Datsun Roadster Forum-311s.org

HI gang, awhile ago I posted elsewhere that I was investigating a noise in the engine, and had found some copper colored metal shavings in the oil pan. Well, figured out where they are coming from: My crank is cutting into the rear of the outer shell of #3 (center) main bearing. The crank is ground down some, as is the bearing. I assume this is what is referred to as "thrust". I note that with the main and rod caps off, the crank moves forward and backwards a fair amount, and I don't really know what stops that movement, but clearly things are not aligned properly or something is missing... (experts invited to reply now!).

How hard is the pressure plate ,clutch pedal?
PPs harden with heat & cool and you put much more pressure on the crank
Rough edge on the crank too would do
Checked play when assembled ...now

Not an expert builder Greg.
It sounds like excess endplay. Endplay is usually set with shims at one ecd of the crank or the other. Thrust faces on the crank have to be polished like the crank mains.
Dan

The centre main usually has thrust flanges incorporated into it.
End float should have been measured when the engine was assembled.

Just as a thought. When you start the car do you push the clutch to the floor and hold it? If so you are putting maximum load on the thrust surface with no oil on it. Once the engine is running the bearing is being fed oil and should handle the thrust load much better. If someone has a better idea I would love to hear it as we had the same problem with the race motor.

I think if the car sits a long time this could be a factor - if driven frequently the oil film should be sufficient since the bearing isn't really dry- Lots of manual cars have a clutch/starter circuit interlock that requires you to push in the clutch to start.

I've only seen this once and it was on a car that ran a long time with no free play on the clutch - in that case the back side of the bearing is under constant load - unfortunately the crank was toast, so not good news - hate to say it but you need to drop the #3 cap and inspect since there is no separate thrust component. j

Greg,

The Nissan shop manual describes an often-overlooked procedure (step nr.5 on page 37) to center the U20 thrust bearing. Do you know if this was done when the bottom end was reassembled?

Anxious to hear some good news on this soon -- please keep us posted.

Jon

Ugh. The back side of the bearing is cut down on #3 and looks very different from the front side. The crank itself is under cut some where it rubbed against the bearing. I will see about posting some pics.
Yesterday I was talking to the place that rebuilt my trans a few years back, as I was inquiring about it being a little notchy going into 1st gear at a stop. After discussing, he was thinking this was more of a clutch/disengagement issue, and indicated that a bad pilot bearing could cause the whole thing to hang up. I have never changed out the pilot bearing, which he said was an often overlooked yet critical part. Crap. In the past I have also noted a change in rpm that was more pronounced when I would activate the clutch, and as I recall it dropped rpms when I put the clutch in, opposite of what I would expect. Several years ago when dynoing this engine, the operator noted it too but we thought nothing more of it.
I really hope this crank is not shot. I spent a lot of money years ago having it balanced by the best guy around (Dave Bliss). He's not around anymore and finding a good crank as well as someone who can balance everything is not easy.
Please keep the info coming. I will be talking to my friend Steve later today.

I did follow the procedure outlined in the manual of hitting both ends of the crank to center the thrust and measuring clearance. I think this is going to be due to a misaligned clutch and/or bad pilot bearing. I hope this $11.00 part has not cost me hundreds/thousands.

That sucks Greg. I really hope it was caught early enough not to be terminal!!

You successfully saved me from 1/2 hour work this morning though....went down the rabbit hole of crank thrust and found a pretty good post about the issue and thought I'd post it here to add to the knowledge base. It's based around SBC but cranks are cranks and thrust is thrust...

http://www.4secondsflat.com/Thrust_bear ... lures.html

Unlikely the the pilot bearing had anything to do with this since it doesn't create thrust one way or the other. All the pilot does is support and locate the outboard end of the trans input shaft-which is why it is just a simple small bushing. The role of the pilot is often misunderstood. The only time it functions is during the shift. That means 99% of the time the pilot bushing has no function at all. When the the clutch pedal is NOT depressed, the trans input shaft & clutch disc are locked to the flywheel and everything is spinning together. The pilot is doing nothing during normal operation.

The only way the pilot would create a problem is if it wasn't pressed in far enough or maybe if the wrong pilot was used (too long). In that case the input shaft could butt up against the pilot shoving the crank forward--probably not likely since there is a generous chamfer on the input shaft.

I can't see the pilot causing this- more likely the crank thrust surface had some imperfections that wore out the thrust surface gradually. Once the bearing gets to the copper it is pretty much all over. If you feel a groove in the crank thrust surface it will need to be turned or replaced (if the crank surface has damage and you just replace the bearing it will simply self destruct again). Sorry, most likely a major bummer.

I have good pics from my pad (3mb size, can't be changed) but cannot post them for some reason (as usual).
So the crank is supposed to run up against this bearing Surface? It is designed that way?
I may try pics from my phone...

It resists the thrust from the clutch spring resistance whenever you disengage the clutch.

Pics of crank, back and front of bearing, thickness difference.

Just talked to Steve... he's got a guy who can weld it up and turn it. We're wondering what caused it in the first place. Stock pressure plate, only change was the aluminum flywheel. More to come.

Could the transmission input shaft be pressing on the crank? 1600 spacer plate? Slave cylinder too tight?