Crankshaft Wear Limits vs Usability
 

I'd like to solicit some opinions regarding the best course of action to take with my rebuild. The motor I'm in the process of rebuilding is a '79SC 3.0 with 181k miles on it (I can't verify this as I don't know the car's or the PO's history, but all indications are that this is the original motor and the odo is correct).

I've completed the teardown and taken the case, crank, rods, etc to the machine shop. Visually, the internals look great....no unusual signs of wear or damage on any of the journals or bearings. I did a quick check with my vernier caliper before taking it to the machine shop and the journals looked to be at the max wear limits for both rods and mains. The machine shop has now confirmed this as just barely beyond the wear limits, and also confirmed that none are out of round or tapered. Overall, the crank appears to be in great shape and the wear seems to be consistent with the amount of miles on the motor.

Because the unit is in otherwise excellent shape, I really hate the thought of locating an unmolested std/std replacement, or cutting this crank to the first undersize (.25mm) as are the typical options. It would be a no-brainer if the journals were heavily worn or damaged in some way, but that's not the case here.

What are your thoughts on having the shop do a light polish on the journals (so as not to remove any more material) and reinstalling it with fresh std size bearings?

Btw, plans for the rest of the motor call for new P&Cs, S cams, Webers, SSIs, etc...usage to be mostly street with occasional exhuberant track time....;)

Please let me know if you think I'm expecting too much out of this crank. Thx

Mike :cool: http://forums.pelicanparts.com/uploa...1178759511.jpg

Many people with race motors run with a little extra bearing clearance to reduce friction and get more power. I'd be comfortable with your idea. Make sure you get pistons that are compatable with the cam selection you have made and check all the clearances when you assemble it.

-Andy

Thx Andy, glad you agree.

What pistons would you recommend? I haven't made any final decisions as to the P&Cs, cams, etc....always interested to hear what others think is a good combination and why.

Well you're limited if you plan on keeping the CIS induction. However if you're replacing P's and C's there's not reason you shouldn't at least do the 98mms for a displacement upgrade! A slight improved cam (20/21 or 964) will help you engine as well...

With Webers as you mention, well the sky's the limit. A good dual-use setup is Higher compression 98mm Mahle's or J&E's (9.5:1) with a DC40 or similar cam. Then you're talking in the 250HP range I would bet.

A vernier caliper, accurate as it may be, is not accurate enough for crank journal measurement. The best Mitutoyo has a resolution of 0.05mm, whereas the factory crank measurements are in 0.001mm.

There is a difference between the high and low limits for a new crank and the wear limit. If your crank is truly below the wear limit, that means it's time to grind it. I would find out from the machine shop exactly what the measurements were, and compare that to the factory spec for an SC crank. On the second page of my (in)famous measurement thread, I illustrated this graphically. Thanks to Tom Butler for the insipiration for the presentation of the graph.

http://forums.pelicanparts.com/showthread.php?s=&threadid=328636&perpage=20&highl ight=metrology&pagenumber=2

The trouble is that a crank at the wear limit will have too great a bearing clearance, leading to low oil pressure. Race engines that are set up loose have the oil pressure relief valve shimmed to compensate, and they get torn down every 25-50 hours and the bearings are replaced.

Chris,
CIS no, Webers yes. I figured it was cheaper to buy used Webers than try to whip the CIS back into shape. ;)
Plus, as you say, now there are fewer limits to the performance upgrades I'd like to make. I agree, 98mm P&C's seem to be a natural choice, but can you tell me what CRs are available with Mahle's? I know JE's can be purchased to whatever CR you choose, but Mahle's only come in limited flavors, is that correct?
What is a 20/21 cam?:confused: That's one I haven't heard of before.

John,
I stand corrected, I meant the low limit, not the wear limit. Now I'm not sure which one the machine shop was referring to, I assumed they were referring to the low limit. I'll call and get the actual dimensions tomorrow.
Agree wholeheartedly about the vernier. I only used it to get a rough idea on the journal sizes to see if they were anywhere near usable range and it seemed sufficient for that purpose. I'd never use those measurements as a basis for my decision. I like the graphs in the "measurements" thread...you just can't beat SPC metrics......;)

At times like this I consider investing in the proper measuring tools, but I just don't rebuild engines frequently enough to justify it. I have a Mitutoyo 0-1" micrometer, a vernier caliper, a variety of dial indicators, but that's the extent of it. I also have access to high quality and regularly calibrated measuring tools at work (I work in the aircraft mfg industry and my buddy manages the metrology labs), so I really don't need to buy them. I will thoroughly recheck the crank again when I get it back home.

Thanks for the feedback guys.

Mike

I've been wondering about this same question myself. Over the past year I have disassembled many early engines for parts and have found several early cranks that measure just ever so slightly below the wear limit. The inspection report below is one example. I have not offered these cranks for sale and I am wondering if they need to be ground undersize or can if they can be reused. Does anyone have any real world practical experience with cranks that are less than 0,0005" under the wear limit?

sorry for the huge images



http://forums.pelicanparts.com/uploa...1178846766.jpg
http://forums.pelicanparts.com/uploa...1178846789.jpg