26/04/2025
A whole lotta shaking going on!
I never write long posts but this is an exception. For a long time I’ve been involved in building Rotax top ends onto Lambretta engines using my own engine cases. I’m not the only person to be doing this, there are several over the years but most recently these conversions are either specifically Gran Turismo or modifications from Holty and the sprinters.
There are different horses for different courses; full off-the-shelf engines get expensive so there are other routes to achieving the Rotax “experience”. One of those routes requires the use of different crank webs from Chisel Speed. These crank webs are development “blanks” based around the 50mm RD250 crank pin. However, with a blank set of webs the piston and rod weights have to be calculated and the webs machined to achieve acceptable balance. The 580 and 583 Rotax engines use 76mm pistons and 64 stroke. It isn’t the biggest but it certainly needs careful consideration. Using this configuration with a Rotax typically requires a Maico 250 conrod.
Earlier this year I was contacted by a Rotax user who had such a crank. He was massively dissatisfied with the engine vibration and couldn’t ride it far without his hands going numb. Relations with the customers crank builder had broken down so I agreed to have a look at the crank and see if there was anything I could do to resolve the problem.
On first inspection it was evident that there were several mistakes in the build that could have been corrected but weren’t.
1. It was clear that a portion of the crank pin was protruding from the web. Rather than take the time to separate the webs a little and investigate the issue the protrusion was crudely ground back.
2. The crank was pressed apart with a peak separation of circa 5 tons, that’s a good fit but not impossible to work with.
3. When inspecting the shims it was clear that the crank had been over pressed as the shims had dished over. The over pressing most certainly contributed to point 1 above but could easily have been rectified
4. There had been no attempt to add any mass off set to the webs, they were untouched. Either the builder didn’t know they had to be machined or couldn’t be bothered. This was very surprising because if you have the facilities to build cranks and you know it is for a Rotax build in a GT case you should certainly have the awareness to ask questions.
5. Even in the short time the engine had been running the shims have worn a distinct groove into the crank pin. This is possibly because they were pinched in place by the webs.
6. Approximately 0.5mm has been ground off the pin (see point 1). This has scrapped the pin (together with point 5).
7. There has been a huge amount of rod big end that has been ground away. Approximately 50% of the thickness of the big end. This was probably done because the builder didn’t know that GT cases have enough machined clearance and casting material for the rod to be fitted un-altered. Either way, such a large reduction in rod material in a stressed area is not a good idea at all.
8. This has reduced the thrust area each side of the rod, this increases the wear on the shims and permits the rod to move side to side and contact the inside of the webs. You can see witness marks where this has happened.
How to correct these errors:
1. The existing webs were modelled on CAD and % out of balance calculated at a staggering 134%, an absolute jack hammer!
2. The CAD model was then modified for simple modification on a manual Bridgeport milling machine and the % out of balance recalculated to 46.5%. Much better but close to the maximum because the web mass and machinery for modification is limited.
3. The existing webs were re-machined to match the CAD model
4. New Maico 250 rod kit supplied with a new RD250 pin
5. Crank was re-built using the RD250 1mm shims
The customer has since re fitted the crank and run the engine for several weeks. He reports the vibration issues have gone and he can ride happily without breaks to recover the feeling to his arms.
The take away learnings:
1. Dunning-Kruger effect (google it), just because you did one type of thing well doesn’t mean you’re an expert on everything. In fact it makes you a liability.
2. If you’re building something you don’t know about, reach out to the manufacturers for advice.
3. If you screwed up on a job don’t let it out the door until it’s fixed.
4. If you screwed up and the job got out the door don’t front it out with the customer, go back point 2 and fix it.
It isn’t beyond the capabilities of anyone with a press and some basic equipment to build Lambretta cranks from a kit of standard parts; this doesn’t make you a master. Where things get tricky is when crank specifications go “off the beaten path” calculations, design and machining are required. In this instance someone had a bad few days at the office, fell asleep at the wheel and drove off a cliff in full Dunning-Kruger mode.
