View Full Version : Crank bearing girdle

07-14-2004, 04:56 PM
Hello everyone, I am going to machine a crank bearing girdle for a engine I am building and I am wondering what would be the best material choice.
I was thinking about somthing like stainless steel, maybe..

07-14-2004, 09:57 PM
T-1 alloy steel plate would not be any more difficult to machine than stainless, and it's got far greater strength than any SS unless the SS is a heat treatable alloy and has been heat treated. Most folks get the idea common stainless alloys such as 304 are strong, but that's not the case. They're just difficult to machine. Yield strength for T-1 is 100,000 psi. minimum. The ASTM designation is A514. You should be able to use this to find a European equivalent. It's economically priced because its produced in large volumes for the manufacture of heavy construction machinery. Price should run about 1.5 times the price of mld steel. It's available in thicknesses ranging from 3/16 to several inches.

11-03-2004, 12:55 PM
Hey Anon,

I designed and made main caps (splay bolt style) for our 525 ci Supercharded Methanol Pro-mod Ford for the last season. The block is a cast aluminium piece, and the caps that came with it were made of 7075-T6. The caps would only last about a half a dozen runs, (not bad with over 2500 horsepower, but it get's expensive). These engines regularly see over 10,000 rpm. The new caps were supposed to be stronger, while keeping close to the originals in weight.

Originally I was going to use 4340, but I couldn't get any in time, so I had to settle for 4140. I machined them in the annealed state, then heat treated them to about 45Rc. They were also stress relieved. The strength at that level is about 150,000 psi, if I remember correctly. I bolted two caps together so that the mating surfaces wouldn't move around that much in heat treating. I also left about 0.010 on the mating surfaces for final grind, and about 0.030 for the final align bore. ON the fastener front, we always use ARP fasteners, and follow their torqueing procedure to a "T". We actually made a jig, and tested their recommended procedure against other commonly used procedures while measuring actual stretch, and it makes a huge difference.

The good news is that they held well, as long as the torqueing procedure was followed. There is only a difference of 2 pounds using the steel caps, and they don't break. Now, the top end.........then a forged block........then a..........and then.......

Good luck,


11-08-2004, 02:57 AM
how hard is it to machine them in non heat treated state?, the only problem I have is that my machine is tiny :)
Thanks for your reply

11-08-2004, 03:06 AM
What kind of machine are you using? 4140 annealed machines just like mild steel. I'm sure if you use positive rake tooling, and utilize some higher feed/small depth of cut techniques, it should be pretty easy on something as small as a Bridgeport.

Let me know if I can be of any help.


11-08-2004, 03:09 AM
hm, I've got a tabletop machine, I 1 or 2 hp spindle...

11-08-2004, 03:10 AM
Hey, what kind of engine is it? What kind of horsepower are you trying to develop? How many rev's? What bore and stroke? What kind of crank?

The reason I ask is that the material you use for main caps will be determined by the forces needed to be controlled.


11-08-2004, 04:10 AM
it's a benchtop machine, with a 1 or 2 hp spindle.. cant remember it now since I am away from the machine