Engine – Repair crank

Did I ever say I invented the uprated Lambretta crank con rod conversions?

Oh I know, yes I did and I’m repeating my self. Bloody hell that was TWENTY SIX years ago, god I’m getting old!

Any way today I stripped a black MEC crank. Why do people say these are great. I’ve never know one stay in line, this is no exception, you can tell by rolling the crank on a table and look at the inner race, you can see where the mag rollers move from left to right.

This ones had a hard life, the old inner race has been heated up to remove it, I prefer to hand grind a flat on one side as shown, this has to be done carefully until a line appears, then you tap either side of the ground race to free it, it spits and drops off.

Do it carefully or you will slip like someone else has done before, some photos show grind marks in the crank. This MEC crank is having a Yamaha 110mm rod fitted so I have to machine the inner crank face as shown.

I’ve also built a SIL Indian crank for a dealer, again with a 110mm Yamaha rod.

Did I ever say I standardised Yamaha 110mm con rod?

I’ve also done a few hand built BGM cranks shown in the photos on the lining up jig. All in a days work!

Indian crank set up on our crank alignment jig

All run out measurements are done at the furthest point our from center

Indian crank modified with a Japanese Yamaha con rod conversion

Crank webs machined, stronger rod and race bearings fitted

Indian cranks look rough but on the whole assemble very well and tight, but some are cracked from the factory

This one is a 110mm con rod, probably the best one on the market

BGM crank on the jig after hand building one for a special engine

BGM with longer rod

A rough old MEC Black crank, reworked to clean up the damage

This shows Blueing where a race was heated up to remove. Lightly grind through the inner race until a thin split occours

Then tap either side with a drift, to shock and loosen the inner race

And if done right there’s no damage to the crank under the race

Except this is where someone went a bit crazy with the grinder

Mark Broadhurst any questions ask mark@mbscooters.co.uk