel the axial movement on the shaft, so I concur with your opinion.

"Do it once and do it right" always wins in the end. Thanks for your input.

[JFranklin]

When the thrust is new the clearance is given as .002-.004, that is on a new or newly rebuilt engine. .004 wear is normal. You will spend a bunch to get it back a few thousandths. Old engines sometimes have .020-.025.  Suppliers also sell a bronze thrust for repair or racing.  

[Gerry Marsden]

Thanks again for sparing the time researching this. Yesterday I spoke to a guy in southern England who has driven a Model A as daily transport since 1968 and 250,000 miles! He is of the opinion that many of these cars are driving around with more than recommended end thrust clearance and whilst not ideal, its a crude engine and quite forgiving. I'm minded to leave it; I will certainly not be replicating his mileage in the few years I have left!

Cheers and best wishes.

[Alfa]

The right decision!! I really don`t think 0.008 clearance is going to cause you any problems.

 

Adam..

[sligermachine]

As a machinist I would think .008 is nice and tight for a used . But some would replace a 80% good tire to . All to the beholder.--kyle 

[Gerry Marsden]

Thanks Guys, nice to have the benefit of your experience. Have begun reassembly, removed some thin shims, "blued" each journal and have a good touching area with a little bit of resistance on each journal. It should sweetly.