Hole Thread Sizes

For future reference, here are the sizes of the holes at the front of the engine, and their locations. Thankfully, only two sizes are used: 5/16-24 and 1/4-28. The two bottom-most holes are the ones that will have counter-sunk machine screws in order to not foul on the duplex timing gear set.

The Chase

Using the appropriate size tap, and a bit of oil on the flutes to capture any gunk, ensures a clean fit of the bolts, without risk of cross-threading.

Tapping the Quarter

These are the lower-most holes, and require counter-sunk machine screws. I want to be sure that these are clean, and won't cross-thread.

Offending Hole

This was the badly damaged hole. The bolt I pulled from this hole was un-usable; and metric, to boot! I re-cut the threads on the bolt to 5/16-24, but they were such a mess, I dug around in my spares until I'd located the proper bolt.

Not Just for Kids

I've not been too proud to employ Play-Doh in this engine build in the past, and am not too proud to do so, again.

Think Thin

I needed the outer diameter of the stick and Play-Doh to be smaller than the inner diameter of the threads in the hole. It took some time, but I got it down thin enough.

Down in a Hole

Inserting the stick into the hole, I pressed and rolled it against the (hopefully) un-molested threads at the bottom. I then carefully removed the stick so as not to ruin the impression I'd made.

Early Impressions

A visual comparison seemed to point to this being a standard 5/16-24, but I waited a day for the Play-Doh to dry enough to press the tap against the threads and confirm.

Tapping

To ensure that I didn't get metal particles into the motor, I masked off the area around the hole so that the masking paper overlapped the plastic bag protecting the rest of the block. A little cutting fluid in the flutes, and I was re-cutting metal!