Crank pilot concentricity

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gzig5

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Is there a tolerance for concentricity of the pilot at the rear of the crank? I set the forged crank from my 340 up in the lathe to check it out and make it a little easier to polish the journals. Before doing that, I turned an T-shaped insert with a center pilot to go in the bronze bushing on the back end of the crank so the center in the tail stock could support that end. The insert was a snug slip fit in the bushing which seemed to be like new inside. Dialed in the snout end to two tenths run-out on the #1 journal. I am getting .006" run-out on the #5 bearing journal, which seems to be a touch high. This is saying that the trans pilot is not in the center line of the crank. There is maybe 1-1.5 thou max from the setup. I don't have a steady rest to support the tail end by the bearing journal so I'm working with what I have. Crank has been ground 10-10 and I wonder if that maybe contributing, if it wasn't centered when ground?
Is this enough run-out to worry about, if it is truly there? I'm going to be putting an A833 behind this motor when it goes back together.

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Could be as simple as the bushing itself. First thing I'd do is indicate off the flywheel register and see what you have.
 
Yeah, that's next. Need to map out all the surfaces. Besides dialing in the front end, I haven't put any time into it. The insert I made in one setup and had no more than a half thou clearance in the pilot bushing. Had to go in slowly to allow the air to escape, so I don't think the hole in the bushing is too out of round, but it may not be centered to its exterior.
 
I tried to post more earlier, the site was acting up. Can you temp. chuck it up at the flange end, use the center at the snout end and double check it that way? I'm thinking, chuck into the converter bore, or outside on the flywheel register? Then check main bearing runout.
 
cudamark said:
How about pulling the bushing and just use the pilot hole?
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That's a thought as well. might have to try the stuff it with white bread trick I saw on TV to remove the bushing.

67Dart273 said:
I tried to post more earlier, the site was acting up. Can you temp. chuck it up at the flange end, use the center at the snout end and double check it that way? I'm thinking, chuck into the converter bore, or outside on the flywheel register? Then check main bearing runout.
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Problem I would have is properly supporting the snout end if I'm not on a bearing surface or balancer mount. I don't have much hope that the bolt hole on the front is very concentric and it isn't a good surface for a center point. I should be able to check the interior and exterior of the flywheel flange/converter bore at the same time as the bearing surface to see if they are in sync in the setup I have. More I think about it, .005 runout on the end of the trans input shaft is about .001 or less on the gear on the end of the input shaft considering the fulcrum point of the bearing. I'd rather have 0.000 but I got to think they've been run successfully with more from the factory.

I've got some more measuring to do and then I'll decide which way to go. I was hoping to just re-ring and put in new bearings with the original pistons and lightly worn stock bores that I had honed. I'll stick with that plan if the journals polish up and the throws measure out correctly cylinder to cylinder. The piston heights above deck before tear down have me concerned that the crank may be a bit wonky.
 
I would use a steady rest on the number 4 main bearing and indicate it in on the 5 main and then check the converter pilot bore.

Of course, if you don’t have a steady rest that would make it more difficult.


EDIT: you can also put the steady rest on the number 5 main bearing and indicate it in on the flywheel register right there on the back of the crank and then just drop the indicator down a bit and take your measurement.

Or you can put it back in the block as mentioned below.
 
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Set it back in an engine block with #1,3 and 5 main bearing and try and dial indicate it that way.
 
Max1196 said:
Set it back in an engine block with #1,3 and 5 main bearing and try and dial indicate it that way.
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There is a guy using his noggin. Good idea. I need to find where I put the old bearings. They are mixed up but that won't affect the concentricity of the crank.

Steady rests for these Austrian EMCO lathes are rare and expensive in the US. I'm in process of adapting a South Bend Heavy 10 steady but that's one of many irons in the fire.
 
I always put the flywheel end in the chuck, then I use a centerdrilled crank bolt and my live center on the other end.
 
You really are registering in the wrong place....if for no reason than the pilot hole is a small diameter. When you hold any part, you want to grab and register off the biggest possible diameter.

The male register that sticks out of the rear crank flange is 'the' place that is specifically machined concentric with the main journals. You should use that point.

As noted, the actual main journal would even be better but IMO it's too risky to run on it with a steady rest. It's 'probably' OK but knowing my luck....

I'd make a very carefully fitted adapter flange that bolts to the crank flange and takes its center from the male register.

Last...if all you are doing is a foo-foo polish job with some emery paper, .006" runout won't make any difference.
 
gregcon said:
You really are registering in the wrong place....

Last...if all you are doing is a foo-foo polish job with some emery paper, .006" runout won't make any difference.
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Agreed. The pilot bushing just supports the input shaft snout. The input bearing retainer bore on the bell aligns the crank and transmission centerlines.
Polish the crank, then dial indicate the bell and correct that (almost guarantee that’s off) and send it.
 
If u actually believe the mass produced machining was spot on back in the earlier days ur living in a dream world. Back then maybe 1 or 2 out of a thousand may have been checked. And if they were checked do u think they were corrections made to the tooling? Now a days most anything can be checked at home, but how many actually do that? Not many unless there is a problem. U less ur gonna measure every last thing and correct it, don’t worry be happy. My nephew always tells me: don’t check it cause u don’t really want to know. This is on used stuff. New stuff I always check. Personally I have never just thrown stuff together and hoped for the best. Kim.
 
Don't pick on old school mass machining. Most modern mass production machining is also not that accurate, either.

Time hasn't changed much in the past 50 years. If you're drilling 10 holes in 2 seconds, they're not gonna be spot on no matter when you do it.
 
gregcon said:
Don't pick on old school mass machining. Most modern mass production machining is also not that accurate, either.

Time hasn't changed much in the past 50 years. If you're drilling 10 holes in 2 seconds, they're not gonna be spot on no matter when you do it.
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BOOM!!!!! /thread.
 
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