The 4200 main caps are located using 2 hollow dowel pins that the main caps bolts pass through. NEVER disturb the dowels. They are not serviced as a separate part. Line honing if its ever needed needs to be done with the block upright - with an aluminum bock and steel main caps the weight of hone has to ride in the caps.

If installing main cap studs a 7/16 ROLL thread tap will JUST fit through the dowels. This needs to be done on a bridgeport type mill.

For high output (turbo - over 15# boost) there is an aftermarket billet main cap support plate available from Steve in London Ont. This replaces the stock girdle and bolts to the block side rails.


Last edited by efi-diy; 12/30/18 03:29 PM.

51 GMC 4.2 turbo
Can't solved today's problems using the same technology/thinking that created them