Aluminum air-cooled and growing

[roverman]

No, it's not British-but... Corvair uses 7/16" diameter, LONG steel studs, to retain the cylinders/heads. These are necked-down to about 5/16" dia., likely to provide enough elasticity, as the engine thermal expands, ( with cast iron cylinders).  Plan is to use Niklsil coated aluminum cylinders.  My concern is, the steel studs will over compress,( barrel shape) the aluminum cylinders, when fully warmed up.  Why not use full diameter 7075 T6 studs ? At full diameter, they would be "stronger" than the oem studs, but none of the thermal diferential problems ?  Your "thoughtfull" input is appreciated, art.

[BlownMGB-V8]

I'd think they'd run cooler than the fins Art, what do the other Corvair (and VW) builders do about it?

Jim

[roverman]

So many forums, so little time. This seems to be the only forum, where they aren't suggesting "lobotomy". Once again, my claimed location is, ' just east of forrest'. All the Corvair specialist I've spoken to, are bent on pushing the products they have in stock. I've tensile tested  7075 T6 aluminum at about 107 ksi., which compares favorably with mild steel. Using all aluminum cylinders, is not where I want excessive thermal preload. Thanks, art.

[joe_padavano]

You're concern over the CTE difference is valid.  Assuming about 10" studs and a 200 deg F delta from room temp to operating temp, my rough numbers are about 0.013" difference in the length of the stud vs. the cylinder (assuming 6.8 micro-inches/in/deg F for the steel and 13.1 for 7075).  I'll let you run the numbers on the stress difference in the stud/cylinder.  

The problem with aluminum studs is that the rated tensile yield of 7075-T6 is 63-69 ksi, per MIL-HDBK-5.  Your 107 number is scarily high. Grade 8 bolts/studs are typically around 130ksi yield. I'd also be concerned about shear failure of the threads on the aluminum studs. I don't have a good answer here, unfortunately.

[roverman]

Joe , thanks for the input. To clarify, I believe Corvair cases, ( studs thread into), are semi permanent mold, aluminum so approx. 40 ksi yield ?  Corvair studs are necked down to  only.312" dia. 107 ksi. is .at break. Are stock Corvair studs actually grade eight ?