The Studeblogger

Sunday, January 02, 2011

Head Bolt Torque, Revisited.

A while back I posted about checking and re-torquing head bolts on Studebaker V-8 engines. The Shop Manual contains all the torque specifications and the bolt tightening sequence (which I documented in my earlier post), but recently a thread on the Studebaker Drivers Club Forum made mention of an additional procedure worth noting. It's probably one that experienced mechanics take for granted, but I certainly don't fall into that category (and I know some of my readers don't either).

Specifically, the heads should be re-torqued when the engine is cold, and should be loosened slightly before tightening. Says Forum member Mike Van Veghten:
Always cold...at least 4 hours since the engine was run, cold.

This is important because the Shop Manual gives conflicting instructions, saying that one should warm up the engine "to stabilise the temperature" before checking head bolt torque. Mike explains that this is outdated thinking, and the reason to torque when cold is simple: the first few bolts torqued after running the engine will be to spec, but the engine immediately starts to cool, causing the metal to begin contracting. Bolts torqued after this process begins will be tightened to a different value than the first bolts you worked on!

Therefore, all bolts should be torqued to spec only when the fastened assemblies are cold. This includes valve adjusters.

Furthermore, Mike says:
Back each fastener out about 1/2 turn (one at a time), then re-tighten to the proper torque value. If you do not back the fastener out, you will not get the proper final torque put into the new tightening operation.

Now (as the saying goes), you know... the rest of the story!

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