STEP 1 – ASSEMBLE THE CRANK
We couldn’t do any measurements until we had a complete crankshaft. For the crank assembly, we went to elusive old crank-master Harry Barlow, of Pro-Porting fame. Harry has been building high-performance scooter cranks since Vespa T5s were a new thing.
As previously explained, the crankshaft parts are from DRT. It’s essentially a full-circle (for reed valve) 54-stroke crankshaft with a billet 105mm con rod, but instead of a full-circle drive-side web, Denis supplied a rotary cutaway web for my abandoned production-class project.
It would have been better if we used a full-circle drive-side web, given that we are running a reed-valve barrel, but equally this project was supposed to use up leftover engine parts, so that’s what Harry was given to assemble.
All the DRT crankshaft parts are lovely quality, but unfortunately, the billet rod intended for full-circle webs did not fit properly into the recess of the rotary web. That wasn’t a problem for grinder-happy Harry who relieved both crankweb and conrod to provide sufficient clearance.
I’ve watched Harry build crankshafts many times and there are lots of little touches that go into a hand-built crank that you won’t get on a production version: deburring of oil-slots and modifications to the webs to improve oil-access to the big-end bearing being just a couple of them.
Next, the webs are pressed together on the crankpin in a hydraulic press. Then begins the brutal black-magic of crank truing. That basically means hitting the crank-webs – very scientifically – with a large copper hammer to get the crank running true. The webs might also end up splayed so corrections are made by either levering the webs apart or squishing them in a vice.
Too much pressure
Typically, this crank proved a git to assemble with a very tight crankpin. That’s bad news for the crank builder but good news for the engine. Given that it took immense abuse to align it, therefore it should also stay in-line more readily too.
After much pressing and hammering and checking the run-out with a Dial Test Indicator (DTI) Harry pronounced it as good as he could get it.
AN AREA OF CONFUSION
One of the things that confuses people is the difference between crankshaft true and crankshaft balance because they are not interchangeable terms.
Crankshaft True is how well aligned the shaft is as it runs in the bearings, and this is adjusted as close as possible to perfect by the crankshaft builder. In theory, the alignment shouldn’t change during the life of the crankshaft. In practice, only the interference fit of the webs being pushed onto the crankpin maintain this alignment in most 2-stroke cranks. If the interference fit is not sufficient then the crank can ‘twist’ out of ’true’ when one crankweb moves on the crankpin. This is a classic problem of both Lambretta and Vespa smallframe engines when you combine a high-revving tune with a heavy flywheel or a rider who is brutal with gear-changes.
Solutions to the twisting problems are:
- Welding the crankpin to the web – this creates other problems as the cooling welds can pull the crank out of true but also prevent correction. It also makes crank rebuilding more difficult.
- Glueing the crankpin into the web with high-strength engineering Loctite. The problem with that is truing the crank before the anaerobic adhesive goes off and trying not to get any in the big end bearing.
- The best solution is having wider crankwebs; which offer more interference fit area to grip the crankpin, but you can’t have that unless you make more space to fit a wider crankshaft. This is the major advantage of Quattrini cases for smallframe Vespas and the CasaCase or Super Misano casings for Lambretta.
Crankshaft Balance is a different subject entirely and worthy of an article on its own. This ratio defines how much off-centre mass the crank has to balance against the reciprocating weight of the piston and con-rod. Crankshaft balance dictates how much the engine vibrates. It is impossible to obtain perfect balance from a single-cylinder engine without additional features such as rotating balance shafts. All you can do is adjust the position and mass in the crank webs – by drilling holes or adding weights using heavier metals – to obtain the best compromise for a specific engine.
Once set, crankshaft balance factor cannot change unless a balance weight falls out, in which case you have bigger problems to worry about than some additional vibration.
Balance ratio changes though, as soon as you adjust another factor – for instance by changing the piston weight (e.g. by fitting a big bore kit). In most situations the engine will vibrate more than it did before.
It is possible to find 2-stoke tuners who will adjust the balance factor of your scooter crank to improve smoothness, but in the real world most of us just fit a crankshaft and piston combination that are known to work acceptably and put up with the consequences. That was all we planned to do with Sam’s crankshaft; after all this is a Vespa smallframe engine with big rubber engine mounts going into a Lui chassis which had pathetically small rubber mounts as standard.
Whatever Harry assembled it couldn’t possibly be worse than a standard Vega.
Words and photos: Sticky
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