Crankshafts: How straight is straight enough?

B

bphillips

Junior Member
I've been working on getting parts together to build up an A engine, and I have a crank that has been ground .010 undersize on all journals, and a block with fresh babbitt to match. Things seemed a little snug when torqued, so I pulled the crank out to check it over. The journals are fine size-wise, but the center main runs out about .0035 when the front and rear are supported on V-blocks. I'm sure there is a straightness spec for A cranks, but I haven't found it in my references ("The Model A Engine" by McRee, and the green cover edition of the Model A service bulletins). Can anyone offer a straightness spec?

Thanks in advance,
Brett Phillips
 
From the print:

With the shaft on the two end bearings the center bearing MUST not run out over .002 total indicator reading and the timing gear shoulder and flywheel pilot must not run out more than .001.

How far out is the rear flywheel flange? That face is to be very tightly machined to keep the flywheel on center and in plane, they only allowed .0005" on the rear face.

Keep in mind the crank was balanced as well as the rods, and then the rod piston assembly. The rod piston assembly would have all been within 4 grams. The rod print allowed +-1 gram for each end of the rod.

You would want to read the book "Know Your Model A Ford" by Murrey Fahnestock.
 
Kevin:

Thanks for your help, I'll have to get a copy of that book! I was so distraught over the center main "jumping rope" that I didn't check the flange runout. At this point, it's kind of moot, as the crank would need to be straightened before use anyway (?). Anyone know of a good shop for straightening, or have a good crank that will grind to -.010?
 
Can the machine shop that ground it not straighten it? In all liklihood, I doubt they can straighten it without problems surfacing later because the journals will no longer be parallel across the length of the crank.

.
 
FYI: :cool:

Thanks Kevin!

I was looking up where to find the book and according to the MAFCA website book was renamed:

http://www.mafca.com/downloads/Indexes/Model_A-AA_Books.pdf

17.KNOW YOUR MODEL A FORD: By Murray Fahnestock, 1958.
This is a book which incorporates two of Murray Fahnestock's
books about the Model A/AA. Those books were KNOW THE
FORD, 1930 and QUESTIONS AND ANSWERS ON FORD
SERVICE which originally appeared as a 100-page pocket-size
reference book encompassing all the fine points in Model A
construction and repairing by approved methods. This book was
retitled in 1975 under the name of MODEL A FORD…THE GEM
FROM THE RIVER ROUGE.
An excellent book in learning all about the Model A/AA Ford.
18.MODEL A FORD…THE GEM FROM THE RIVER ROUGE: By Murray
Fahnestock, 1975.
Same as KNOW YOUR MODEL A FORD above, just retitled.







Kevin in NJ said:
From the print:

With the shaft on the two end bearings the center bearing MUST not run out over .002 total indicator reading and the timing gear shoulder and flywheel pilot must not run out more than .001.

How far out is the rear flywheel flange? That face is to be very tightly machined to keep the flywheel on center and in plane, they only allowed .0005" on the rear face.

Keep in mind the crank was balanced as well as the rods, and then the rod piston assembly. The rod piston assembly would have all been within 4 grams. The rod print allowed +-1 gram for each end of the rod.

You would want to read the book "Know Your Model A Ford" by Murrey Fahnestock.
Click to expand...
 
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Brent:

I haven't asked them about it yet, as I wanted to have a spec in hand before calling them. I am also doubtful that it can be straightened to run correctly. It looks like I'm probably going to be looking for a crank or a bearing job in the near future...
 
Oh

An important consideration.

When grinding the cranks the person doing the grinding may have centered on the wear. A very common practice to minimize the amount of material that is taken off. This means the center main will be ground 180* off of the two ends. There are also quicky tooling that lets you round up a scored journal that may bring the center off center.

If you try to straighten a crank ground off center you will only make the rear flange go out of plane and cause the front and rear mains to wobble.

My brother has learned quite a bit between trying to get cranks ground and learning how to operate the one he bought. There are a lot of ways to subtly screw up a crank. I did not even touch on getting the centers on the rod journals all in the same radius. Nothing like having pistons stop at different heights for an even running engine.

So you need to really look close at what is happening with your crank. If it is bent then you should expect a wobble on the outer journals and the face of the rear flange. If it was ground in a bent state, then it would need to be reground when straightened.

You could always just say it is just an old Ford and it does not matter and wonder why the engine is not right later.
 
A Model A crank is hard to grind. They are very flimsy, the center main will give as the crank is being turned. You have to bring the steady rest up just right. This is not a problem with most cranks.
My advice is find some one that grinds A cranks all the time. I have seen this on many cranks, but most people do not check it. Yes I used to grind cranks many years ago.
 
