I've been using various examples of the FROG249 BB for almost 50 years now, with complete satisfaction. I have never experienced a problem with the cases splitting, nor have I previously heard that this was an endemic issue with them, as it was (for example) with the AMCO 3.5 BB models. A lot of reputations of this nature get started because someone has a problem with one example, immediately concludes that it must be endemic and spreads the word. This may be such a case. I recall several engines which acquired undeserved reputations for weak wrist pins, for instance, simply because a few owners over-compressed them or put them into a hydraulic lock.

I've always found the FROG249 BB to be a well-made and well-mannered engine with a more than acceptable performance by the standards of its day. In fact, at the time of its initial release in late 1955 it ws the most powerful mass-production 2.5cc diesel on the British market. The earlier ones (up to 1962) were made by International Model Aircraft, while the later ones were made by Davies-Charlton Ltd. In my experience, the IMAexamples are the superior products, although the D-C ones are generally quite acceptable. You can tell the difference by the serial number - the ones made by D-C have a "T" prefix. What are the serial numbers on your examples?

There are two versions of the 249 BB. The "standard" ones with plain cooling jackets have Oliver-syyle porting with the transfer ports drilled upwards at an angle through the pillars between the exhaust ports. The "modified"versions have red-anodized heads and feature horizontally-sawn transfer ports below the exhaust openings, along the lines of the A-M diesels and many others. The "modified" engine was supposedly more powerful, although the difference in my experience is not that great. But more to the point, the "standard" version is far more amenable to careful tuning by someone who knows what they're doing. My own tuned example is installed in a "Chaos" vintage diesel combat model and hauls that large airframe around at a rate of knots! It's very little down on a standard Mk. III Oliver for performance in my direct experience.

I wouldn't have any reservations about using these engines. I've doneso for years, with no problems whatsoever.

Good luck!!

Interesting! As stated earlier, I've been using these engines for almost half a century, as have a number of my mates. Between us, we have yet to encounter this issue. Sounds as if the experience of ChrisM (ffkiwi) is similar. Clearly it can happen, but I still don't believe that this is an endemic issue - if it was, more of us would be experiencing it!

I have also yet to encounter a shaft breakage with one of these units, either with my own engine or anyone else's. I think that Raglafart was a bit unlucky there - I'm satisfied in my own mind that this too is not an endemic flaw. That said, any model diesel can break a shaft - all it takes is a manufacturing flaw in the component or some heavy-handed treatment in service.

I had a look, and I can't readily see any design differences between the red -head and plain-head cases. Perhaps they switched to a stronger casting alloy??

I agree with Raglafart that the switch to sawn ports from drilled ports with the so-called "modified" engie was a step backwards in design terns. My own tuning efforts have clearly showed that the original Oliver-ported version had far more potential. They should have stuck with that and developed it further. A well-tuned one of these can give a Mk. IIIOliver a good run!

Finally, the "finger-biter" issue noted by John C is not unfounded - in fact, it's a common issue with ball-race diesels, in particular those with short strokes. Short stroke engines tend to snap over faster than long-stroke ones in any case, andthe addition of ball-races simply exacerbates this problem by reducing the snap-over resistance! This is why Olivers are so user-friendly - that slightly longer stroke.

Cheers!