Bob,
I read all that and, I'm sorry, but I still agree with Mike Clanford and Richard Dalby (two ol' Londoner Frog experts). A few general remarks:
1. Your "quotes" are largely an Oz/Canadian opinion... OK...
2. I think we mainly have a terminology problem.
3. No Mk I engine is ever called a Mk I while it's advertised and available. It only becomes a Mk I "posthumously", when another version is released, and these MK I, Mk II designations are always applied afterwards, by people other than the manufacturer, who was just "improving his engine". It's precisely this point that makes the "Marks" business a bit subjective, i.e. people other than the manufacturer are deciding what constitutes a significant difference, and the various enthusiasts, "experts", testers, etc. won't necessarily all have the same opinion on this point.
4. To close my case(!) I'm reproducing another of GH's photos, of two different Frog 50s.
Your assumptions, and those of MEN etc, imply that these two Frog cylinders/heads are both a Mark I... The bigger head and considerable internal differences go unmentioned, an example of my observation above that assigning another Mark number (or not) is highly subjective.
5. Have a look at the head on the engine tested by RHW, and in the early Frog adverts (there's one in the link you gave). Those two heads are nothing like the head that you're calling a Mk I, and the differences are exactly those shown in GH's photo.
I'm now going back to building my Slicker!

Originally Posted by brokenenglish

No Mk I engine is ever called a Mk I while it's advertised and available. It only becomes a Mk I "posthumously", when another version is released, and these MK I, Mk II designations are always applied afterwards, by people other than the manufacturer, who was just "improving his engine". It's precisely this point that makes the "Marks" business a bit subjective, i.e. people other than the manufacturer are deciding what constitutes a significant difference, and the various enthusiasts, "experts", testers, etc. won't necessarily all have the same opinion on this point.

Yep....when attempting to acheive a true "chronology", maybe we shouldn't bother the "Mk" numbers too much, as such things are easily mixed up by "historians"...
We now know, however, that the shift from the "low" to the "high" cylinder configuration did doubtless take place some time during the period of the "crankcase No 1"...





But wait, there's more.....
Before we take a break: Brian, the comp.screw that came with my "short head" Frog 50 is not ED Baby. The diameter is too large to fit the ED, and what's more interesting, it's way too small to fit an "ordinary" Frog 50....!...this screw fits the Frog 50 "short head" only....
Not even the most notorious bodger do SHRINK a hole...

As I have a vague memory of seeing a Frog 50 shown with a "single-bent" lever, somewhere on the "net" (I think, a scan of an early ad...), I am not yet ready to dismiss this screw as a non-original.....
EDIT: FOUND THE AD !

What's your thoughts on this...?



Pic below:
I do own a thread gauge, but some pitch templates are missing, i.e. a slight guesswork had to be done...the guesswork beeing fairly close, though....
Please correct me, if something looks wrong....


Nerdy stuff, isn't it...?...but sometimes such things lead to new, useless knowledge...

Bosse, The answer to your compression screw question is that I wrote something stupid!
I've never seen a Frog 50 with the single-sided comp. screw. Even Warring's test, which is on the first model (smaller) cylinder head, i.e. a real Mk I, had the normal T-shaped screw.
However, when I checked the very early Frog ads, there it was, just like yours!
But I maintain the rest of my post.
Another point is that in Bob's references, it's stated that the later "strap" was to strengthen the crankcase...
I don't think the Frog 50 ever had a crankcase strength problem. I think Frog's first intention was to use that crankcase as a basis for an .049, and the strap was to allow for a future longer stroke, but they dropped the idea and went for the Frog 80. I didn't "invent" this theory, it's always been the general opinion of English Frog specialists (which I'm not) and I think it probably originated at the factory.

[QUOTE=brokenenglish;23112458
I'm now going back to building my Slicker![/QUOTE]

Glad to hear it Brian! I was wondering when we might see some progress

Thanks Brian, for your input..
Yeah, it may be a real early example, like the comp.lever indicates....
However, as I already run 3 (!) Frog 50 projects, all near completion, I've decided to put a stop here, not starting a 4:th....
Therefore I posted the bits in the "parts" thread, hopefully to find a backplate that I miss, and also a good home for this "project"...

Brian, Re. this FROG 50 discussion, please let me make it perfectly clear that I have no wish to upset you in any way, but you are clinging stubbornly to a notion that was false from the start and now unfortunately, is set in stone. My main concern is not to prove myself right, but rather to set the record straight, so that in 50 years time someone may be reading all this and know it is 100% correct and not hearsay. To achieve this, I will rely on the single greatest model engine expert in history, ie. P.G.F. Chinn, a person who was there at the time. If you feel that you know more than Peter, then please provide suitable documented proof. The first two scans below are from Peter's column "The Latest Engines" which appeared in Model Aircraft magazine, August 1955. The scan on the end is from "How to Make Model Aircraft" (also by Chinn) dated March 1956. Please read them carefully before refuting them, as you are getting in deeper by the minute. BOB

First scan from the FROG catalogue dated 1st March 1956 - last two from Model Aircraft dated February1952

Showing just how long that the single sided comp. adjuster lasted, here's Chinn's photo from April 1952 ("Accent on Power" column)

While we are on the subject of Frogs, I'm looking for some info on a couple of Frog 150's I have. The first one was sold to me as the first model 150R. The other one is clearly a late model DC made Frog, but I don't know whether its an R. How do you tell an R from a standard model? I wouldn't know my R's from my elbow.
That collar around the cylinder of the later one is built into the cylinder, not stuck on. I've seen a picture of a Frog 150 with a circular style muffler (or was it a throttle?), so I'm assuming this is the remnant of a muffler long since removed.

