Frog 500 Glow motor

Picked up a couple of model aircraft engines from a flea market yesterday, an Elfin 249PB, beam mount, and a Frog 500 glow-motor, at a good price, wasn't expecting much. Elfin is locked up so soaking away in solvent. Frog came apart OK but big end of con-rod badly oval so will need to make another, otherwise it looks like it might be a runner, screw heads and fins etc. not butchered. Can anyone help with accurate hole centres for the con-rod please. Quite a bit of info on the web but not that dimension. Cheers Graham W.

Cross section x stroke = cc, so

stroke = cc/area of the piston

Sadly only have a 349 diesel...but I saved it from my mates skip !

He has 2 petrol Mighty Midget ? engines of his late fathers he would part with.... reconditioned , with invoices from a long time ago , and never used....

Stroke not same as con rod length. You do need a rod or estimate from the old one?

With some measurements from the engine and the timing info from here (page 121):-

http://rclibrary.co.uk/files_titles/1561/Model_Aircraft_1950_05.pdf

it should be possible to compute (or measure from a cad drawing) the required length.

Am part way through a similar exercise with a DC Spitfire. Having settled on a dimension I ran an animation in Fusion360 to check if it made sense. Now just need do final assembly, find a prop and then we see whether I got it right.

All you need to know is the protrusion (or not) of the piston and the stroke. The rest is then easily calculated from the parts in hand.

I read the interesting history, by Adrian Duncan, of that engine. i see there are copies, spark ignition and glow plug variants, and the later replicas had slightly different dimensions (metricated). Piston to head clearance may be the most important tolerance.

If a petrol variant (these were convertible to glow plug) it would appear you may have a quite rare and more valuable version.

The one I have has a centre distance measured over a big end pin of 0.2187 and little end of 0.1875 = 1.395" . Maybe a bit of ovality but should get you somewhere near the mark. You can have this one unless you want to make one for the fun of it! Just pm your address if you do want it.

DH

OZ

PM's sent guys

Returned pm. In the mail tomorrow for you

Just sat in the workshop looking at the Frog 500 I picked up recently, holding another con rod sent all the way from OZ. I can only say that the generosity of people using this forum, either with advice, or in this case providing a scarce part is pretty overwhelming. . Thanks to everyone but particularly Dallas.