Categories: Technical

by chris


If any of you are mad enough to re-build your 105e Watermota engine, this might be a rare situation you might come across.

Having spent a small age stripping down, re-machining, replacing parts and painting my engine ready for re-assembly with the incredibly helpful Martin Festa Bulldeath. We started to fit replacement oil seals and got to the point of re-fitting the pistons with new rings. Only two of the pistons would go in (1 and 4), however the remaining two (2 and 3) had no intention of going in ! It was a real head scratcher as the rings seemed to gap perfectly in the bores. What did appear odd was that the two ‘interchangeable’ compression rings had different depths. Having explored the issue in depth, it became apparent that two pistons were original and two were replacements, with the replacements having shallower grooves or lands. This meant that the ‘fatter’ rings supplied as standard simply didn’t have enough back clearance.

I’d considered a couple of options (given that times are tough on the old wallet) including buying a ‘spares engine’ of which I found a couple for around £100 in the hope there might be 4 identical pistons present in good repair. This threatened to be a bit of a lottery, so decided to get a full replacement set of pistons, rings and pins along with big-end bearings.

So I’ve not had the chance to try re-assembly yet, but fingers crossed.

What I would say is that knowledge of the engine that comes from stripping it down to nothing is absolutely invaluable rather than thinking there’s some kind of black magic going on in that engine block. The confidence I’ll have in my engine will be second to none knowing exactly what’s going on.

Update 26th November 2022

The good news is that the new pistons, rings, pins and circlips all fitted however I made a basic mistake in assuming the crank and therefore the big-end bearings were standard. Whist assembling, it became apparent that in the past, the big end bearings must have spun out due perhaps to oil starvation requiring the crank to be machined and then requiring undersized bearing shells. This also accounts for why two of the pistons were different on this engine with the con rods most likely having taken damage and being quickly replaced, pistons and all, with ones from another engine.

So now it’s another wait for further bearings to arrive and the engine assembly can continue.

New 105e Standard pistons
Pistons actually fit !
however the big end bearings didn't fit as the crank had been machined by 20 thou to make up for the previous bearings having spun out in ancient history