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Trevor, It's probably because of the continent I grew up on, but I didn't understand your comment.... Charles, The Trimble method doesn't include any machining after the heat treatment. The results of his mathematical analysis are that the correct size spreader dowel during heat treatment will...

I use George Trimble's method to make my piston rings. The only change I've made to his process is to use a normalization temperature of 975F rather than his originally published 1475F. In addition, I use a 200 lumen flashlight to check the fit of each finished ring inside a cylinder before it's...

A shroud for a pair of 60 mm fans was machined from a block of aluminum and painted to match the radiator. When mounted, the fans stand 3/8" off the rear of the radiator, and the air they draw in around the sides of the shroud is blown over the top of the engine. The 6V fans are powered by the...

"Regularly I use 6086 T6 alluminium for regular stuff cause it's easy to get where I live in Ireland." ... Just curious, what is 6086 aluminum and why is it so easy to get in Ireland?

Chuck, Yes, the clamp dampened the tall skinny workpiece during cutting. - Terry

The radiator was painted with matte black Gun Kote which requires a 300F oven bake-out to cure. In the past I've had trouble with JB Weld out-gassing underneath this paint and creating bubbles in the finish. So, after the epoxy had been allowed to cure for a full day, the radiator received a one...

The three parts making up the radiator were machined from 6061 aluminum. Both tanks and the core's exterior were machined on my Tormach, but I did the core's interior manually on my Enco Bridgeport clone. Hogging out the interior required a 3-1/2" cutter stick-out, and so a 3/4" diameter end...

Ghosty, Yes, I could. At first I thought that using a couple of them would be the solution to the starter shaft problem as well. The problem with pc radiators is that their inlet and outlet are on the same end of the core making them awkward to use in a model engine. My earlier Offy coolant pump...

The effectiveness of the model engine coolant systems I've had experience with don't seem to scale very well with engine displacement. There are probably some hard-to-fix reasons for this, but there may also be a few things that can be improved with a re-think of the radiator. The Offy's block...

See posts 423 and 424 above ...

As mentioned at the very beginning of this build, in addition to a slight repositioning of some of the head bolts, the diameters of the liners were reduced by .030" to improve the robustness of the head gasket. These changes in turn required changes to the pistons, and a sketch shows their...

Jon, The rods and caps were machined together at the same time, but there were two separate pieces of aluminum bolted together with the seam between them being the seam between the rod and cap. The blanks for the bearings were turned a calculated amount oversize and after parting off, the...

Elliot, I used a spark ignition with two CDI's. The distributors were disguised to look like magnetos. - Terry

A simple fixture was made up to support the rod caps while their rear ends were finish machined. All went well, but then I discovered the diameters of the previously machined big end bores had all come out different. After retracing my steps I found the bores had been growing by a thousandth or...

My rods were made to Ron's drawing with only a few minor changes. I've never actually seen an Offenhauser rod, but I suspect they were probably designed for clearance inside the close-fitting crankcase. Here is a photo I found of one being offered for sale on ebay:

The connecting rods were machined from 7075 aluminum. The big ends will be fitted with bronze sleeve bearings, but hardened steel wrist pins will turn directly in the aluminum on the small ends. An oil passage through each rod will connect the two. One of the earliest operations in the workflow...

Those jaws are hardened steel (real hard). You would have to anneal before drilling any holes and likewise before welding. Brazing will destroy the heat treatment which will need to be redone, and then re-grinding will be needed afterward. I don't think a repair is practical. - Terry

Edit: problem solved with an ipad reboot. Sorry for the noise. Terry

See this thread: https://www.homemodelenginemachinist.com/threads/looking-for-ron-colonna-offy-plans.27301/page-3#post-321750 Ron Colonna designed this engine and sold the plans as a book and more recently on a USB thumb drive. He still might be willing to sell you a copy. - Terry

That spline-looking thing on the front is actually a spur pinion that drives the oil and water pumps and the camshafts. The smooth portion at the very front of the crankshaft is a hardened sleeve that's one-half of a sprag clutch that is built into the front of the engine for starting. If the...