MORLEY’S WORKSHOP

The real red story

It seems a lot of people don’t understand what goes into making an engine or what manufacturers do to control clearances (ie: bearing and piston sizes). Let’s look at piston sizes in a Holden six or V8: These could go from size one to size 25 with one through 16 being `Standard’ and 21 to 25 being 0.005” or 0.006” oversize. From what I can remember there was no size 17-20.

Pistons were placed in a controlled-temperature room for 24 hours before being graded for size. The difference in size was measured in microns and that’s why there were so many variations. If you look at the pan rail of any of these engines you will see that the piston size is stamped adjacent to the bore so that the pistons could be selected and prepared long before insertion on the production line. As far as I can remember there were no oversize rings to go with the 0.005” or 0.006” oversize pistons.

Now for the bearings: If you look at the pan rail again, you will see that there are centre punch marks adjacent to every main bearing cap. One dot equalled a standard bearing, two dots indicated a 0.001” bearing in that location in the block only.

Then, the crankshaft will have coloured daubs of paint next to each main journal indicating standard or 0.001” on that particular journal allowing for the correct bearing to be installed into the main