stainless vs 4140 for headbolts with aluminum threads

[marinop]

We only use 25lb/ft on 3/8" x 24 x 4" headbolt bolting aluminum heads onto aluminum cylinders. I am looking to keep compressive strength (torque on the copper headgasket) as close to constant as I can. With this being an air cooled engine. There are load of heating and cooling cycles. I'm looking at 303 stainless since the torque is so low but have been told that 4140 is closer to aluminum in its expansion! Aluminum is much prettier though!

Any comments or advise will certainly be appreciated

Marino

 

[TVP]

Austenitic stainless steels like Type 303, Type 302, etc. have a CTE closer to that of Al than martensitic alloy steels like SAE 4140. Preload on the fastener/joint assembly is the important parameter, not torque.

 

[Metalguy]

303 is just about the last stainless I'd use. Great for the machinist, not for the user. Use 304 or 316. You can find them, with some difficulty, in various strengths-from cold working.

 

[marinop]

Hi Metalguy,

What is the reason you would not use the 303!

Marino

 

[NickE]

Probably the extra sulfur added to improve machinablity ruins the fatigue life in this application. (just a guess)

 

[Metalguy]

Yes,in general, 303 is not recommended for "critical" applications for the same reason that 416 isn't either. They usually work OK, and most SS rifle barrels are 416, but I don't like all that S in there.

302/304/316 and 410 have a lot less and are fine. For REAL critical/fatigue applications such as steam turbine blades there is an extra low S mod. of 410, called 403.

Correctly tightened cyl. head bolts don't see much cyclic loading, but I'd use something else.