Adjusting Main Bearings

Unknown

I have a 1940 Hudson 6 that has a dry knock. I have tighted up the center main by removing shims, which helped, but I want to try to eliminate it. Someone said that it could be the front and/or rear main that is loose, although the center is the usual culprit.

Does anyone have direct experience with the end mains needing tightening? Or is it almost always the center?

Also, the service manual describes a puller to get the end caps off. Other than a factory puller, what can be used there? And finally, where does one obtain new packing material, and how does it go in, again without access to the factory tool? Or should I just forget about it? Thanks again for any help.

Geoff

You can ascertain if it is the front or rear bearings by shorting out 1 and 6 spark plugs in turn, which will lessen the knock if the bearings are loose. However, you do need to also ascertain if it is actually a main bearing or connecting rod, or a piston knock. Generally, a main bearing will knock under load, with a heavy pounding type knock when pulling hard. A con rod will knock worse when cruising, with foot lightly on the accelerator. A piston will emit a hollow knock when sharply depressing accelerator. You can drop the front and rear mains by inserting a sump bolt in the cap and using a claw hammer or pinch bar as a lever against the front stud, or with a wedge under the crankcase flange for the rear. For re-packing, make sure the holes are dry, and just insert a silicone gun into the hole and squeeze hard. Use high-temp silicone. Wipe any extra silicone away from the sides afterwards. Wait for this to set before you start the motor. Good luck,

Geoff.

Unknown

Thanks Geoff, I will try shorting out #1 and #6 when starting. Also, when the engine is fully warmed up, and driving along cruising, there is a noise like a tappet ticking, perhaps a bit stronger. This is with the accelerator at a neutral position, at about 35mph. (Car has 3:55 gears, non-stock rearend). But that noise is separate from a sharp rapping when starting.

Park_W

That noise at cruise sounds a lot like a loose rod bearing.

Geoff

I agree with Park. I would check the rods. Unfortunately your model does not have adjustabel bearings, so if there is a loose rod you will have to replace it. The easiest way to check rod clearance if you don't have plastigage, is to fold cooking foil four times over, cut just a 1/4" wide section the length of the bearing. Drop the cap and slide the piece of foil into the bearing and tighten, then check how tight it is to turn the engine over. Foil is .0005" thick, so a four-fold of this is .002, which should make it real tight. If you can still turn the engine easy, then you have a problem. However, it is perhaps wisest to drop each cap, and lift the rod and inspect the actual bearing material in the rod - usually if the bearing is failing it is probably starting to crack or part away from the rod. good luck,

Geoff.

Unknown

Well, thanks to Geoff's advice, (I never would have tried this without it), I pulled down the front and rear mains, and took .004 shims out of each. With a .002 shim stock 1/2" wide at each journal, the engine just started to turn over tight. No wonder it was knocking. The bearings looked decent, not all pounded out. Next I will take each rod down, and check them. If a rod needs replacing, are they available in Standard size, or how does that work? Thanks again guys.

Unknown

Put the engine back together, and it runs quietly, no knocks or anything. That other noise that perhaps was a rod is gone also, who knows? Thanks again for all your help.

Geoff

Another satisfied customer! Glad it all worked out. Sometimes it is hard to diagnose a knock, but if all three mains were loose this can set up all sorts of strange noises. Best wishes,

Geoff.