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

If you have access or know someone who has access to a Bridgeport mill you could probably get it damn near perfect on the knife edging part, then of course have her polished and balanced to get rid of those nasty stress risers.

 

yeh I guess just go for it...as long as you follow the pics fairly closely I dont see you could go so far that it couldnt be rebalanced.
God there must be a lot of hours in that one in the photo..Id guess a hundred at least, but you can just feel it cuttin thru the air....
To develop a flathead to the point you NEED to do that would take some biiig bucks, but if your a freak like me and just like grinding and polishing stuff..have fun!

 

The trick grinding would be the hard part. The polishing could be done with ********y finishing equipment pretty easily- I sure wouldn't want to have to stand at a pedestal buffer with that....

 

The first thing I would recommend doing is talking to your machine shop about how much material you can remove. If you take to much off of the counterweights it might require some "heavy metal" to balance it, depending on wich pistons etc. you are using. That can get a little $$

...go by your local supply house and pick up some good "flapper" discs for your 4 or 6" angle grinder....they make them starting at 36 grit. Makes taking off the bulk of the material a little quicker. I also have made a crankshaft holder out of some plywood (pictured in just about all flathead rebuild books)....basically some V-blocks to support the crank, keeping it from sliding all around the workbench sure helps.

 

I'd also recomend a flap disk over a hard stone disk They tend to grind easier without gouging the metal. As far as technique, the best I can recomend is see if you can get in contact with AV8. I remember his posts on the subject, but don't know if he ever did an actual TECH on how to do it.

 

I took a die drinder and played around on a factory rat crank for a minute or two. That steel was too hard to mess with. Really tough.

You sure you wouldn't rather just add some more main caps instead?

Ha!

Dave
http://www.roadsters.com/

 

Somewhere I have some slides of what Whitey McDonald does with his cranks. It's amazing how much he takes off and where he takes it from. Now I guess I'll have to look for them and dig out my ****py-*** scanner.

 

Hey Jim:

If you happen to run across those pictures, let me know . . . would like to see them. I have a polished blower crank - but it has not been lightened like the one shown. I'm sure that the removal of weight could help it rev faster -- but I'm also sure it depend on the car's weight as to whether you gain anything from this amount of work. That said -- I'd trade mine for that knife edged one anytime!

Dale

 

Hey Champ,

Is this for Merc crank or its smaller cousin? Just wreck a few cheapie non-merc ones and see what happens. Is the real purpose of the knife egde is for cutting through the air? Oil?

Danny

 

I think it's all about weight. Sure the knife edge will cut better, but the weight removed is what really makes the difference. The polishing eliminates stress risers, and probably makes it shed oil easier.

 

Send AV8 a PM or give it a day & see if he chimes in - I'm sure he's done a short step-by-step somewhere...

 

One of the major benefits of knife edging is a reduction in oil aeration, as oil droplets in the crankcase are no longer being impacted by the blunt end of the crank counterweights. This should help your bearing life. If you were really mad, add a crank s****per and then reduce your oil fill!!

Good luck.

BTW is that a forged or cast crank?

 

Back in the "old days", we knife edged a lot of cranks to reduce oil friction. As I remember it, a lot of time spent for not much gain if any. The same goes with polishing rods. Sure looked pretty though.

 

Somewhere, there are some pics of what I did to mine...

Maybe on this thread?
http://www.jalopyjournal.com/forum/showthread.php?t=54676&highlight=crank

I decided that while it would be nice to go all the way to AV8's level of lightening/finish/detail, I wasn't quite ready for that yet. I also figured that the benefit vs risk (cost) ratio started getting out of wack the farther beyond simply cleaning up casting flash you go. Also, just like any rotating ***embly, the more material you take from the outer edge of the rotation, the more you effect the rotational m***. So I didn't think it was necessary to go nuts trying to remove material down by the mains.

Therefor I chose to clean up the flash, polish the surface and knock down any edges I could get to easily, without risking a total destruction of the crank.

I started with a flap disk (I think it was like 100grit but I can't remember) on a 4" angle grinder to knock of the flash and (****er) Knife-edge the counter weights.

Next came a die-grinder with some red stones that I flame-shaped. I pretty much hit every surface with these to give it a nice even surface for polishing.

Next came 60grit abrasive roles on the die grinder. Again, I touched every surface.

Lastly, I hit the whole thing with a flap wheel on the die grinder.

I then took it to the machinest, where it was bead blasted, machined and ballanced.

This is the result...

 

The main gain for a street flatty I think is the resistance to chipping in the event of a failure. Beyond that, you're dealing with perception as far as I'm concerned.

If you say you can feel the effects of a polished crank in the seat of your pants, I'm gonna tell you to get new pants... And stay the hell away from me with those...

