The crank from my 26 has a .005" run out at the center main. I'd like to hear how the DIY bunch have done this repair. PK
Place the model T crankshaft in a lath on centers and carefully push on it while observing how much the dial indicator moves.....you will quickly discover how weak the crankshaft is and easy to bend. The professionals hit it with a chisel like tool....I can not bring my self to straighter one that way.
Here's one thread with a lot of opinions from experienced mechanics: http://www.mtfca.com/discus/messages/411944/417680.html?1390456208
Pat, I wouldn't bother. Just replace the thing with a Scat and end the worries.
This reminds me of a problem I had when I had my main bearings poured. The bearing man said to have the crankshaft checked for cracks. I took it to a machine shop who said it has no cracks, however it is crooked. I asked if he could straighten it and he said, "yes, but we will need to turn it." So I said just take off the minimum amount because my bearing man can bore the bearings to fit. Then I took the crank to my bearing man who miked it and said,"are you sure you want to use this crank? The mains are 30 under and the rods are 50 under.
After spending about $150 on the crankshaft, I decided to get a SCAT crankshaft. Should have done that first!
I did the same as Les, but put a steady rest on the tail, so when you push down it has a place to go. I bent mine by running the tail center in to tight. Steve
So, two wooden blocks and an 8 pound maul is possibly not the best way?
I straightened my '26 crank successfully a few years ago, after reading up on previous discussions and opinions here on the forum.
First photo shows the basic setup, second is a closeup of the 20-ton ram applying pressure to the centre main.
I used a junk block with upside down front and rear main caps to support the ends. Pressure was applied to the centre main through a bearing cap and hunk of old bolt.
It was necessary to apply a LOT of deflection to achieve any permanent change — photo below shows 0.320" (not a misprint) at the centre main.
After several trips between the lathe (for measuring) and press, I got the total runout to a miniscule 0.0005". Having said that, Les makes an excellent observation that very little force is required to deflect the crank a few thou, so maybe your 0.005" runout is nothing to worry about.
That's the way I do it Cris. Seems like you push a hundred thou past your goal to remove one thou of bend on a Ford crank. I have two file drawers of cranks to sort out and check before which ones go to the grinder.
Here is the set up I used to check the run out. The first and third mains are at </0005". This gives me .005" at the center main. I tried pushing down with one finger and got 1.5 thousands deflection. Pretty amazing! PK
When straightening a piece of steel it's best to go slightly beyond and then back again.
It takes VERY little pressure to go back the other way.
The reason to do it that way is it removes stresses.
The original crankshaft is the most significant Achilles' heel in the Model T's powerplant. _Even though I have a high-compression head, a high-volume intake manifold and an NH carburetor, which in combination should make significantly more power available to me, I don't dare open the throttle wide and put that extra power through my 100-year-old crankshaft. _As long as your engine is out of the car and opened up, it makes sense to take advantage of the opportunity to install a SCAT crankshaft. _I'm reminded of something my dear old Dad used to tell me: "Cheap is expensive."
HI ; Can someone explain me why the Crankshafts are always bend in the middle? When a Crankshaft is mount in the block,with the three caps in place,then it is nearly impossible to bend a shaft.But they are always bend in the middle.
Thanks for your answer
They are not bent in the middle! When you fix the ends only the middle can be indicated. You will find that the fly wheel end has bent.
Actually the true description or term is, crankshaft warpage, and not a bend but warped in a bow shape, that's what makes the center bearing out the most.
I believe most shafts get bent not by being in a block, with 3 support points, as you describe, but by being laid down on their sides for dozens of years, allowing gravity to warp them over time.
Why would a crankshaft, a heavy piece of forged steel warp by gravity over time? Seems odd to me. Other pieces of steel doesn't warp by less than a force that is above the yield strength of the material. Only exemption would be if Uri Geller came by and gave it a few strokes
I was always taught to store crankshafts on end and to hang cam shafts to avoid distortion. I assume gravity and the weight of the object will eventually cause it to distort but expect that there is a more complicated scientific explanation.
I hung one up once for so long that it straightened out like a piece of cold rolled! :-) KGB
Sorry, with all respect, I just can't buy that. Plastics and other soft materials may creep over time under moderate pressure, but not steel. Steel does creep, but that's only under considerable strain or at high temperatures, so if the crank hasn't been through a house fire, then it shouldn't bend over time by just gravity.
Here's a scientific diagram showing the temperature when steel may start to creep under more moderate pressure - that's about 800 degrees Celsius or 1470 F.
Measuring runout can be done in various ways, maybe the results can vary between two times the same crank is set up to be measured due to the methods used - especially if it's been years in between?
I will defer to those who know more than I do about this issue but what I do know is that I have had to put a fair amount of pressure on a number of crankshafts over the years to correct run out that came from someplace. It may well be that the problem arose because someone pulling shim out of the center main caused the deflection but for the time being I think I will keep my spare crankshafts standing on end because like they say, it may not help but it sure won't hurt!
I put the crank in the press and the run out is now less than .0005". I'd love to go with a SCAT crank but it can't happen this year. I will have the crank checked for cracks before use. Thanks for all the info and help. PK