Crank cutting into #3 main bearing

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david premo
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Re: Crank cutting into #3 main bearing

Post by david premo » Tue Nov 13, 2018 2:53 pm

The problem you are seeing is caused by keeping the clutch depressed for long periods of time while waiting for a light change or sitting in traffic with the clutch depressed. What happens is all the oil gets displaced from the thrust surface when the clutch is in use and cannot lubricate the bearing and begins to gall and overheat the surface resulting in its damage to the crank thrust surface and the bearing.

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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Tue Nov 13, 2018 3:06 pm

spyder wrote:
Tue Nov 13, 2018 2:10 pm
Could the transmission input shaft be pressing on the crank? 1600 spacer plate? Slave cylinder too tight?
Stock 2 liter spacer. The slave had about 1/4 free play at least... I could wiggle the arm. I think I sold or more likely gave away my old lightened steel flywheel so I don't have anything to compare it to. Just trying to imagine this... so, if the flywheel itself were thinner, could this move the trans input shaft further forward and then subsequently be putting more forward pressure on the crank/pilot bearing (maybe making contact against the forward travel of the hole) thus making the thrust wrong such that it never gets relief? I gather than when you engage the clutch the crank shifts forward some and will normally run against that side of the bearing by design, but that it is not designed to be in constant contact?

I'm just glad that Steve has a guy who knows what he is doing and that this can be fixed. Also glad I listened to myself.

Thanks for the thought above Dave, but I rarely do that. I know because I would always watch the lights with the thought that "I may need to pump the clutch a couple times to get her to slot into 1st, be ready" cause I hate being the guy who doesn't go when the light TURNS! That was part of the reason I was going to have the trans looked at in the first place.
Last edited by Gregs672000 on Tue Nov 13, 2018 3:13 pm, edited 1 time in total.
Greg Burrows
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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Tue Nov 13, 2018 3:10 pm

I think the difficulty getting it into 1st gear is related. I think there's gonna be a flywheel issue. SOMETHING is forcing the crank forward constantly. With the rpms dropping when I engage the clutch (as I recall anyway) that makes even more sense. But...?
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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Tue Nov 13, 2018 3:15 pm

Somebody wanna measure the thickness of a 2liter backing plate? Maybe the PO switched them years ago before I owned the car and I never knew. Not the original engine, trans unknown.
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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Tue Nov 13, 2018 3:25 pm

GeoffM wrote:
Tue Nov 13, 2018 12:25 pm
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
Very helpful article, thanks!
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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Tue Nov 13, 2018 3:30 pm

I guess the thinner flywheel idea does not make sense as I imagine this, but the thickness of the backing plate could as the distance that the trans input shaft travels is based on how much distance there is between the trans and the block, determined by the plate thickness, right?
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Re: Crank cutting into #3 main bearing

Post by GeoffM » Tue Nov 13, 2018 4:14 pm

Uh Oh, has Lorna hacked into Gregs account?? :) :) ...I kid.

Remember, the input shaft doesn't move (axially). The clutch disc is engaged on the splines of the input shaft so it can float when the pressureplate/flywheel sandwich it.
On that note, didn't I read in one of the posts here about someone installing two pilot bushings by accident? Just a thought about a potential cause of thrust caused by stack-up..... If , say, there was two pilot bushings and because of that, the shoulder of the input shaft pushed up against the sholder of the pilot bearing, causing it to thrust the crank forward when the bellhousing gets tightened. Not sure if this is possible, as I'm not sure how much relief (depth) the pilot bore has compared to the length of the pilot bushing.

Just a thought.
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Re: Crank cutting into #3 main bearing

Post by JT68 » Tue Nov 13, 2018 5:39 pm

I think if there were two pilots something would have failed a long time ago- not enough room for two.

Nope, moderate differences in flywheel thickness would have no consequence. (As long as proper clutch free play was obtained) If there were ZERO free play, or riding the clutch— those could kill it. More than likely though, it’s just that the thrust surface was not perfect and wore the bearing prematurely.

