We recently brought you the first part of this Pinasco 251 feature. For anybody building a large frame Vespa engine, or indeed a Pinasco 251 there will be plenty of useful info and tips contained in the articles and videos so it should prove to be a very good reference guide.
This time we get the engine bushes in, as well as the bearings and seals.
Video: First Things First – Engine Bushes
The joy of starting with new casings is that everything is clean and that makes it easier to assemble.
The first step on this motor was fitting the engine bushes and the steel washers that sit below them. I’ve seen these mounts hammered in before but instead we chose to pull them into position using threaded rod and some ‘found’ spacer washers. You can buy lengths of threaded bar suitable for this job from most hardware superstores.
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Apparently, it’s not good to lubricate the elastic engine bushes with grease when fitting. Instead we used a little tyre-fitting soap to help ease the bushes into position. Easy enough.
It’s worth noting that the brake shoe pins are only rusty because the casings were powder coated before use
Steel washers are fitted before the engine bushes and using threaded steel rod we pulled the engine bushes into position until the steel tubes inside the bushes met in the middle.
The rear shock bush and central steel tube can be pushed into place using a bench vice.
Once the engine bushes were in position, the main engine casing could then be placed into the workshop engine cradle, so it could easily be worked on.
Crankcase alignment
One of the differences between the 251 engine casing and a normal T5 one is that there is no raised lip around the crank on one half of the crankcase to align with a matched recess in the other side. The Piaggio engine uses this lip and a single dowel in the bottom of the engine casing (between the drain plugs) to align the gasket faces correctly.
Instead, Pinasco use tubular dowels around two of the fixed crankcase bolts to align the casing. The instructions then make it clear (IN RED) that you must not try to fit the standard dowel at the back of the crankcase – despite there being a hole for it and even a dowel supplied in the early kits. This third hole was not generated in the same CNC process and therefore if you try to fit the 3rd dowel it will likely cause alignment issues.
On Iggy’s engine the crankcase halves aligned well just with the supplied two tubular dowels, leaving a flat cylinder gasket face with no step from one side to the other. This is good news, and very important in order to avoid air-leaks.
Video: Drive Side Assembly
The first step on the drive side engine casing is to fit the supplied studs to hold the crankcases together.
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We know from other 251 engine builds that there can be an issue of gearbox oil leaking up one of the studs and out under the flywheel. The gasket face area around this particular stud (arrowed) is smaller than on the Piaggio casing. The best solution is to apply plenty of sealant around these studs when assembling the cases and allowing time for the sealant to harden before testing the engine.
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If the engine does suffer a gearbox oil leak up the stud after building then the solution is to clean the stud hole and apply silicone to both sides of the washer before fitting the nut. This is the same solution as used by some Lambretta engine builders on cylinder head nuts and washers where minimal cylinder base gasket area is a problem.
We measured a small clearance of 0.1mm between the bearing and the circlip on Iggy’s casing using a feeler gauge. Alan from Diablo Moto (who Iggy seemed to drive to every 10 minutes for parts) provided a suitable shim washer to take up the slack.
Drive bearing shimming
The Vespa Largeframe engine layout has its roots in the 1940s and this motor was never conceived with performance in mind.
One of the worst aspects of the design is that the clutch is fitted directly onto the end of the crankshaft. This has two undesirable consequences.
1. The clutch acts as a second flywheel. When the clutch was small and lightweight, like the 6-spring PX clutch, it didn’t matter too much. As clutches got larger and heavier in a bid to handle more power (like the 7-spring P200 and Cosa clutches), then this mass needs to be considered together as part of the total mass of the flywheel.
2. The clutch being on the end of the crank also means that every time you pull the clutch lever you exert pressure on the end of the crankshaft, which in turn puts a side-load on the drive-side main bearing. The stiffer the clutch springs the higher the load on the bearing.
The second of those issues creates a common real-world problem in movement of the main bearing in the casing. If there is space between the bearing and the circlip, then the whole bearing can start to move (together with the crankshaft) towards the flywheel side when the clutch is pulled in.
ONE TO WATCH:
On some Vespa engines you can actually see (and hear) this in action with the engine ticking over if you pull the clutch lever then you will notice a change in engine tone and the fan will visibly move closer to the flywheel cover.
The worst outcome is that the bearing seat wears and the outer bearing can spin in the casing. In that situation the casings are soon wrecked.
TOP TIP:
The best way to prevent this issue is to take up any excess clearance by fitting a special shim washer before the circlip. Excessive clearance is surprisingly common; even on Piaggio casings. Ideally there wouldn’t be any appreciable clearance on new casings, but on Iggy’s set there was a small amount.
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The shim washers suitable for Vespa Largeframe engines are 50mm inside diameter and 62mm outside diameter. Alan’s were bought in the UK from The Bearing Company, but packs of various sizes of shim are also available from SIP Scootershop (part number 90012010). If you are building lots of Vespa Largeframe engines then having a set of these in the workshop is a good idea.
