Why the 15mm bore size on Diesels?
 

Hello All:

It appears that the bore size of our Diesels have the standard 15 mm dimension. Anybody have any ideas as to how this size became so prevalent?

Tia,

Franchi

Many glow engines are the same, or close to it. I guess it is a nice round number for a slightly over square .15 engine (15.0mm bore and 14.0mm stroke).

The Rossi .15 glow came out in the late '60's and I believe that it's generally considered to have had the greatest influence on the design of high performance 2.5cc glow engines ever since. It had the 15mm bore 14mm stroke. Since then most similar engines have shared the dimensions because of Mr Cox's reason above. Very often a Rossi .15 p/l will be a drop in fit into a later engine of another brand.

Well this predates the Rossi by a long shot-and the culprit is the 2.5cc FAI limit adopted in the 50s-for both FF and several C/L classes-and it was Europe driven. Bear in mind that Europe (other than the UK) has been metric for a long time so it will be normal to think in terms of mm and cm for cylinder bore and stroke dimensions. Likewise bear in mind that there were already existing 1.5cc and 2.5cc classes in tether car racing [and interestingly this sport STILL has a large following in Europe, despite largely dying out in the rest of the world] so it was merely a case of the FAI ratifying and adopting what were already existing engine sizes-with 2.5cc being the most popular of the small capacity classes. There are only a limited number of 'easy' bore and stroke options that will give you 2.5cc-and engines were already going down the route of 'square' or 'over square' layouts for revs and power-so 15mm bore and 14mm stroke evolved quite early on-and its not as if you can patent the bore and stroke dimensions, so it fairly quickly became the standard European layout for 2.5cc. You may or may not be aware though-that a school of thought held that while 15x14 bore and stroke was good for power, it did not lend itself to the best fuel economy-and a number of TR engines used 14mm bore and 16mm stroke..the Hans Hornlein Taifun Orkan and Webra Mach-II being good examples. Interestingly this was a return to early 1950s 2.5cc bore and stroke measurements-the 14mm bore and 16mm stroke being more common back then-eg the early Webra 2.5s (before the Mach-1....which was an extreme case having a 15.5mm bore and only 13mm stroke)-the Jaguar 2.5, and some of the earlier Hornlein 2.5s

qazimoto is quite correct regarding the Rossi influence-and the subsequent interchangeability of some components from brand to brand as a result (Cox Conquest, Taipan 15 Goldhead being two examples) but the 15x14 layout was established well before the Rossi 15 was even conceived, let alone put into production.

ChrisM
'ffkiwi'

Most of the modern F2A and F2D glow engines have both the Rossi's layout and the 15x14 bore/stroke, as do many of the .15 size buggy motors. The Soviet era Eastern European engines (MVVS etc) are a good source for replacement ABC Rossi 15 p/l. Same with the CS Rossi clones. The cheap Chinese ASP/Magnum .15 glow engines have a p/l that'll fit a Rossi.

Hello Mates:

The reason that I asked this question is that a "diesel" expert" informed that the reason was due to the following fact. Post WWII due to a shortage of steel, the diesel cylinders were made from frames of bicycles. This tubing would yield a cylinder that was 15mm. This tale while romantic, seemed to be a bit over the top. If true, why had I not heard about it through out the years?

Perhaps this is a well guarded esoteric secret know to only a few. In any case, has anybody learned about this idea?

Be well,

Franchi

No doubt there's a grain of truth in that notion Frank.

WW2 Chromium-molybdenum steel (CrMo) alloys have been used for aircraft fuselage tubing, rifle barrels, steel tube bike frames, vehicle gears, and a host of other applications. You can look up 41xx series steels for the whole gammit. I understand that model diesel engine cylinders, have been made from 4140 tubing grade in the past. Model engine crankshafts have used the grade in cylindrical rod form as well. I vaguely remember reading that surplus WW2 4140 rod was still available for sale and was used in a local 1980's model diesel production. I'm sure that there are better alloys available today.

Model diesel pistons have been made from the cast iron in old style auto engine camshafts, so it's likely that a lot of WW2 scrap was recycled into postwar model engines. An older gentleman metallurgist (Neil Molloy) once suggested to me that many post war British Model Diesel manufacturers used Magnesium alloys for crankcase casting because so much was available cheap via the scrap market due to it's abundance from shot down German aircraft.

I don't think that the CrMo tubing inside diameter determined the bore size though. Cylinder bores are carefully prepared with numerous machining operations. As pointed out above there's not too much practical variation in the dimensions of a 2.5cc bore anyway.