Re: Crankshaft Drag Desired after Engine Rebuild

Posted by Joe on March 04, 1998 at 10:37:16

In Reply to: Crankshaft Drag Desired after Engine Rebuild
posted by Gordon Biggar on March 04, 1998 at 00:18:32

: Is 58 foot pounds of drag too much?Where have I gone wrong?Where did Les obtain his 35 foot-pound figure?Do I go back and increase rod bearing clearances to .002 inches, which would be outside of specifications?
I would consider 58 foot pounds a bit high but not impossible.I am not sure how Les came up with his number not having read the book or done it exactly according to his methodology.I have been to the school of hard knock engine rebuilding, however.
If you think about it, a high reading in this area can only come from one of three sources.
1. Surface roughness of the grinding/honing marks on the cylinder walls.These are normally made to present a 45 degree angle to the axis of the cylinder, this being that which self hones or polishes to a correct surface on operation of the engine during break in.A coarser grit than is normally used might leave a surface roughness greater than usual causing increased friction.You could check with your rebuilder on this.He will probably give you assurance that "I have done hundreds of engines just like this and they all worked" and he is probably correct.
2. A bent crankshaft is another possible cause for the symptoms you describe.(worst case)
In my case I found the crankshaft bent after completing a total rebuild of the engine. I speculate that the crankshaft grinding machine in holding the crankshaft between centers caused the crankshaft to bend slightly, as I remember about .018 inch on the center main journal.How the machinist was able to get the center journal round under those conditions, I have no idea, and neither did he when confronted.(It must have bent afterwards, he said - very unlikely, said I.)
I was able to have the crankshaft straightened to within .001 (by another machinist) and the bend has not returned the last time I tore down the engine.The old timers I have talked to about this seem to feel that the Model A crankshaft is a "sensitive thing" and not immune from "moving around some on it's own".I can say having a crankshaft straightened is a "heartstopping" exercise, not for the weak of heart or spirit.It's absolutely amazing the amount of spring you have to subject a crankshaft to to get it to take a permanent "set".
3. The difference in torque may be a result of the radius of either the crankshaft journal where it meets the webs or the radius of the babbit as it has been poured and machined.A small difference here would dramatically affect the turning effort required.But it is not a big thing.
Most modern line boring equipment and some of the old ones present a "square shoulder" on machining the babbit hole and the radius has to be manually relieved to match the radius in the corner of the crankshaft journal.This used to be done with bluing and a hand-made radius tool although I suspect a commercial tool is now available or adaptable to the boring bar.Small differences here may make a big difference in turning effort but most differences will wear to a proper fit on breaking in the engine.

Failing in finding a cause from those I name above, I think here is an occasion to "look to the fundamentals" and examine in your own mind your rebuilding process.No engine rebuild is ever 100 percent done correct: you only hope that you have eliminated or compensated for the major points of wear and that the minor discrepancies won't raise their ugly heads and
preclude successful operation.If you have done everything the best possible in each of the foundation elements, then likely everything will come together and work.From the way you describe it, it seems you have done everything just about right.
I might even question the 35 foot-pounds as a guideline, it being one number for one particular engine under one particular circumstance.A small difference such as temperature at the time of test or the amount or type of lubricant used during assembly would dramatically affect this number.
I would not bother to increase the clearance to .002 or above, this will happen soon enough on it's own.
Henry, when he originally put the engines together, used to break them in on a test stand first driving the inoperative engine by an electric motor and then (purportedly) using the electric motor as a kind of "prony brake" and getting some useful power from the now functional engine.The purpose of all of this was certainly to improve the break in condition of the engines as delivered but also to "weed out" the inevitable errors and mistakes.
I might recommend something along the same lines.Set up a stand and test run the engine.Or at least do this in the frame prior to putting the body and everything back in place.
: It's a great learning process, but oftentimes frustrating!Thanks for any advice that one may have on the subject.
It is indeed.My class is still in session although I am a bit less frustrated with the teacher than I was at the beginning.Hope this all helps.
If anyone else has any horror stories about bent crankshafts, post them.You'll have at least one student reading.
Best regards,
Joe

: Gordon Biggar
: Houston, Texas

E-Mail Comment to Webmaster