Looking for any input on polishing/knife edging a Merc crank.

I have kinda wondered about the overall benefits of a lightened crank given the whole flathead package...

There already is the idea that if you're running a 4+" crank, piston speed is a limiting factor, so do you really want a stroker motor that 'spins-up'?

I built my motor to utilize the inherent torque characteristics of a flatty with a long crank... I'm running a high compression for the street along with staying with 1.5" valves. I don't plan on having anything past like 4k. But it's a street car.

 

The lightening isn't about raising RPM, it's about increasing acceleration--the lighter the innards, the QUICKER the thing will rev, and the effect of a pound removed from rotating/reciprocating stuff is MUCH greater than a pound of dead weight removed from the car. Circle track people or NUTS about this, doing things like cutting away unused gears on the cluster and such. Also--the greatest loads on an engine (a normal engine, not a Formula one or a fuel dragster) are the inertial loads of the ***emblies at the top of the exhast stroke, NOT any loads caused by power!

 

Thanks Bruce.

You helped me conceptualize something that I was having trouble with...

I guess the piston speed is the same regaurdless of how fast the motor gets to say 3k rpm.

 

Lowering the rotating m*** in the engine helps acceleration a lot. They'll also decelerate faster, which in real world driving with hills and turns can be a down side. Flathead Jack's has several specialty cranks from 18 to 25 pounds lighter than stock for the big spender who's got to have every bit of zip they can wring from a flathead. Overall I'd say that Kevin would get more effect from polishing than all the grinding, maybe a whole 1/2 hp. NASCAR builders claim 3 to 7 horsepower gains from polishing all the internals (even the block!), but those aren't exactly flatheads anymore.

 

I have a Merc flathead crank (4") done by Mike Bishop. (looks like the one above)

The polishing DOES relieve "stress risers" in the body of the crank.
Ford used ingots of metal that were dropped forged to create the basic shape of Merc cranks. The forge mold "marks" (the parting lines) once they are smoothed out; WILL reduce the stresses left behind. (Ford did VERY LITTLE cleaning up of these "marks")

"Knife edging" the counter weights just lets the crank "cut" through the oil rather than "splash" through it. (it can also slow down the "whipping" of oil in the pan)

Mike uses grind stones - starting with co****, working down to finner - to begin the smoothing, knife edging and lightening of the cank, then finishes with "flap wheels."
Because he removes an equal amount of material all around the crank as he works on them; the final balancing takes very little added (or removed) to achieve the final balance.
Mine needed 1/2 gram removed from one large counter weight to arrive at a final "+/-" 1/2 gram balance. (this was with the shot peened matched rods and weight balanced pistons/wrist pins/rings)

My crank has been reduced about 4 lbs. in weight. Any more weight removed for a crank used in a street engine (low to medium RPM's) will cause the engine to "bog" between shifts. (lift off on the gas; and the engine loses rotating inertia) He just finished a cank for a Bonneville flathead where he took off about 12 lbs. (sustained HIGH RPM's)

Time? Mike said he spent OVER 150 hours to do mine. And this is after "refining" the process over about 100 cranks.

 

BTW. When I did my crank ,I chucked it in a lath,free wheeled the chuck and use the lath as a holding fixture to work on it .Also it help to keep things true,don't put power to the Crank just let it float and work it by hand. Mike your workmenship on those flatmotor cranks is beyond motal men.FWTW I think it is a good idea.

 

great great thread.... really liking it. too hexy for any motor i'll be building but it never hurts to know... just in case i win the lottery!

danny

 

digger dave - what is the intended usage of your flathead? sounds like you are prepping for something that will take some abuse

 

The crank is going in a C69A (Canadian block; same as a USA 59A) with some mild porting, (learned from Mike) main bearing straps, and a S.C.o.T blower on top.
I mainly intend it for street use, but it might see a few "p***es" at the local strip. It will "power" a '28 A Tudor Hiboy.

The picture Kevin showed at the begining of this thread I believe is a picture of my crank; but when they're that pretty who can tell the difference!!

I met Mike at Bonneville when he was hanging around the Tardel / MacKenzie LSR a few years ago, and he was showing me how a polished crank withstood TWO engine failures. (holed the blocks; big time - in BOTH cases) It was cleaned up; and returned to a third engine.

I talked him into some big time "horse trading" (parts for polish) to do one for me. (he got SEVEN MORE cranks as part of the deal!!)

I have always wanted a flathead that could stand the odd "buzz" without blowing it to pieces; so the crank treatment seemed like the best way to go!

 

it is a thing of beauty really, anything to squeeze a few more HP out of the old girl!

 

amen!

 

I thought my ears were burning! Someone talking about the ol' pigpen again. I was just looking at the pretty polished crank. It's my humble opinion that to polish a crank like that, you have to have a lot of time on your hands, and you will be getting almost negligible gain for your efforts. Overkill. Here's a shot of my Merc crank, cleaned up, bone stock, with a fresh 10/10 done at the local machine shop. When the two cranks are covered with black sludge after a few miles, you won't be able to tell them apart.

pigpen