A Prototype FROG 175

By Miles Patience.

Since with the help of others, Adrian has put a lot of time into creating the very detailed pages on this website about FROG engines, it may be of interest to see a surviving prototype of where it all began with the first production engine from International Model Aircraft (IMA), the spark ignition FROG 175.

My late father and I believe this to be a 1945 prototype of the FROG 175 made by George Court, whose story is told in Adrian’s main article on the early FROG engines. We purchased the engine from an eBay seller some time ago. There were a lot of bidders for it, so clearly others agreed with this thought. Regrettably, as is all too frequently the case, the seller had no real history for this particular engine, so that has been lost to time.

As many readers will know, the FROG 175 spent around 3 years in production, becoming very popular. It really set up IMA in good standing for the next 15 years of model engine manufacturing up to 1962. Despite its notably lightweight construction, it was a surprisingly spritely performer that started and ran very well, fully justifying its popularity. Its main drawback was its vulnerability to crash damage.

Having had time to look over the engine and compare it with a new-in-box production example, it quickly became clear that the prototype was a lot more complex to manufacture. As is only to be expected with transitioning from a prototype to a production-ready design, a lot of subtle manufacturing simplifications were made.

The most notable weaknesses of the first production engines were the small “mouse ear” mounting lugs, as they were extremely prone to breaking off. The prototype lugs are a good bit thicker and shaped for strength, although mounting the engine would be just as tricky.

The production version of the 175 was sent out with several items of paperwork. This included an instruction booklet, a parts price list and a detailed drawing of the complete engine. Looking carefully at that illustration, which was also used in their adverts, the engine looks to be the prototype model now under discussion.

As can be seen from the photos, there are lots of subtle differences, the more obvious being as follows:

• The crankcase and front housing are polished sand castings, the corresponding components of the production engines both being die-cast.
  

• The timer arm is fabricated from folded brass and held in place with a pair of nuts and bolts, rather than an aluminium stamping held on with rivets as used in the production models. It has been painted silver, suggesting that they already had plans to make the production component an aluminium stamping.
  

• The prop driver and nut are different. The prototype prop driver features two spikes that have been brazed in place, while the production engine uses two very short cheese head screws. The prototype nut is quite a lot flatter in profile.
  

• The prototype’s cylinder fins are turned from aluminium as a slip-on sleeve. The only fin which is integral with the cylinder is the lowest fin, which locates the separate cooling jacket in position. The jacket appears to be shrunk-fit into place. The fins on the production engines are all integral with the rest of the cylinder, presumably being easier and quicker to make that way.
  

• The cylinder head has much deeper side slots.
  

• The needle valve adjusting arm is machined all over from solid with a clamp having a square cross section. The production engine was made using round bar stock.
  

• The conrod is machined from solid. The production engine uses a casting.
  

• The piston is much more complex, using an internal aluminium wrist pin carrier. The production engine has the standard wrist pin arrangement.
  

• The crankcase is stamped number 1. The piston crown has number 7 stamped on it, however the internal aluminium wrist pin carrier is clearly marked as number 5.
  

During the course of my research, I came across a couple of printed price lists. The first, an undated list, offers the early brass fuel parts, including the aluminium “cleaning plug for jet”. The second list, dated February 1^st, 1947 offers the second and much more frequently-seen style of the short turned and knurled brass fuel nipple.

I hope that early FROG aficionados will find this material to be of interest.

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Article © Miles Patience, England

First published November 2021