Top tool: Compression tester

It’s no coincidence that this item is the top tool this month, as it was used in the diagnosis of Mick’s seized Vespa. The scenario was that his engine had ‘soft seized’ and then freed up but wouldn’t restart. Mick felt that it still had compression, so was busy changing plugs roadside and trying to kick-start the scooter, even trying to bump start it at one point. Once he got the scooter to the garage via his pal’s trailer and explained the situation to me, I said straight away: “You’ll have smeared the piston over the rings and so while it does have SOME compression, it’s not enough to get it started or running.” Within 60 seconds I had the plug out and the compression tester in, throttle open and gave 5-10 ‘man-sized’ digs on the kick-start lever. The engine showed a compression reading of around 70-80psi. I note, on my tester, that anything under 100psi is a pig to start, and anything under 90psi usually doesn’t run. So it’s a very quick and simple way to diagnose that compression is the culprit without having to strip the top end roadside.

The other good use for a compression tester is to gauge wear on a cylinder. When you have freshly built a customer’s engine you can make a note of the reading. Then, say, for example, a year later when it is in for a service, check the reading again. If you see a significant drop in readings, then you know the piston rings may be wearing quickly and could perhaps do with a change. Whereas, if it’s retaining compression well, then you can perhaps save the top-end strip for another day.