Twisted crankshaft nightmares
Major drama with this modern classic
When this muchloved 1980 Z1000 started running badly a few years back, the owner parked it up. Eventually, he dragged it out and took it to M&M Motorcycles to get it running sweetly. Mechanic Jason Atreides at the Cambridge-based dealership, said: “The bike fired up and it was almost running perfectly, but not quite. Cylinder one’s reading after a dry compression test was low, so I did a ‘wet’ test to narrow down the possibilities.”
To do a wet test you put a teaspoon of oil down the plug hole. If the reading is now higher that indicates that the oil has sealed any wear in the ring area for a couple of spins. If it stays low, it’s a valve issue or a hole in the piston.
Jason added: “There was no difference, so I popped the head off and put cylinder 1 at top dead centre (TDC). The lack of carbon on the exhaust valve cut-out suggested the valve had ‘kissed’ the piston and got bent, but it still sealed during a leak test, so it must have been just the right side of the tightest of tolerances. “Then I looked across at cylinder 4, and it wasn’t at TDC. Bring it up to TDC and cylinder 1 is now slightly off. Oh no, we are looking at a twisted crankshaft.” These bikes used a pressed-together roller bearing crank and one of the journals, probably the one between 1 and 2 cylinders had twisted, throwing the timing out and meaning one valve was slightly open at TDC, hence the figure on the compression test. So, now instead of a couple of hundred quid, it’s close to a grand as it’s three to four days’ work to do a full engine strip.