Crank checking and a soldering tip from Nick Prince | WORKSHOP
It’s been a while since our Mad Manx, Nick Prince showed us some of his favourite tools but he’s back sorting out twisted cranks and soldering on in the field. If you’ve got any workshop tips, favourite tools or handy hints share them with us…
True not blue
There’s nothing quite like being balanced and true in life. To this end and whilst meditating, it occurred to me that it’s the same for a scooter’s engine. One critical part of any engine, which you need to ensure is trued up is the crankshaft. A crank which is out of shape can alter the characteristics of any scooter and cause symptoms from annoying vibrations, through to more serious component and bearing failure.
The right tool for the job
It took me a few years to find a tool which would enable me to check how “true” a crank was. That was until I visited a show where I found a setup being sold by JB Fabrications. It comprises of a slotted open box section base onto which a wheeled upper assembly bolts, along with slotted mounting points for a dial gauge (which isn’t supplied).
The wheels are actually bearings and are positioned to allow your crank to be placed upon them and rotated (make sure the wheels and the crank are clean). You can then move the upper part of the assembly and/or the dial gauge to the position you need, to check your crank alignment.
A word to the wise
Needless to say, if you don’t know the origins of a crankshaft you are about to put in your freshly built engine, you are best to check it for alignment. There are loads of reasons why a crank can go out of shape: an engine seize, a bad gear change, or Neanderthal engine-building. There are other external factors like clumsy postmen or courier personnel who aren’t always as careful as they could be with vulnerable parts. As such it’s even worth checking new cranks for true if they’ve come through the post. If you don’t check and the crank is twisted, it’s a major engine strip down job, so you’re probably best off to check it anyway.
A tangled web
This tool is used to check if the crank is balanced. You spin the crank and watch the dial gauge indicator and you will see the high and low points – you then need to either open or close the webs. The webs are the circles either side of the con rod and these have to be in line for your crank to be true. The front has to be in line with the back one.
Find a chisel that is bigger than the space between the crank webs. Ensure your chisel just fits in-between the webs. This will let you open the webs (slightly) to true the crank.
Place the chisel between the webs; it should just sit right at the top of the webs rather than going down between them. Now hit it lightly with a heavy lead shot or copper hammer and re-check the crank. I’ve lent my lead one out, hence the copper one in the pic!
If you went too far opening it, squeeze the webs with a vice grip and then tap the outer web. You need to repeat the open/close process until you get the tolerance you need. If the webs are out of line hit them again.
It’s worth remembering to wear a good glove to take some of the shock of each blow, hold the crank very tight on the flywheel side and hit the back slow and hard. Try and get as close as you can to 0.05mm. The better quality the crank the harder it is to get the web to move.
I use a Vernier to check the top of the crank near the pin and the bottom of the webs opposite the pin and try and get the two numbers to match up or as close as you can, keep checking. When truing up the webs you hit them with a hammer and remember where you have hit it and then keep checking the trueness. Always practice on an old crank until you get the hang of it!
Watch the Video
Iggy made the decidedly blurry video (above) a couple of years ago, it gives you some idea on how to check and deal with a misaligned crank. The video features veteran Lambretta mechanic, Malc Anderson. Malc has dealt with a twisted crank or two in his time.
SLUK reader offer
Where do I get one from?
The crankshaft checking tool Nick uses is made by Jon Betts at JB Fabrications and is priced at £55 plus £8.95 P&P. Jon has given a £5 discount to SLUK readers. Just send £58.95 via PayPal to: email@example.com
State: SLUK OFFER when you send the payment.
His latest version of the tool is fully adjustable and tall enough to allow a Lambretta crank to spin without the Conrod fouling the base.
I borrowed a type of soldering iron I’d never used before for a stator repair in the middle of a field, as you do. It did a very good job, much better than the alternative of heating up a nail on a camp stove! It heats up by burning newsagent purchased lighter fluid and when full runs for about 20 minutes. It’s also light and easy to carry. It’s also very versatile in the workshop too, with no hindering power flex to get in your way.
Essential piece of kit
The Draper one I own was quite expensive at £33 but I have seen them selling for as little as £20. Needless to say, it’s now become an essential part of my kit.
Words and photos: Nick Prince
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