In the last episode Boris started cobbling together a scooter ready for a forthcoming rally. When I say cobbling together, I mean it’s a ‘bitsa’ from his garage. He’s using a PX 80 chassis and LML engine, then doing a bit of tuning and fitting a new BGM 177 kits. Once he’s built it he’s got to get it TUV approved (the German equivalent of MoT) and then ride it to Venlo for the Bank Holiday.
Time is tight so we’d better let him get on with Part 2. If you need to recap, or missed part one click: BGM kit part one
With all the necessary parts available, it is now time to get the spanners out. Or rather the Dremel, as some minor adjustments will be necessary to improve gas flow. First is the carb box – as a larger carb will be fitted a small edge needs to be taken off to get a smooth intake area. Very minor indeed and easy to see if you use the 24mm carb gasket as a pattern.
Our next area of attention is the transfer area at the cylinder base. As already mentioned, the BGM’s transfer ports at the gasket face are intentionally small, so it can be fitted ‘plug-and-play’ on standard casings, but actually the transfers are much larger inside the cylinder.
There is only a thin edge covering the full size of the transfers, so it is relatively easy to grind this away in order to adapt the cylinder to your required design – from completely standard to taken out to the max, everything is possible. In my case, I will follow the shape of the original transfers in the LML casings.
These are significantly larger in size than the BGM’s original shape, but nowhere near the size of what is theoretically possible. It’s certainly a compromise – but again I don’t want to do any grinding of the casings at the moment as there is no time to take them apart. This is something to keep earmarked for when the next overhaul of the engine is due.
For the moment, I’m using the original base gasket of the LML to mark where the transfers need to be opened up at the cylinder base. The major difficulty when actually grinding the necessary bits away is that the spigot of the cylinder is in your way when getting into the edges, so besides the Dremel, I used a variety of small files for the final details. The result is sufficient in terms of aesthetics and – more importantly – will most definitely do its job.
Finally the BGM’s base gasket has to be adapted to suit the now enlarged transfers.
Why not use the LML gasket when this already has the correct shape? Squish clearance is the answer.
This term refers to the distance between piston and head at top dead center (TDC). BGM suggests a clearance between 0.8 and 1.2 mm. Obviously that is not a lot of a difference, so being accurate matters more here than when bolting on a standard cylinder that can have any clearance of up to 3mm.
In order to get the squish clearance right, the BGM kit comes with three different alloy base gaskets plus one (optional) head gasket. So we definitely want to use these rather than an LML-paper gasket, especially because we don’t know how it would survive in the long run.
Before we get to the point of measuring squish clearance, the piston and cylinder kit has to be fitted, which works basically just as any other kit for the Vespa.
The BGM alloy kit and piston are matched according to production tolerances, indicated by letters A, B, C etc. stamped into piston and barrel, so it is a good idea to check whether you have matching letters. These grading letters represent very small differences in piston size Depending on manufacturer, plated alloy barrels pare preferably use the same letter piston, but are often ok to run with one letter either side. So in a ‘B’ cylinder you can fit A, B or C, but you can never use and ‘A’ piston in a ‘D’ cylinder, and vice versa.
The BGM piston has C-shaped gudgeon-pin rings plus recesses in the piston itself to fit them, a little awkward as they have a tendency to shoot off into the most distant corner of your workshop, however once fitted, they sit super-tight in their grooves.
Another peculiarity of the BGM kit is that the head is machined so that the combustion chamber slots into the barrel rather than sitting on top of it, which both automatically centralises the head on the barrel and ensures a tight fit. Nice.
First thing that has to go on however is the base gasket, and this of course will later determine the aforementioned squish clearance. I chose the middle-one of the three to start with, which is 0.4 mm thick.
After fitting piston, barrel and head you’ll have to torque down the head to the required 16NM. This is not a lot. In this case, more isn’t merrier as alloy barrels have a tendency to get warped when too much torque is applied – not good. Also using the massive torque wrench which you normally use to fit the wheel nuts of your car will do no good as they usually do not produce reliable readings at such low figures. So the advice here definitely is to get yourself a good quality torque wrench suitable for the required torque range – it really makes life a lot easier and can be used for a multitude of other applications as well.
Don’t forget to squish!
Once the head is torqued down correctly, it is now time to measure the squish clearance. This is absolutely vital if you want a well-running machine, as you’ll either lose power when the squish gap is too big, or even worse you’ll run the risk of the piston hitting the head if it’s too small.
Despite this, people still ignore that procedure rather frequently – don’t ask me, why.
The easiest and fastest way to do this is to enter a length of solder wire through the spark plug hole, so that it is squashed between the piston and head once you rotate the engine. You can then retract the solder and measure its thickness where it has been squashed.
It’s good practice to do this several times in slightly different places in order to be sure you have an equal squish band all around. (NB: there are also more elaborate ways of doing this, however for the time being, this should suffice). It turned out that with the 0.4 mms base gasket alone, I had only 0.6 mms of squish clearance, which is definitely not enough. I therefore took the head off again and fitted the head gasket, which took the squish clearance to just about 1.0mm – perfect.
Dog clutch modifications
With the cylinder kit in place, fitting the clutch is the next step.
The Ddog clutch basket works just like a standard one, however due to the thicker material, it requires two minor changes: At first the fin inside the clutch cover needs to be machined down in order to stop the basket rubbing there with its outer edge.
Most people will know this is necessary when fitting a large 115mm clutch to a 125/150 engine, but even the smaller lip in the 200/T5 clutch cover needs some work to clear the Ddog basket.
Secondly the small lugs of the upper clutch plate need to go as the Ddog basket is thicker than the original and the lugs can’t reach around it. Their original purpose is to hold the clutch basket together against the centrifugal forces at high rpm – a task now left to the Ddog basket on its own.
The rest of the clutch-rebuild is business as usual, and also the 23-tooth DRT clutch cog normally doesn’t require any alteration. (Normally means here unlike mine, which was riveted to its base-plate off-center, so this had to be machined in a lathe first to run true… so make sure you check this before fitting.)
Finally I fitted the reed-valve and carb box.
In a (probably vain) attempt to improve the performance of the reed-valve, I replaced the original steel petals with carbon-fibre ones, still I’m sure that this is a less-than-ideal solution.
Anyway, the reed sits directly under the carb box and requires specific gaskets to suit, but apart from that it’s a straight fit.
The rest of the build is like any other Vespa engine: fit the electrics, covers and cowls. With my stator plate and flywheel originating from a 1980s PX EFL, it came as no surprise the wires were as brittle as my mother-in-law’s chocolate pie.
As being stranded with no spark is one of the lesser desirable ways of ending up on a hard-shoulder, replacing them is a no-brainer.
This is even easier nowadays as there are ready-made stator repair looms available in the right length with pre-fitted wire connectors.
Ready to fit and set up
And here we are, the engine’s put together ready to be married to the chassis.
This has been prepared as usual with new cables, petrol hose in order to avoid some obvious sources of failure when on the road. Also electrical connections for lights etc have been checked – I’m a big fan of tidy electrics as I cannot recall how often we’ve been standing at the side of the road trying to find the problem amongst a variety of loose cables of the same colour held together by terminal strips.
In my opinion, there are enough things that can fail on a long distance trip without any prior notice – so anything you can take care of relatively easy should be done.
Ruling out potential problems and troubleshooting those that pop up when test riding will be on the agenda for the forthcoming part three of this little series, plus getting the engine registered for the scooter.
To follow: The journey to Venlo
Words and photos: Boris Goldberg