Not going into first was probably a result of this happening. As the thrust was failing, the crank was being shoved forward so full disc release may not have been occurring. 1/4” is a ton of free play, so good chance the disc wasn’t releasing fully. That would make going into first rough on the synchro for sure.
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Re: Crank cutting into #3 main bearing

Post by GeoffM » Tue Nov 13, 2018 8:28 pm

1/4" end play would take up almost all the clutch travel, correct?
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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Tue Nov 13, 2018 8:36 pm

JT68 wrote:
Tue Nov 13, 2018 5:39 pm
I think if there were two pilots something would have failed a long time ago- not enough room for two.

Nope, moderate differences in flywheel thickness would have no consequence. (As long as proper clutch free play was obtained) If there were ZERO free play, or riding the clutch— those could kill it. More than likely though, it’s just that the thrust surface was not perfect and wore the bearing prematurely.

Not going into first was probably a result of this happening. As the thrust was failing, the crank was being shoved forward so full disc release may not have been occurring. 1/4” is a ton of free play, so good chance the disc wasn’t releasing fully. That would make going into first rough on the synchro for sure.
Makes sense JT, and I think the article mentioned earlier confirms that the thrust surface must be perfect. Must not have been, or something got messed up last build. That would also explain why I was always trying to adjust the clutch better for 1st. I had the clutch set so the spring pulled the arm against the plunger, but could pull the arm forward about 1/4 inch free play. I had replaced the slave, master and flex line. It was fine in all other gears, but I always double clutch and rev-match so it may not have been as pronounced.

Taking the crank to Steve tomorrow and then off to the grinder for evaluation and repair.
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Re: Crank cutting into #3 main bearing

Post by redroadster » Wed Nov 14, 2018 8:49 am

I think the trust washers/shims are different thicknesses although they look the same , if crossed or swapped that would cause it,

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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Wed Nov 14, 2018 12:29 pm

redroadster wrote:
Wed Nov 14, 2018 8:49 am
I think the trust washers/shims are different thicknesses although they look the same , if crossed or swapped that would cause it,
One side is untouched by the crank (you can see the oiling groove etc). The other side is completely ground down. The bearings have locating slots that make them fool proof for installation, so no incorrect install.

Assuming my trans plate is stock thickness, I think JT is going to be right... the surface MUST be perfect or it will cut into the bearing and over time wipe out its ability to lubricate enough to protect itself. See the article above that was posted (thanks by the way!).
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Re: Crank cutting into #3 main bearing

Post by greydog » Wed Nov 14, 2018 12:40 pm

I also read in a separate article that too much end play does not retain the oil film. In fact, that article went on to say that performance builders sometimes set end play at half of slightly more than OEM but that cutting it further will cause lubrication problems.
From that I conclude that:
It's important to be "within a range"
It's better to be at the low end of the range than the high end.

Everything I've read about end play talks about the importance of the finish on the thrust surface of the crank. One even talks about why they machine polish rather than simply polish with emery on a lathe. They talk about why it's difficult for the crank grinder to grind that surface (has to be done with the side of the wheel which must be dressed accurately and often.
I'm about to take an H20 crank to my machine shop to be checked and polished, you can bet I'll be talking to him about this subject.
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Re: Crank cutting into #3 main bearing

Post by Gregs672000 » Wed Nov 14, 2018 12:59 pm

Agree 100% Dan. Steve trusts his guy, Tim, and said the same thing. Even how the swirl marks are left on the surface can make a big difference in how well it will oil. Once it is compromised, the protective film of oil gets scraped away like a squeegee and it just gets worse over time. Crank grinding is an art. Folks who can do that right are becoming harder to find. So glad I have my friend Steve.

I reviewed the engine's history yesterday and found that the crank has not been ground since I built the engine the first time 32 yrs ago. The whole assembly was balanced much later by Dave Bliss, but Steve said he does not grind cranks so it was not touched then. I know I checked bearing clearance etc and just replaced the old bearings with new because they were still in spec. I hit the ends of the crank and checked endplay as described in the manual, but I know I didn't think about anything else. I assembled the engine and did not know how critical this surface could be, so this is likely on me. Ignorance is not bliss. Always something to learn!
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Re: Crank cutting into #3 main bearing

Post by JT68 » Wed Nov 14, 2018 2:16 pm

Your grinder will probably need the new thrust bearing itself to size it. Some sets have oversize thrust surface-some do not. Be sure to give him the actual bearing you will run//
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