68 mag case, crank binds in one direction, help?

[davidcz...]

When I turn the motor clockwise, it binds up with a decisive thump! It's not the rods getting hung up, checked. It doesn't seem to be the chains binding either. However when I turn the motor counter clockwise, this bind-up doesn't happen.

The guys at the shop say this has to go in to the machinist for a line bore. My question is: which way does the motor spin when running? Should it be able to spin in both directions? Should I stop the rebuild here and fix the binding?

Here is a photo, the green arrow indicates the direction that isn't binding-->

[K Sykes]

How are you making sure it's not the chains?. I don't think a lack of alignment would cause an abrupt stop. Also, is there anything hitting the engine stand?

[Steve@Rennsport]

These engines turn CW.

If you didn't send the case out to have the parting surfaced machined and the main bearing saddles bored back to standard, you're about to do this.

ALL mag cases distort once the through-bolt torque is relaxed, and they all need this operation in order to prevent the crank from binding in the bearings. In short, they are no longer in alignment.

[davidcz...]

I did have the case machined and line bored. Guess it needs this done again?

Thanks for the answers Steve. I am contacting my machinist

[BURN-BROS]

Your rods are not supported at the small end. The big end of the rods are at angles that make contact with the oil pump.

[Steve@Rennsport]

Originally Posted by davidcz...

I did have the case machined and line bored. Guess it needs this done again?

Thanks for the answers Steve. I am contacting my machinist

Nope,..if that was done correctly, you SHOULD be golden provided the case doesn't need to be disassembled.

I'd be looking at the intermediate shaft assembly as a potential source for your problem, assuming nothing is jammed in the oil pump.

Excellent suggestion from Aaron,.....I've assumed you were supporting the rods as you rotated the engine.

[Walt Fricke]

Yeah - you've got the engine on a stand. Rotate it so it is on one side - cylinder spigot bores up. Then get a bunch of rubber bands. Run them through the rod small end bushing, and hook them onto the cylinder studs. You can do this with two per rod, or maybe get away with just one.

These will keep the end more or less in the center of the spigot as you rotate the motor. Most likely problem is that the big end of one rod is hitting the case because it is cocked over. You don't have hands enough to keep them where they need to be (as if they were attached to a piston and thus in the center all the time), especially with the engine horizontal, where all six want to droop down.

I assume you weren't supporting all of the rods. But the fact that you didn't know which way the crank turns suggests you may not have realized that this happens with the motor partly disassembled like this.

I have had this sort of bump/bind happen often, and only fairly recently saw a picture here showing the rubber band trick. A Eureka moment, even if someone else found it, although I always realized what it was after the first time it happened, and just juggled rods around until I could move more. I guess I didn't realize the oil pump was the main culprit, but the issue is the same. Not much room to spare in that case. And don't lose sleep over why it happened only when you turned the motor backward - unless this really is a problem with the chain. But if the rubber band part doesn't work, and you get binding with rod ends right in the cylinder axis, that would be next.

If you forget clockwise, think of which way tightens the bolt which holds the flywheel pulley on. That's the direction.

[davidcz...]

I wasn't supporting all of the rods. I'll go to the garage tomorrow and try the rubber band trick. Hoping this works, thanks guys.

Is Walt correct in saying that the motor spins the same direction as tightening the bolt that holds the crank pulley on? ( I am a little confused because he said flywheel pulley, pretty sure he means crank pulley ).

[BURN-BROS]

Yes, motor turns clockwise in relative to the crank pulley.

[Walt Fricke]

Oooops - crank pulley.

Luckily, some mistakes are so obvious they don't lead to real confusion.

[Gene Smith]

When you look at the cooling fan remember it blows into the engine. That indicates the direction of rotation. It is never a god idea to turn an assembled engine backwards. It is tough on the valve train. Simply look at the fan when turning the motor and you'll always be spinning correctly.