 

Kevin a cranks a crank.

The angle grinder is the way most of 'em get done, but i do have access to a bridgport if you decide you need to go that route.

The main thing to remember is to not go near the counter balance weights. it costs a ton to have weight put back on a crank.

I'd offer to do it for ya but you'll want to do it yourself. I'll come and watch you though if ya want.

 

do you think that shot peening the crank would be better then polishing as it supposidly closes the molecules on the outside of the metal? does polishing do the same thing?

and would the surface of the crank being bead blasted or shot peened be more prone to holding smaller particles of oil?

supposidly a port and polish job now days has eliminated the polish as the air i guess flows better over the surface with a slight texture, better???

i dont have experience with this...just thinking
Zach

 

i believe a textured surface would do the opposite of what is intended here.
with texture, you add a place for oil to hold onto - texture causes friction... the smoother the surface, the less friction is created.
the forces exerted when a crank is spinning at 5000rpm are pretty extreme, you can imagine how much additional energy is needed to get oil off the crank and in context to compe***ion racing, every little bit helps... at least, this is what has been said in various articles.

av8's crank treatment seems more logical for an all out race car which will see the benefits in MPH and throttle response at high RPM... perhaps he can help clarify.

Kilroys treatment would seem to be the most logical for the novice. i dunno, again more speculation.

 

Yes I have polished a flatmotor crank ,lot of work.I also have run a crank scarper type of pan,though it worked but no definete word.I have adapted a drag racing vacuume pump to the 303 in the car now.Can,t run tray because of size of crank.Think pump works but have decided to stop racing the coupe that where I got data from.Hope to try many expearament with the altered in future.

 

I'm definately a novice...

I was inspired to buy my first die grinder by AV8's flathead porting. I never built a motor before this flathead. I sure as hell never thought about grinding on a $400+ crank before. I was scared ****less...

That being said though, the shot I posted is a little decieving....

Between the poor quality of the pic/the angle/ and the blasting that was done to the crank after my polishing, the crank looks a lot more 'stock' than it is. There's actually quite a bit of material missing from it.

From what he said, AV8 takes his cranks to just about the 'limit'. I don't even think he would say it's more or less necessary to go that far. It's better but it's a personal call on the work-risk/reward thing.

Also, my machinest suggested the bead blasting of my polished crank to give it a more consistant surface, to add a little surface strength, and to help the oil release from it. So I guess at least his thinking is that a perfectly polished surface isn't the best for going through the oil.

Another thing to think about is that most of the oil in the crank environment is in the form of mist. So perhapse the same thinking that dictates a slight roughness to the surface of the intake runners for example, applies to the crank surface.

But then the debate might be about what level of 'roughness' is best.... I'm thinking a VERY faint golfball pocking.... No rough edges, just tiny dents.

 

Actually part of the idea behind polishing is to remove the stress risers. Most factory parts or better said a lot of factory parts are full if scratches and parting lines and the like. The fact that the oil slings off easier is just a fringe benifit.

if I'm building a throw together motor i don't take the time but if I want one that will take abuse and be abused it costs nothing but time to polish it. Knife edgeing is something that is strictly race as far as i'm concerned but a lot of street motors get raced right???

On the head treatment, that was mentioned, the polishing treatment has gone by the wayside on the intake side but a lot of guys are still polishing the exhaust side. Rough intake will atomize the incomming charge better and a smooth exhaust side won't collect carbon as quickly. Well that and there is no need to berak up spent gas.

 

thanks
Zach

 

There is more to consider here, any else thought about light cranks in a flatmotor?Pro and cons?

 

Some stuff:
1How to Build & Modify Chevrolet Small-Block V-8 Pistons, Rods & Crankshafts (Powerpro Series) by Dave Vizard

is a very good read on this; one of the highlights is the lengthy and thoeoughly illustrated preparation of a 400 SBC cast crank for racing--polishing, knifing, hollowed journals, everything with zillions of pics and thorough discussion by a racing engineer. This is the story of a $50 crank eating a thou in labor...
One of his interesting thoughts is cutting journal size to max undersize whre proper (not thick soft layer) bearings are available in order to allow heavy fllet radii to strengthen the thing. Buick GN turbos got their cast radii heavily rolled for extra strength.
I have one Merc crank with the fillet area cut into a GROOVE! The mad grinder said that putting a radius there UP OR DOWN had the same effect on strength! Engineering comment from Zach J??? Looks scary to me!
Lightening: The bandsaw article KL refers to is an early 50's one on Senter at Ansen lightening the hell out of a flathead crank. I'd think this would have to depend partly on finding suitably light pistons and rods to allow balancing? In flatheads, racers oft used 221 journal sizes and rods to cut weight.