TOOL OF THE MONTH
Our Ralph gets quite boring this month!
If you are going to rebuild an engine you need to measure what you have got, to determine what is and isn’t within the manufacturer’s specification. The fit of the pistons in the cylinders will have a massive impact on the performance of the engine and must be within quite close tolerances to be healthy. When you see a bike belching out smoke, the chances are that the pistons are not a good fit in the bores, which leads to myriad problems. Whils st the engine is in one piece there are tests that can n give indications of trouble with the mill, includin g the ubiquitous compression test, though a leak down test is more reliable.
The only real way to quantify what you have is to take accurate measurements where the factory manual dictates, and by accurate, I do mean accurate. The only affordable tool that will do this to a sufficient level of precision is a dial bore gauge. There was a time when such kit was very costly and the preserve of engineering shops using them on a daily basis, but as with many tools, they are now far more available at a sensible price. As with all tools, there is the caveat to avoid poor products often sold on ebay.
To use this tool, one very gently grips a suitable micrometer screw gauge in the soft jaws of the bench vice.vice At this point the micrometer is set with the nominal bore size which will be shown in the manual; for ex xample, the nominal diameter of a Z1 900cc b bore is 66.000mm. The reading head has a suitable extension fitted and adjusted so that the plunger is midway through its travel with the reading head between n the anvils of the micrometer. At this poin nt the DTI (Dial Test Indicator) is zeroed. In the case of the analogue DTI or dial gaug ge the bezel is rotated until the po ointer is at zero.
In a fit of extravagance, I treated myself to a Mitutoyo digital DTI. I had tried a cheap Chinese one which was rather predictably a massive disappointment so I went to the other end of the scale. This just requires a press of a button to zero. Regardless of which type you use, once zeroed and the gauge is inserted into a bore, the DTI will indicate how far over nominal the measurement is. To get the reading head perpendicular the handle is rocked backwards and forwards until the smallest reading is shown. The factory service manual will inform you at what places the bores should be measured and what the tolerances are. The Z900s have their measurements taken 10mm from the top, 60mm from the top and 20mm from the bottom of the liner. The measurements are taken in line with the crankshaft and at 90° to it in all positions. When I built my latest Z900 the greatest measurement was 0.04mm which, when added to the nominal 66mm, gave me a total size of 60.04mm and well with the service limit of 60.10mm.
In the case of my Z900, I then used the 5075mm micrometer to measure the pistons, only to find that they too were well within tolerance. I bought a new set of standard rings from IMD as I would never consider re-using piston rings unless I was 100% sure that they were very low mileage to my certain knowledge. They not only wear, but also lose spring as they age.
If money is tight then search out a good quality, old school used instrument like the one I am using in the photographs. Whilst I did hunt for years to get the Japanese Mitutoyo digital DTI and micrometers because I love nice toys, they were still pretty expensive and, in all honesty, no more accurate than their old school analogue predecessors. New cheapo Chinese mics and DTIS are not a bargain, but what are worth hunting out are old school British manufactured instruments from the days when we had engineering factories all over this country. The old imperial mics and DTIS cost buttons and assuming they haven’t been misused will be accurate and can be calibrated. There is an app I use on my iphone called ‘Converter’ which changes pretty much anything to anything, so changing 66mm or whatever to inches isn’t an issue, though my Kawasaki manuals seem to give the measurements in imperial as well anyway.
Another benefit to having cylinder measuring gear is that you can check that a re-bore you have paid good money for has been done properly, but sadly that really isn’t always a given.
Far better to check before spending money on gaskets putting a motor back together, only to find it smokes like your nan did...