Connecting rod end play

[Ken MA]

Trying to track down a thumping noise on this motor accelerating under load.
Motor was rebuilt 2 years ago and driven 2000 miles.Pulled the pan and made some measurements


Reground Crank .020
Rebored with new pistons .060
Re-babbited mains .0015-.002
new babbit rods .002
Crank end play .009


There is a lot of side to side wear on the rod bearings .010-.024 between the thrust end of the rod bearing and the crank . Thrust surface on rear facing #2 bearing is worn thin. What is an acceptable end play clearance on rods? Is #2 pounding against the crankshaft causing the noise, or is the .009 crank end play causing the problem? Before this block was rebuilt most of the babbit on the original rods was worn away . Any ideas on the cause or fixes is appreciated!

 

[Chuck Sea/Tac]

Are the rods bent? If they’re still attached when you crank the engine they shouldn’t be moving back-and-forth on the journal. They should be centered. 009” crank thrust is Pretty much the outer limit. There is a Brass aftermarket thrust retainer that you can get from the vendors.

 

[Kohnke Rebabbitting]

Trying to track down a thumping noise on this motor accelerating under load.
Motor was rebuilt 2 years ago and driven 2000 miles.Pulled the pan and made some measurements


Reground Crank .020
Rebored with new pistons .060
Re-babbited mains .0015-.002
new babbit rods .002
Crank end play .009


There is a lot of side to side wear on the rod bearings .010-.024 between the thrust end of the rod bearing and the crank . Thrust surface on rear facing #2 bearing is worn thin. What is an acceptable end play clearance on rods? Is #2 pounding against the crankshaft causing the noise, or is the .009 crank end play causing the problem? Before this block was rebuilt most of the babbit on the original rods was worn away . Any ideas on the cause or fixes is appreciated!

.009 end play is wore out. We set from .003 to .003-50, on the rear main.

The center main should have About the same amount of clearance, on either side of the crank, when the crank is set.

Pictures would help!

Rods have to be checked for Twist, Bend, and Offset, and most builders don't, thinking their rod machine, or mill will bore them straight, or they are using wore out machinery. The same goes for the Rod alignment machines, either wore, or the cast has moved, as in Wilson Rod Aligners. The babbitt should close to equal, on both sides. But still the most important thing, is Alignment.

The side play in a Model A Rod should not be over .012 thousandths, Never under .003. We set the clearance at .005 or .006. Some cranks when reground, the standard has been opened up, on the width.

Herm.

 

[Ken MA]

I cant 'be sure the rods are not bent, the engine is in the car. The babitted rods were bought from a well known machine shop in Ohio and installed by a local rebuilder. Major reason for this rebuild was that the original rods had no babbit left on the thrust surfaces 018-.035clearance. Now #2 has .024 clearance and sits on the rear facing thrust surface when the crank is turned. Factory spec for crank endplay was .007 do you think .002 would contribute to the connecting rod wear? #1&3 .018 , #4 .012, #2 .024 Who sells the brass thrust retainer?
Thanks Ken

 

[ryanheacox]

Well, I know on my old engine the thrust surfaces on the rods were mostly gone and the rear thrust play was around .080.... it measured over 1/16" using a tape measure. Did the rear thrust go and wipe out the rods? Maybe, makes sense to me. I don't think .009" would do that though.

 

[George Miller]

The noise sounds like a center main knock, not a rod.

 

[Ken MA]

To Ryans comment, was the engine thumping when accelerating or climbing a hill? Common wisdom suggests mains but would crank or rod end play sound the same?

 

[Jim Brierley]

Herm and George have it right. I would suspect the noise is from a main bearing too. Rod thrust doesn't usually cause noise. On my race engine, which is full pressure oiling, I have .015" rod thrust to allow oil to flush thru the bearing for cooling. Is it noisy, don't think so but hard to tell with an open exhaust. -

 

[ryanheacox]

To Ryans comment, was the engine thumping when accelerating or climbing a hill? Common wisdom suggests mains but would crank or rod end play sound the same?

Mine got quiet under load. My center main was pretty well shot as were the rod bearings. Mine only really made noise at idle and on deceleration.

 

[denniskliesen]

What is the balance of your crank, rods, pistons, flywheel and clutch? Believe it or not it doesn't hurt to balance the pulley up front. You could also have a bent crank. I had an engine with a thumping sound under acceleration and pulling steep grades. It had a bent crank .006" and was out of balance. Check your flywheel is perpendicular to the crankshaft also. Get it as close to no runout as possible and check the runout of your crankshaft flange. Check your journals for out of round.