Originally Posted by Scaledown

While we are on the subject of Frogs, I'm looking for some info on a couple of Frog 150's I have. The first one was sold to me as the first model 150R. The other one is clearly a late model DC made Frog, but I don't know whether its an R. How do you tell an R from a standard model? I wouldn't know my R's from my elbow.
That collar around the cylinder of the later one is built into the cylinder, not stuck on. I've seen a picture of a Frog 150 with a circular style muffler (or was it a throttle?), so I'm assuming this is the remnant of a muffler long since removed.

The blue head was the outwardly distinguishing feature on the 150 Type R, internally of course, the main changes were to the cylinder porting. The DC made example is not regarded as an "R".

Yippee Bob, we agree entirely on the 150R!
However, for the "serious business", I think you've trapped yourself...
Look at your full-page scan on the left in post 4251...
PGFC gives four very detailed changes between the Mk I and the Mk II...
Don't you find it strange that these four detailed changes don't include the strap under the case?
Do think that the reason may be that the strap came later, i..e. after the Mk II?
For me this indicates that you've pulled out all this Mk II documentation, but that there was another very significant change afterwards.
I certainly don't think I know more than the Great Peter about Frog engines (ridiculous!), but please admit that your "Mk II documentation" above dates from before the strap and therefore isn't the end of the story.
Thus, I still think that, in your documentation, Peter is describing exactly the Mk I / Mk II difference shown by the two heads and cylinders in GH's photo, and that the strap (Mk III) hadn't yet appeared. When describing the Mk II differences, it's inconceivable that Peter wouldn't have mentioned the strap if it had been present...
Again, I think Mike's book is dead right, that's all

Originally Posted by brokenenglish

Yippee Bob, we agree entirely on the 150R!
However, for the "serious business", I think you've trapped yourself...
Look at your full-page scan on the left in post 4251...
PGFC gives four very detailed changes between the Mk I and the Mk II...
Don't you find it strange that these four detailed changes don't include the strap under the case?
Do think that the reason may be that the strap came later, i..e. after the Mk II?
For me this indicates that you've pulled out all this Mk II documentation, but that there was another very significant change afterwards.
I certainly don't think I know more than the Great Peter about Frog engines (ridiculous!), but please admit that your "Mk II documentation" above dates from before the strap and therefore isn't the end of the story.
Thus, I still think that, in your documentation, Peter is describing exactly the Mk I / Mk II difference shown by the two heads and cylinders in GH's photo, and that the strap (Mk III) hadn't yet appeared. When describing the Mk II differences, it's inconceivable that Peter wouldn't have mentioned the strap if it had been present...
Again, I think Mike's book is dead right, that's all

Brian, it says in the factory FROG catalogue "Mk. II.......reinforced casting"

Brian, if you believe that Mike Clanford's engine book is Gospel, then you probably also believe in the tooth fairy and belong to the Flat Earth Society. Here is just one of his mistakes - the company that took Gordon Burford to court was not Davies-Charlton (with regard to their 1.49cc Sabre Diesel) as Clanford states, but North American Aviation (with regard to their F-86 Sabre jet fighter).

Wow guys - I feel I should call "seconds out - round 5" I'm certainly not getting involved in this heavyweight match between these experts over the Frog 50, I did have one which, as it had the thicker comp screw and angled needle I think was a Mk II, but I lost it when my Frog Zephyr hooked a thermal and flew away forever.

On the 150/ 150R though I do feel qualified to make some comment. We (my brother-in-law and I) had an "silver onion head 150", a "blue onion head 150R" and later a "blue taper head" 150 which presumably dated from after the move of production to DC. What I can say for certain from many hours of flying all these motors in models like the KK Gazelle and Frog Talisman is that, in ascending order of performance they were (1) the silver onion head 150 (2) the blue taper head 150 and (3) - best by some distance - the blue onion head 150R, which was an outstandingly good plain bearing 1.5cc diesel. Coincidentally we also had two examples of the later Frog ball raced rear induction 1.5, the Viper, and they were superb, as powerful as the 150R but with wonderful handling and fuel economy when used in 1/2A team racers.

Being that it is a sunny Sunday afternoon I decided to do a straight comparison between my two Frogs. Same fuel (unnitrated) and same 8x4 prop. The peak rpm of the two was almost identical! Obviously engine condition is a factor but they feel like they have the same compression.

Then I tried a chinese Silver Swallow 1.5 from the mid 70's - similar externally to the Frogs. The Silver Swallow was an absolute mongrel to start but when I did get it going I got 600rpm more than the Frogs. I wasn't expecting that.