PINASCO 251 BUILD PART 3 – GEARBOX & PRIMARY DRIVE BUILD | WORKSHOP

So you’ve watched the previous part of Iggy’s Pinasco 251 engine build where we delved into the shady world of drive bearing shimming (yes, that’s a thing) and fitting of drive-side oil-seals. If not, go back to Part 2 here.

You can also revisit the first part in the series here.

For this episode we are gnashing teeth and grinding gears. Actually, we aren’t. This bit all went together quite smoothly.

Measuring the gearbox end float (clearance)

Fearing the gearing

Pinasco’s engine-build instructions are pretty sparse, but they do recommend an upgrade of input gears (also known as a ‘Gear Cluster’ or ‘Christmas Tree’ for obvious reasons) to a special Pinasco close-ratio part that meshes with the four original Piaggio loose gear cogs.

Putting up our Christmas Tree early

The recommended part (part no 25270810) has 12-13-17-20 teeth and is designed to mesh with a standard 35-tooth gear from a PX200. This provides a shorter ratio for 4th gear and works well in conjunction with a 23/64 primary gear.

Following instructions

Iggy clearly did not get this memo or the required parts. Not only did we have to salvage his Piaggio Christmas tree from his old engine, but he also bought a new BGM 36-tooth loose gear to use with it. Hopefully that’s better quality than the later Piaggio production ones. I’m glad he’s experimenting with this configuration. I’m sure it’ll all be fine…

Shimming up

The late PX (‘EFL’ or ‘Arcobaleno’) gearbox has 2 shims (one at either end) which can be varied in thickness to set a desired clearance of 0.15mm to 0.4mm (according to Piaggio). The clearance should be measured with 2 feeler gauges on opposite sides in order to get a true reflection of the gap without tilting the shim.

Why is it important?

This measurement is important because too tight a clearance will waste power in friction as the gears spin against one another. Too large a clearance allows the gears to tilt on the shaft and promotes gear jumping.

ONE TO WATCH:

Iggy had bought a GR Gears output shaft for this project, but thankfully, we elected to reuse Iggy’s original Italian shaft rather than fitting the new Indian shaft. GR Gears were an Indian supplier to the LML factory so you’d hope that their products would be good quality. However, this twisted LML output shaft found recently in an engine by Dan at Total Revamp suggests that their recent production may not be up to scratch. This is not the only one that we’ve seen that has distorted like that!

VIDEO ABOVE: Gearbox assembly and shimming

Rebuilding the Primary Drive

Given that we were re-using Iggy’s original Christmas Tree (after we removed the fairy from the top) it meant that we had to remove the original Piaggio helical primary drive cog, and replace this with the Pinasco straight cut one.

I started to get giddy with the pillar drill, removing the rivets that sandwich this assembly together before realising that this was a waste of time as we were replacing the cog anyway. It is infinitely simpler just to grind off the tops of the rivets and strip the cush-drive assembly that way.

Cush-Drive Rebuild You Say?

Yes I did. This infernal assembly of springs, plates and rivets holding the primary drive cog to the gear cluster is a cush-drive. The springs inside are there to absorb sudden forces and protect the engine and transmission from clumsy gear changes or sudden clutch releases.

ONE TO WATCH:

It is not uncommon for hard-used large frame Vespas (especially ones that jump out of gear) to suffer from cush-drive failure. If abused, eventually, the springs fracture and the sandwich plates push apart off the rivets. If you have a snatchy on/off clutch action and the scooter wants to wheelie every time you let the lever out then there’s a good chance the cush-drive needs rebuilding. Clutch and cush-drive problems on large frame Vespas go hand-in-hand.

Cush-Drive rebuild kits

Vespa PX cush-drive repair kits are cheap and you need to rebuild one if you ever change the large primary drive gear. No matter what experts say, it’s not the easiest job to get right first time at home with basic tools. If you assemble the cogs the wrong way around (see video) or rivet incorrectly then the job can end up a mess. The plates can warp, the rivets can stick up proud and rub the engine casing or it can all come loose and need redoing.

TOP TIP:

Timo Drack from TD Custom in Germany came up with an excellent tool to hold the primary drive assembly together correctly for riveting. I used it for the first time in the making of this video and the results, using a concave-nosed punch, were very good. It’s certainly a lot easier to do a good job using this tool than by riveting freehand or trying to crush the rivets in a vice. I’d highly recommend this tool.

Video above: Primary Drive Repair

This process is exactly the same for the Pinasco 251 engine as any other Vespa large frame.

Lubrication

After checking the bearing surfaces for wear on both the shaft and Christmas Tree itself, the original Piaggio shaft was deemed fit for re-use. This thinner end of this assembly runs on loose needle bearings. This arrangement is normally very reliable but if starved of lubricant the hardened surface of the shaft and tree can start to crumble and break down.

Refitting the drive shaft is not too much trouble if you use the tried and trusted method of slapping loads of grease into the shaft and using it to hold the needles into place while you push the shaft home into the casing.

We put a magnetic tray covered in clean paper below the shaft to catch any needles that fell out but the grease did the trick.

Video above: Primary drive fit

The gear input shaft nut should be torqued to 22-25 lbs.ft (30–34 Nm). It should be secured with a fresh tab washer.

ANOTHER WAY:

Some engine builders suggest only fully tightening the input shaft nut once the engine casings are together to aid shaft alignment.

Text: Sticky
Videos and main images: Iggy
Additional images: Sticky

Thanks for help and assistance to Alan at Diablo Moto, Dan at Total Revamp, Christiaan at PLC Corse and Piergiorgio and Vittorio at Pinasco for help with this article.