Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 12:42 pm

I'm totally rebuilding my 1972 Cub and just put a rebuild kit into the engine, new pistons, rings, valves, bearings, etc.

I installed the crankshaft with new bearings and it all came together well, the crankshaft was turning freely when bolted to the bearing races, but when I connected the pistons to the Crank with the new bearings on the connecting rods, then torqued to the required 16 ft / lbs it locked up the crankshaft, it won't turn. If I losen the connecting rod end cap bolts then I can turn the crank again, so something is happening when I torque the connecting rods, with the new connecting rod bearings .
Any ideas on what I be doing wrong?

Any help is very much appreciated.

:help:

Neil

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 1:31 pm

This may get involved:
1. Is the engine truly locked up? A new engine can be really difficult to turn over by hand.
2. Have you tried torquing 1 rod at a time to determine if it's one or all rod bearings.
3. You should get some plastigauge and check clearances on each rod--directions for use can be found in the service manual.
4. Do the bearings match the journal? IE 0.010, 0.020 etc
5. Do the numbers on the rod caps match the numbers on the rods? Also, are the caps installed correctly
(numbered side of cap to numbered side of rod.

Once the above items are determined/completed, the problem should be solved. I am sure there will be others chime in with additional information.

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 2:48 pm

I agree with Bill E Bob regarding the plasiti gage. I would also pull the main bearing caps off and measure the clearance with the plasiti gage too. Never assume that the clearance is correct, always verify that the clearances are to specification. If the clearances are to tight the oil will not be able to do its job and you'll be doing the bearing and crankshaft work all over again.

Again like Bill E Bob said, make sure the caps match the rods and the bearings are the correct size for the crankshaft. It sounds like your rod bearings are to tight. Another thing to check is the radius on the ends of the crank journals. Look for any witness marks on the bearings edges that might indicate the bearing is being pinched by the journal radii.

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 3:26 pm

One more thought. I'm assuming you put the new pistons in fresh bores? If not, other possible causes could be a ridge at the top of the bores, rings not properly gapped for the minimum bore diameter.

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 3:41 pm

Thank you, Ricky.
Great suggestions. I will definately get some plasti gage and check all of the tolerances.

Any suggestions on the best source for plasti gage, is it available at auto parts stores, or do I need to find it at a tractor shop?

regards,

Neil "The Riot"

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 3:53 pm

There were two sizes of rod bearings. You need the later ones. Connecting rod changed at ENGINE number 261718. Bearings and rods must match.

anywhere for plastigauge

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 4:12 pm

It's available at any auto parts store. It comes in different colors for different clearance ranges. Check your manual for the recommended bearing clearances and tell the parts man what range you need. If I recall the clearance on the main / rod bearings is .001" - .002 or.003" and that plasiti gage is green. Double check me, that's just going from memory. It's cheap and easy to use. Just tear off a piece about the width of the bearing, put the bearing cap back on and torque to spec.. Remove the bearing and compare the width of the smashed plasti gage with the markings on the plasti gage package to determine the bearing clearance. If it smashes more at the far edges of the bearing, it might be the radius on the crank is not to specification.
If the radius is out of spec. take it back to the machine shop that ground the crank ( assuming that was done) and have them repair the journal radii. The crank doesn't always need to be ground so if the crank was in spec. and not ground, the kind of cancels out the journal radius. If it was good before, it should be good now.

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 9:15 pm

Well I ran down some plasti gage and picked up a digital micrometer. I was able to check the # 1 journal to connecting rod bearing and it measured at about .002 right in tolerance. I didn't get a chance to check the journal but will tomorrow. Since it was checked by my machine I didn't spend a lot of time on it, but will check it now.

So now that # 1 is checking out within tolerance I'm kinda baffled, but won't throw in the towel until I check all of the bearing and connecting rod journals.

On the question regarding the piston bores, yes it is new piston in a fresh .020 over bore on the cylinders. The pistons and the cylinders seem to be a really good fit and they travel well with what I would call normal ring drag.

In checking the bearings I appear to have the correct ones for my engine, but will double check with the supplier to make sure they are correct for my Serial Number.

Gents, thank you again for your very kind consideration and input on my little situation.

Regards,
Neil "The Riot"

Re: Cub Engine Rebuild - Stuck crankshaft

Mon Jun 10, 2013 9:22 pm

Can you move the individual pistons in the bores, did you check the ring gap? did the machine shop turn the crank and sell you the appropriate bearings?

Re: Cub Engine Rebuild - Stuck crankshaft

Tue Jun 11, 2013 6:48 am

The rod bearing numbers must match the crank and the rod, not the engine. The later serial numbers had a wider rod bearing and would rub on the sides of the early s/n crank. The rubbing gets tight when you torque it.

Re: Cub Engine Rebuild - Stuck crankshaft

Tue Jun 11, 2013 7:34 am

Thanks, outdors4evr.
That appears to be what is happening. I am getting some witness marks on the I.D. of the new bearing right on the edge about 3/32 wide on both edges.

I ordered a complete rebuilt kit for my engine and these are the bearings I got. So how do I match the bearings with the Crankshaft? Is there a particular parts supplier who would know how to match these up properly?

Re: Cub Engine Rebuild - Stuck crankshaft

Tue Jun 11, 2013 7:37 am

outdoors4evr wrote:The rod bearing numbers must match the crank and the rod, not the engine. The later serial numbers had a wider rod bearing and would rub on the sides of the early s/n crank. The rubbing gets tight when you torque it.

It is the other way around. You can use new, narrower rods on an old crank but you cannot use old rods on a newer crank. If his 1972 engine is intact he has the newer, narrower rods and the new crank revision. The connecting rods were changed in 1968 at the engine number I cited. They were backward compatible as a service item as long as the new bearing was used.

Re: Cub Engine Rebuild - Stuck crankshaft

Tue Jun 11, 2013 7:51 am

ndtheriot3030 wrote:Thanks, outdors4evr.
That appears to be what is happening. I am getting some witness marks on the I.D. of the new bearing right on the edge about 3/32 wide on both edges.

I ordered a complete rebuilt kit for my engine and these are the bearings I got. So how do I match the bearings with the Crankshaft? Is there a particular parts supplier who would know how to match these up properly?

Order bearings from TM tractor (Sponsor, link below) or give your parts supplier the engine number and they can cross the part number. Any auto parts store should be able get these items out of their warehouse. What bearings did you GET? Look up the manufacturer's part number and see what the catalog application range is.

Re: Cub Engine Rebuild - Stuck crankshaft

Tue Jun 11, 2013 8:24 am

Since this is a repair request, I moved this thread from the How To section to the Cub forum for more exposure. It looks like Bob and the others have you going in the right direction.

Re: Cub Engine Rebuild - Stuck crankshaft

Tue Jun 11, 2013 9:17 am

Thank you, Sir. I'm still a bit new to the Farmallcub site and appreciate the help with the nav & protocol.