The missing pieces of the jigsaw
Sadly, the process of building and photographing an engine over several weeks and several venues means that pieces do inevitably get misplaced. When it came to hunting for the cylinder base gaskets (and the Pinasco instructions) for the 251 we may as well have been hunting for a Swiss naval submarine in a bowl of Alpen.
A supplied selection of base gaskets in different thicknesses are what you typically use to set the squish – the gap between the piston and the cylinder head – on any particular kit.
On the Pinasco kit, no squish clearance is given in the instructions (we did find them later). Typically most builders would then aim for around 1.3mm to ensure that the piston was in no danger of touching the cylinder head. The larger the squish the lower your compression ratio.
VIDEO: Top end build and squish check
Squish
On a previous build of a similar motor, even using the slimmest of the supplied gaskets, we ended up with a 1.8mm squish, which Vittorio from Pinasco suggested was fine for a road motor. Sadly, with only a thicker, left-over base gasket to use, Iggy’s squish ended up closer to 2.2mm; which is ‘generous’. It’s something we can always adjust later. Beggars can’t be choosers.
The bad part about building any of the exotic engines for Vespa or Lambretta is that they’re not widely stocked and there are very few shops you can just ‘nip to’ to pick up spares like you could for the kits of the 1980s.
Luckily I have a couple of local scooter workshops – Diablo Moto and Total Revamp – both of whom have built 251 engines and one of whom had a spare ‘thick’ base gasket.
Local scooter shops are a rare resource that you should support to ensure their continued existence.
Catch up on the series so far
Before we get stuck in, if you need to recap what we’ve already done you can click on the images above for the first four parts of this feature. Each episode contains videos and useful info for anybody building a large frame Vespa engine.
Last night a local scooter shop saved my life
Top end build
The assembly process for the Pinasco 251 cylinder is no more complex than any regular kit. The main difference is the inclusion of a thick brass head gasket aimed at improving cooling by aiding heat transfer between the cylinder and the head.
The 251 has a very secure head fitting thanks to 8-point fixing and the use of two dowels to correctly align the head and barrel. Two of the cylinder stud holes are larger to accommodate the dowel. On Iggy’s engine, the head gasket needed a little filing to one hole to get it to fit correctly over the dowels.
Leak-Down Testing
This is a process that is now widely used in the Lambretta world, but less so by Vespa engine builders. That’s a mistake because the results of air leaks and resulting weak mixtures are just as damaging to any 2-stroke engine.
Essentially you can conduct leak-down testing with little more than a blood pressure gauge and a few home-made adaptors.
Where to find out more?
We covered leak-down testing on SLUK in this article.
First attempt – not holding any pressure at all
How we did it?
On the Pinasco engine, it was easy because the kit we made up has an adaptor plug that is simply clamped into the Pinasco rubber inlet manifold. We then used an adjustable rubber bung to block the stub manifold for the exhaust system.
How bad is bad?
There’s no need to make sure an engine is perfectly air-tight. It might not be possible to achieve perfection without changing major components. What we are looking for is such a slow loss of pressure that at running rpm there is not enough time for it to make a difference.
- Really slow deflation is not ideal but can be acceptable if slow enough
- Slow, steady deflation can usually be traced by spraying around probable leak sites with soapy water
- Almost instantaneous deflation is harder to trace because there’s no time for bubbles to be detected
VIDEO: Leak-down test
In the case of Iggy’s 251, the pressure was lost almost instantly. The video above covers our journey to finding and solving the leaks.
The stator screw hole leak was solved with a grub screw and silicone sealant
The stator screw hole leak
Should brand-new casings have a leak? The answer to that is definitively no, but perhaps you should first address that question to Piaggio.
The Pinasco 251 is based on the Piaggio T5 engine casing, but this motor suffers the very same issue. The design is such that if the factory drill and tap used to make the flywheel screw holes goes a little too deep then it can break through into the crankshaft area.
VIDEO: Fixing a stator screw air leak
Once a stator screw is fitted then very little air can leak through, but ideally that figure should be zero!
The easy way to do this is simply to Loctite the screw when fitting the stator, but in a quest for a permanent fix we instead cut off a short section of M5 thread, cut a slot into it and used that as a grub screw. Other types of grub screws are available.
The initial test with the grub screw, retained using Loctite hydraulic sealant did not hold, however adding silicone sealant before the grub screw did the trick.
This was a permanent solution to a problem that Pinasco simply inherited from Piaggio…
Linishing reveals where the exhaust flange isn’t flat because it distorted during factory welding
The Exhaust flange leak
The bigger problem proved to be with the stubby exhaust flange supplied with Pinasco’s box exhaust for the 251. While the cylinder has very little flange area for the exhaust to seal against, the exhaust stub itself has even less mating surface.
What appears to have happened is that the relatively thin steel plate for the exhaust flange had distorted when the tubular part is welded into it, meaning that the surface is no longer flat.
Given that Pinasco do not supply an exhaust gasket it is vital to have a flat mating surface so this was achieved by linishing (using a fixed belt sander) which in turn makes the flange even thinner.
Making the exhaust flange thin is not ideal because it makes it even more liable to bend and warp if you over-tighten the two securing nuts.
Again, in the end we managed to get Iggy’s exhaust flange to seal after linishing flat and using high-temperature silicone.
It’s a shame that Pinasco didn’t choose to use the same conical exhaust manifold fit as the original T5 cylinder. That’s a solution that simply seems to work.
After linishing flat then there’s nothing more you can do. We’ll have to rely on high-temperature silicone to maintain a good seal
VIDEO: Making your own gasket
The good news
The positive part of this saga was that the engine and top end build all went together fine and the main parts held pressure once we’d made a new base gasket for the reedblock (we’d damaged the original whilst trying to find the air leaks).
Next time – we get deep down and dirty with Pinasco’s Bull Clutch system… Read about it here.
Words: Sticky
Images and video: Iggy
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