ORIGINAL: Diesel Die-hard
Well. although I was somewhat hesitant to actually run this clearly unused example, I decided that the search for knowledge more or less obligated me to give it a try! It turned out that the “good” compression was somewhat illusory, being largely created by the presence of assembly oil in the engine. When diluted with a fuel prime, much of the compression went away. Despite this, I was actually able to hand-start the engine fairly easily using an oil prime. However, after six to eight seconds of running the oil prime had run through and the viscosity of the oil in the fuel had become reduced by heat. Not only that, but the metallurgy is such that the piston fit becomes looser as things heat up.
As a result of these three factors, after around 8 seconds all vestiges of compression completely disappeared and the engine simply died - no amount of fiddling with the comp screw would keep it going. I couldn't even keep it running long enough to get a picture! After a short cool-down period it would re-start easily with the oil prime, but the same pattern kept repeating itself. Oh well ..... at least it upheld the Gotham tradition apart from the fact that it did actually start!!
There’s no doubt at all in my mind that this engine would run fine with a tighter piston fit. All other fits on this example are excellent. I could of course rebore it and thus get it going, but it would then be highly non-original. So I’ve decided to leave well alone, thus preserving its original NIB condition.
I do have my original Gotham Deezil with the precision-made replacement replica replacement parts, so at least I can experience the Deezil as it should have been!!
Fit is perfect, but there is no taper in the sleeve hence the engine will run a short time before the engine stopped cause the sleeve was expanded by heat of combustion..
Do it as i wrote here:
Lets take a look at a typical lapping job - that of producing a fine finished bore and piston for an IC engine. In fact, piston and bore are both lapped in separate operations (NOT both together). All of these operations will be carried out in the lathe (and I need hardly mention the importance of keeping lapping compounds off the machine, particularly the chuck and slideways). For the bore an expanding lap is ideal, and this should be some 3-4 times the total length of the bore. The first grade of abrasive would be mixed with light machine oil (10W or lighter) and liberally coated on the inside of the workpiece. Similarly, the slurry would be added to the outside (and inside assuming it is of the ventilated type) of the lap. The lathe would be started at about 300rpm (for a nominal 1" bore) and the lap passed rapidly through the bore, keeping it moving back and forth without it coming out the bore. How to hold the lap? well, perhaps the best way is with a 'floating' tailstock holder, and failing this holding with the hand is a method as good as any. Be careful when holding the lap by hand as it's possible it may jam, hold it lightly and expect the unexpected. Remember also that unless the lap is maintained dead parallel with the bore (an almost impossible task) it will tend to bell-mouth the bore a little - hence the reason for making the work a little longer than finished size and trimming to length later. When the inside of the bore has achieved an all-over grey appearance, with the fine scratches appearing even and criss-crossing both ways, and with no evidence of any deeper scratches (as might be left by the reamer) it's time to move onto the next finer grade. The work will have to be removed from the chuck to clean it properly, and this should be done with clean paraffin oil followed by hot soapy water. The same procedure applies to the lap and all traces of the abrasive must be removed. The process continues until you reach the 'flour' grade of abrasive by which time the finish on the workpiece should be very fine indeed. A final polished finish, should this be deemed necessary, can be achieved using metal polish (diluted Autosol, or some liquid chrome cleaner). The lap should be a separate 'finishing' lap so there is no chance of contamination with the coarser grades of abrasive which might be embedded in the main lap. The piston is treated in a similar way except of course the lap is female. Work will continue with the coarse abrasive until (using the un-trimmed bore as a gauge) the piston will not *quite* enter the bore. At this stage finer grade abrasives are used and work continues until the piston will just enter the bore tightly. At this stage, it is usual to finish mating the two parts by using metal polish and briefly using the piston to lap the bore directly. Great care needs be taken but this method ensures that the fit is good for the entire length of the bore.
To test the fit is correct:
1:The sleeve and piston is dry and free for oil = Piston is tight in the sleeve
2:The sleeve and piston is oiled = The piston is moving easy in the sleeve
3: The last and important! The piston is loose in bottom of sleeve and tight a bit before the piston is passed to TDC.