Vintage Workshop
Services for Brough Superior motorcycles and their contemporaries

KTOR crankshaft last update: 03/2022

After making the crankcase and cylinder drawings I made a drawing of the original crankshaft. Having done a number of structural calculations I could just wonder why these crankshaft did not fail much more often - I found only negligible safety margins in the critical connections i.e. big end to flywheel, drive side mainshaft to flywheel. Thus I ended up increasing the mainshaft diameters by 1/8" -see drawing- and using heat treated 42CrMo4 steel for the flywheels. By the way: The bearings I used for the increased diameters are LRJA 7/8 and LRJA 1-1/8. These are single lipped roller bearings so no problem to assemble. And they have the same outer dia. as the original ones.

So, already back in 2010, I obtained two slabs of 42CrMo4 and rough machined them.

This is how things remained for quite some time until I made friends with Hannes from Austria. He was building a KTOR engine as well, but lacked a lot of information. So I helped him out with drawings etc. and he machined my flywheels.

One of the problems is machining the tapers for the shafts. Hannes solved this by CNC diving down the bores under the appropriate angle on quite a high number of lines around the circumference. This worked quite well.

I was very pleased when I received the flywheels and shafts. On a preliminary assembly the shaft could be trued to less than one thou runout which I consider good enough.

I found there was a trifle: on one side, Hannes had apparently forgotten to machine the smooth radius between the boss holding the big end and the flywheel body. This was easily put right on my trusty Deckel FP1.
Ok, I am a bit hard to please: I thought the taper bores were not smooth enough so I made a number of tapered laps and lapped them with fine diamond paste until a smooth finish was obtained. This worked quite well, but (of course) it made the crankpin go farther int to flywheel which resulted in zero end float to the conrods :-(
I am now faced with the decision to take a bit of material off both flywheels or to make a new crankpin. Perfectionist as I am I think I will do the latter as the assembly is already 0.8mm narrower than intended...


Any kind of feedback to is appreciated
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