Banger rod knock

[Special Coupe Frank]

Hope I don't say something stupid here...

When you say # 4 Rod has "approx 2 mm play", do you mean from side-to-side on the crank journal, or "up-and-down", in the direction of rod travel ( perpendicular to the crankshaft) ?

Suggest you remove the rod-caps, one at a time (keep track of how many shims are on each bolt, and which side), and inspect the babbit: there should be an " X -shaped " groove in the bottom of each cap, and the babbit surface should be smooth and shiny. A bearing that is "wiped-out" will often have no groove left, or the babbit may look like it has "smeared-into" the groove: this is from the bearing getting warm enough to soften the babbit, which then "runs" or get mashed into the groove.

If the babbit looks decent, the next step would be to replace the cap and see if you can adjust the bearing clearance, using either Plastigage or the foil-square method. If you wind-up with all the shims removed and STILL have too loose a bearing, then you would have to change the rod. I may be wrong, but I do not think it is possible to remove / change a rod without removing the piston and rod from the block, which requires removing the cylinder head.

After checking all the rod bearings and journals, do the same check on the main-bearings.

If a main-bearing is wiped-out, there's little you can do to fix it without pulling the engine, and sending the block out for new babbit or conversion to insert bearings; at this point, you might as well do a full rebuild.

I would NOT seriously consider mixing an inserted rod with babbit rods - it will probably not balance with the other rods/pistons, and would cause destructive vibrations.

If you were "feeling" the knock through the pedals, that sounds to me like a loose main bearing or a loose flywheel. Unless the engine is knocking so bad / loudly that it literally sounds like a jack-hammer, rod-bearing knocks are not usually "felt".

Another very common source of "knocking" in the Model A engine is the camshaft: frequently, the thrust-spring and plunger in the timing cover fails, allowing the cam to "walk" fore and aft in the block; each time it hits against something (block, or timing cover) it will make a heavy knock. An easy check to to remove the side cover from the cam gear, and use a screw-driver to try to move the cam gear back and forth: if it moves more than 1 or 2 mm, the thrust plunger is either stuck in its bore,or the spring behind it is broken. Usually this knock is most pronounced at idle speeds / low RPMS.

Hope this helps some... if the problem turns-out to be a failed rod-bearing, you might get by with swapping-in a decent rod, as a short-term repair. More importantly, try to determine WHY the bearing failed: mos common cause is lack of lubrication. Make sure all the dipper-wells in the dipper/baffle tray hold oil and that there are not any holes rusted in the wells. If there is a hole in the well, oil will run-out, and there is nothing for the dipper to catch, and the rod bearing will starve for oil.

It would probably be a good idea to remove the valve cove, clean out any sludge, and make sure all the drains are clear. The oil circulates by being pumped up to the valve chamber, and from there it flows by gravity to all the engine bearings (cam, mains, rods).

Good luck !