I'm beginning to think that poor crankshaft grinding jobs are the real reason that poured babbitt bearings have gotten a bad name in recent years... This engine is going to end up on the custom side of things, and I'd like to keep the build in house, otherwise I'd send it to one of the reputable builders on this forum. Can anyone recommend someone who grinds a lot of Model A cranks and does a good job?
 
They must be straight before grinding as stated earlier. All it takes is a hand press and dial gage to do a good job .We used to get them within .0005 on the center main and checked at the shop before grinding.
 
bphillips said:
I've been working on getting parts together to build up an A engine, and I have a crank that has been ground .010 undersize on all journals, and a block with fresh babbitt to match. Things seemed a little snug when torqued, so I pulled the crank out to check it over. The journals are fine size-wise, but the center main runs out about .0035 when the front and rear are supported on V-blocks. I'm sure there is a straightness spec for A cranks, but I haven't found it in my references ("The Model A Engine" by McRee, and the green cover edition of the Model A service bulletins). Can anyone offer a straightness spec?

Thanks in advance,
Brett Phillips
Click to expand...

By the way .003 is about how thick one hair from your head. I would not worry to much about it. But if you want it closer it can be straighten.
 
bphillips said:
I've been working on getting parts together to build up an A engine, and I have a crank that has been ground .010 undersize on all journals, and a block with fresh babbitt to match. Things seemed a little snug when torqued, so I pulled the crank out to check it over. The journals are fine size-wise, but the center main runs out about .0035 when the front and rear are supported on V-blocks. I'm sure there is a straightness spec for A cranks, but I haven't found it in my references ("The Model A Engine" by McRee, and the green cover edition of the Model A service bulletins). Can anyone offer a straightness spec?

Thanks in advance,
Brett Phillips
Click to expand...

jandm-machine.com Send it to J & M Machine Co., It will be done perfect.

Phone 508-460-0733 Herm
 
Herm: Thanks for the tip. I will check with them.

George: I thought about that too, but what concerns me is that the oil clearance should be less than half of what I have for runout at the center main (assuming .0015 or so clearance). I added that tidbit to the fact that many A's have problems with the center main anyway, and the fact that this engine will be working hard, and I began to get nervous.
 
Hi Brett, All of the above advice is very good and solid material. You are only 0.0015 away from the acceptable tolerance of 0.002 runnout, and you do not really know where your problem originated from. It might be interesting to support your crank on the vee blocks on an iron machine bed/table or granite test stand, and indicate the trueness of rotation of all of your mains and the crank pulley shaft and your flywheel flange. If you're really good, you could indicate the top rotation of each of the crank pins at TDC (or BDC). This will reveal a good deal about the condition of your crank, and even if it is worthwhile trying to correct. If you get very lucky, you might be able to straighten it to 0.002, and find that you can use it effectively in your engine.
Straightening an A crank is a bit of "black magic", and requires that it not just be stressed in the correct direction, but that the counter-bend be stress-relieved by tapping it at the correct points on either side of the center main. Sometimes the resultant "wobble" indicated in the message above can be avoided by correct stress-relief. Also, J&M, mentioned above, is a great outfit...

----------------------------

burlingtoncrankshaft.com
 
I am wondering: If you mount an A crank in V-blocks at each end, how many pounds of force would it take to deflect the center 0.001"?

Doug
 
Chris:

Thanks for posting. What you say makes quite a bit of sense, and I feel that you have revealed something that I was holding out on! Before setting the crank up on V blocks, I tried putting it between centers in the lathe to see if I could indicate it there, but I quickly found that the original centers did not run true with the main journals. I then retreated to the V-blocks. I'll set it back up and indicate the outside of the throws tomorrow evening and post what I find. I'm beginning to think that the person who ground it centered up on the worn journals, rather than working from a centerline and known stroke. It might be junk, but a dial indicator won't lie...
 
George Miller said:
By the way .003 is about how thick one hair from your head. I would not worry to much about it. But if you want it closer it can be straighten.
Click to expand...


That's what I was thinking.... if you are measuring the difference between the correct .002 and .0035 them you are taking too much time looking :D.. Yeah I know "to each his own"..

D :cool:
 
If you think .003 is bad, jcrosson's crank was out .013 and would vibrate till you couldn't see out the mirror. It, as indicated earlier in another thread, destroyed the center main and rear. My grinder straightened it to within .001 and reground it so the run-out at the rear flange was .0005. The engine runs great with inserts and is smooth as a Model A should be. Cranks can be straightened by the right person and my guy does cranks of all kinds for many different applications from stock to full bore race motors. Offset grinding is a specialty to stroke many race motors done at his shop. All balancing is done on a Hines computer balancer that balances the crank rotationally and lengthwise. The last crank I had done was out 7 grams rotationally at the flange end. It is now 0 grams overall. It can be done.
 
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