Crank polishing

[MAURICE BLENDHEIM]

Originally Posted by Marvin Robinson

Got a crank that needs polishing to get another .001-.0015 on the rod throws. Anyone have a method for doing this in house, or is a machine shop the ONLY way to get this done... trying to meet a deadline.

Marvin…Do it right, the crankshaft grinder will finish it for your desired clearance. No point meeting a deadline with questionable clearances. MB.

[Marvin Robinson]

Mike,

I'm looking for .003 - .0035 on the rods, and I'm at .0023. My simplest solution would be to have the machine shop take the throws to 2.4985. Right now they are 2.5000 - 2.4999. I don't want to go to .010 under, that would require getting all new bearings as well, and put me into a 3-4 week wait. (I also wonder if .010, or even .0015 would take out enough metal to warrant a rebalance).

[MAURICE BLENDHEIM]

Originally Posted by Marvin Robinson

Mike,

I'm looking for .003 - .0035 on the rods, and I'm at .0023. My simplest solution would be to have the machine shop take the throws to 2.4985. Right now they are 2.5000 - 2.4999. I don't want to go to .010 under, that would require getting all new bearings as well, and put me into a 3-4 week wait. (I also wonder if .010, or even .0015 would take out enough metal to warrant a rebalance).

No you will be fine. A good crank grinder should be able to have the crank back to you in a few hours. MB.

[kansas stocker]

If your crank grinder can take .0007-.0012 off each journal he's much better than mine and I doubt that. I'd run it like it is before I'd try to polish it off. Just another opinion.
Pete

[Alan Roehrich]

Most new racing connecting rods of good quality come in with a big end bore 0.0002" under the low limit as given in most machine shop manuals, for maximum bearing crush, which allows for the correct eccentricity with a racing bearing. Yes, you can get away with opening them up. But I prefer to avoid that. And I never use Scotchbrite on either side of a bearing, just a personal preference.

A good machinist can usually set up and take as little as 0.0005" off of the main journals. However, it will take a great machinist with a really good, tight grinder to take 0.0007" off of the rod journals. Asking a guy to do that, I'd think no less than $250 would be a fair price. Very few will even attempt it. There simply are not that many dedicated master machinists grinding crankshafts anymore, it does not pay what it is worth, given the low cost of the economy aftermarket crankshafts.

It will be next to impossible to polish a journal as much as you want and not have it out of round and tapered, possibly "hour glassed". No way I'd do that, either.

Are you measuring the bearings with the coating already applied? Because the coating is about 0.0002" per bearing half. If you are, that would put you at 0.0027" actual clearance, which, if everything is good, should be okay on a 2.5" journal. It might be on the tight side of ideal.

[Paul Precht]

I'm just wondering why so much clearance is needed, I run barely over .002" on BB Chryslers. My buddy was running 3.5-4 on his blown alcohol hemi drag boat with 70 weight oil, that I understood.

[Mike Taylor 3601]

I would run the .0023,like Alan said the coating takes away some clearance,your where I would set clearance on that size journal and would use around 20wt. oil.
Mike Taylor 3601
Paul,
.0035-.004is actually on the tight side for blown hemi w/70 wt. alot are set .005-.006 due to started cold and zinged wide open

[Marvin Robinson]

I was looking for .003 - 0035, seems to work pretty well on the BB Olds motors. The measurement was with the coating. I checked the bore on the rods, and they were slightly under spec (.0002), a very light hone put me right on the money. Thanx for the suggestions and advice, guys.

[Marvin Robinson]

Originally Posted by Alan Roehrich

Most new racing connecting rods of good quality come in with a big end bore 0.0002" under the low limit as given in most machine shop manuals, for maximum bearing crush, which allows for the correct eccentricity with a racing bearing. Yes, you can get away with opening them up. But I prefer to avoid that. And I never use Scotchbrite on either side of a bearing, just a personal preference.

A good machinist can usually set up and take as little as 0.0005" off of the main journals. However, it will take a great machinist with a really good, tight grinder to take 0.0007" off of the rod journals. Asking a guy to do that, I'd think no less than $250 would be a fair price. Very few will even attempt it. There simply are not that many dedicated master machinists grinding crankshafts anymore, it does not pay what it is worth, given the low cost of the economy aftermarket crankshafts.

It will be next to impossible to polish a journal as much as you want and not have it out of round and tapered, possibly "hour glassed". No way I'd do that, either.

Are you measuring the bearings with the coating already applied? Because the coating is about 0.0002" per bearing half. If you are, that would put you at 0.0027" actual clearance, which, if everything is good, should be okay on a 2.5" journal. It might be on the tight side of ideal.

Alan,

I know a machinist would have to be crazy precise to cut rod journals that close, the setup would take hours to get right... much easier to just cut to .010", but that would mean new bearings/coatings, too, and the Clevite 542HXN doesn't come in that size. Calico informed me that their coating is .0003", and I was measuring .0023 on the rods, which bothered me. A light hone on the rods and scrubbing the backside of the bearings got me in the ballpark. Thanx