The Oil Seal Conundrum
Vespa PX drive-side oilseals are not the world’s best design. They are very large diameter but the outer rim that grips into the casing is narrow. If the interference fit between the seal and casing is weak or the casing is worn, or the wrong type of seal is fitted, then the seal can come loose and spin in the casing.
How do you know you’ve got the right type for your engine?
The rule of thumb is as follows:
If the casing has a machined ring in the seat for the oilseal, then these casings are designed to use the type of oil seal that is fully covered in rubber (or Viton).
If the casing has a plain recess for the oil seal, then it is designed to accept the bare metal type which just has a bonded (normally Viton) centre section.
Which oil seal?
- The fully rubber-covered seals were normally fitted to earlier engines with the groove in the casing. Bare metal seals were used in most T5 casings and also later PX engines where there is no groove in the seat for the seal.
- Typically, for Piaggio, there are exceptions to this rule. The first series of T5 engines for instance, had an oil seal seat in the casing with a machined groove in it but were fitted with a Viton-lined metal seal. Perhaps it’s no coincidence that these engines were some of the worst for the seals coming loose.
- According to the ‘rule of thumb’ these early engines should use rubber-covered seals, but fully-covered Viton seals were not available at the time. They are now.
- Later T5 motors – like later PX engines – moved to the plain oilseal seat and are all intended to use the metal oilseals. This is the layout used for the Pinasco casings as well.
The Ethanol in Fuel Situation
Ah, here’s the subject that is causing a lot of stress, but it doesn’t necessarily need to. Ethanol-resistant oilseals have been around for Vespas for a long time and were even fitted from new since the 1980s. There’s no need to panic.
The ethanol percentage of modern UK E10 fuel (5% – 10%) is higher than before (0% – 5%) but classic Vespas will still run on it, and while it will attack and harden old, black nitrile-rubber oilseals, they won’t fail instantly. Upgrading to ethanol-resistant fuel pipe and fuel tap washers is probably more urgent.
The sensible thing to do on a rebuild now is to future-proof your engine with oilseals made with ethanol-resistant materials made from a Fluoro Polymer Material (FPM) also known as FKM materials. Viton is the name of one particular brand of FPM produced by Dupont, but rather like Hoover, the name is often mistakenly used generically to cover all FPMs.
Another common mistake is to assume that all FPM seals are brown in colour. In fact Viton and other FPMs can also be made in black and other colours. Since people assume that brown = Viton, many manufacturers are now switching to brown-coloured seals.
Also note that there are various qualities of Viton and competing materials with differing percentages of fluorine. These can be better or worse at dealing with alcohol fuels (like methanol). It’s not as simple as just fitting a brown oilseal. Ideally you want a good quality one with a decent reputation like Corteco.
Malossi now make a Viton seal with a PTFE sealing lip that is highly regarded. For Iggy’s engine he bought the genuine Piaggio steel Viton seal from SIP.
The Clearance Question
The other issue with fitting PX drive-side oilseals is one of clearance between the seal and the bearing.
Ideally you need 0.5mm clearance between the seal and the centre of the bearing to ensure that they never come into contact. Since the drive-side main bearing is subject to side forces from clutch actuation, the more the bearing wears (or moves) the more likely the bearing is to come into contact with the seal.
ONE TO WATCH:
If you have fitted a dive-side PX oilseal and there is less than 0.5mm clearance between the inner part of the seal and the drive bearing, or if the seal actually touches the bearing then pull the oilseal out and try again with another seal. This is a job that is vital to get right due to the inconvenience and damage that can be caused by a seal failing. Even the most expensive seals are cheap compared to a wrecked engine casing.
Vespa PX seals vary in design from one manufacturer to another and some are more ‘domed’ than others. Harry Barlow of Pro Porting developed a tool to slightly compresses the ‘dome’ of the seal to increase clearance between the seal and the bearing. Dan from Total Revamp made his own version of this tool which we used to compress the seal’s ‘dome’ by around 1mm in order to avoid possible contact with the bearing.
Video: Drive side bearing and seals
There are lots of different opinions on the best method to fit PX-type oilseals, but most people seem to agree that coating the outside of the seal face with Bearing-fit Loctite is helpful in keeping the seal in position.
In addition I gently roughed-up the outside of the seal face with a file to provide a surface for the adhesive to key to.
For fitting the seal itself I use a modified Indian assembly tool obtained from BGM to tap the seal into place until the outer lip of the seal was flush with the casing.
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As supplied these tools are intended for fitting the main bearings and the outside diameter is actually slightly too large to correctly fit the seal without forcing it into the casing. My tool has been machined in a lathe to slightly reduce the outside diameter to provide room for the seal lip to compress as it is pushed into the casing.
Heat and Cold Are Your Friend
While I do have a selection of special tools for the PX bearings you can fit most of them (as shown in the video) without tools. You can do this simply by getting the casings up to 100 degrees C (easy check – it is hot enough once it will boil spit) using a blowtorch or electric heat gun. The bearings can be stored in a freezer for an hour in a freezer bag. In most instances a cold bearing with simply drop into a hot casing, but the correct workshop tools (or similar drifts) are useful if the bearings get stuck and need to be tapped home.
Text: Sticky
Images and video: Iggy
