VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 thread standard and spanner sizes

can anybody enlighten me on the types of threads used on the 1927 Capitol and what type of imperial spanners fit, i've tried AF imperial and Whitworth spanners but nothing fits and i normally have to resort to the old adjustable...

I'm not sure of sizes because I don't have a '27 Chevy however this information might help if you remove bolts and measure them. Also you have to take into account that old bolts and nuts corrode and therefore the proper wrenches might not fit anymore.

http://www.baconsdozen.co.uk/tools/conversion%20charts.htm

Steve

All the threads will be either SAE UNF or UNC threads,which definitely take AF ring,open end spanners.

Head bolts are whitworth 1/2" X 12 with a 55 degree pitch vs 1/2" X13 60 degree pitch USS (UNC)
Steve

The head bolts on the 27 are an old American standard dating back to the early 1900s. Current 1/2" UNC bolts are 13 TPI with a 60 degree thread angle. At the time the Chev 4s were designed there was an alternative US standard coarse thread of 12 TPI 60 degrees, the same pitch but a different thread angle to 1/2" Whitworth. This still exists as a USS (US Special) thread, but it is not common.1/2" USS 12TPI taps and dies are still available. The original bolts take a 3/4" AF spanner - I just checked mine.

Because of the rarity of the old USS 1/2" bolts many twenties Chevs have had the head bolts replaced with 1/2" Whitworth. This is not ideal because of the difference in the thread angle, however it is probably better to use new Whitworth bolts than stretched or corroded originals. Since the torque specs for the head bolts are low, the Whitworth replacement seems to work OK - I had one engine on which all the head bolts had been replaced with Whitworth with no apparent ill effects.

Frank.

thanks Frank, i knew there was something i didn't recognise. i've got 60 degree thread cutting bits for the lathe so i should be able to turn up a few bolts to replace a few odd ones i have.

Allan.

That would certainly be better and more original than the Whitworth compromise. Do you have an original to copy? From memory the bolt heads are deeper than standard Whitworth or UNC. If you don't have a sample I could measure the depth for you.

Frank.