Below page from the book "From Milan to Molsheim" about the different T39 engine dimensions:



The results of the 1926 Klausenrennen:


In class F, racing cars (Rennwagen) 1100-1500 cc, we find Georg Kimpel from Ludwigshafen/Germany with a Bugatti having following specification:
- Year built - (19)25
- Cylinders - 8
- Bore - 60 mm
- Stroke - 66 mm
- Volume - 1492 cc
- HP - 80
- Revs - 4500
Without any doubt an unsupercharged T39, of which I am convinced it was #4603 which after having been used as works car at Montlhery and Monza was sold in March 1926 to Edgar Morawitz of Prague, then to Georg Kimpel, and later to Willi Cleer.

If the final version of the T39 with dimensions 60 x 66 mm has been developed only in 1927 - how to comment these data? Please consider that the a.m. result list was published by Automobil-Revue shortly after the event in 1925, so it is a prime source.

What was the question?

If the final version of the T39 with dimensions 60 x 66 mm has been developed only in 1927 - how can an T39 built in 1925 can have such engine as early as 1926?

Or in other words - how reliable is the engine data shown in "From Milan to Molsheim", and what is their source?

Hi Michael,
also I don't understand your question, as we to know the type 39 1925 was already driven

1926 Bugatti catalogue presenting the 1.500cm³ type 39

Seems that nobody took the time to read the page I posted above as graphic.
So here are the essentials Hucke/Ulrich/Kruta are stating in their book:

The original 5 unsupercharged T39 from 1925 had a bore x stroke of 52 x 88 mm, by using a T35 block with reduced bore. For 1926 a supercharger was added with bore and stroke remaining unchanged. For 1927 then the cars prepared for the GP de l'ACF had 54 x 81 mm, but the power output was still disappointing, so they finally changed the engines of these cars to 60 x 66 mm by using the same crankshaft as that used for the T36 (1100 cc) model.

I don't know Michael Ulrich, but the late Uwe Hucke surely was one of the most knowledgeable Bugatti historians, and also Julias Kruta is no nose picker. For me "From Milan to Molsheim" was always some kind of a bible, despite some small mistakes.
So if even the official Bugatti catalogue for 1926 shows 60 x 66 mm, how can they write such b...s...?

Comparing any bugatti book to the bible is very appropriate like the bible most are full of mistakes mis translations and dogma that has become fact until proven wrong by looking at the original source documents.
Perhaps ask Julius directly he is a lovely chap and presumably as official factory historian he has access to the factory records ie he could check the engine book.

I tried to buy a 66mm crank there are more around than you may guess as most are now running as 2.3's even the 51A's are rebuilt as 2.3's sad really.

GRAND PRIX BUGATTI [Conway]

Hucke/Ulrich/Kruta imply that Bugatti used the T35 crank with 88 mm for the first T39 models, and the T36 crank with 66 mm for the later ones. For me this sounds unlogical, as it would have been easier to use from the beginning the T35 block with 60 mm bore, but a shorter (66 mm) crank. The next step then is also logical, to take the T39 engine and reduce the bore to 52 mm in order to create a 1100 cc unit for the T36. In my opinion an engine with dimensions 52 x 88 mm as extreme long-stroker would have been no competitive high-revving racing engine.