Hi folks,
I have a question for the engine guys. I'm working on a JAP Model 5 Type
53 single cylinder petrol engine mounted on a Ransomes "Mastiff" cylinder mower. The engine dates from 1962. Cubic capacity is 412 cc with a power output of 4.5 hp at 2,200 rpm. I particularly like this engine because it's solid yet very precisely built.I want the engine to be reliable so I can use the mower frequently. When I first turned the starting handle to test the compression, it seemed poor. I then removed the cylinder head and found that the piston seemed loose in the bore. I could see a gap around the piston and thought that I could even see the top compression ring through the gap (but it was hard to be sure because of the reflections). With the piston at the top of the bore I estimated the gap to be 0.025" on the diameter with a feeler gauge. The piston wobbled in both directions, but slightly more perpendicular to the crankshaft. The only other engine I've seen with a piston this wobbly would never run reliably (I just realised that the unreliable engine used a cast iron piston in an aluminium bore, whereas this latest engine uses an aluminium piston in a cast iron bore - perhaps a wobbly piston is a much bigger deal in the first case because of the differing coefficients of thermal expansion?).
But the bore looked good. I wondered if perhaps the wear was much worse on the piston than the bore, because the aluminium piston is softer than the cast iron bore, and obviously the piston is in contact with the cylinder wall all the time, whereas parts of the wall are only intermittently in contact with the piston. So I bought a bore gauge to measure the wear. I now feel a bit dumb, because I later discovered two other problems that were causing the compression to seem poor: a slightly loose spark plug and a deliberate blockage in the inlet manifold left by the previous owner to stop dirt getting inside. I have now checked the poppet valves too. The seats look good and the valves are closing properly. The compression is now better, but not as good as in some other engines I own.
I went ahead and measured the bore. Here are three pictures. The first shows the bore (that ridge looks far worse in the picture than in reality; it's actually only visible on one side of the bore, and can barely be felt). The second shows my Etalon vernier calipers, which I used to set the zero on the bore gauge. The third shows my Mercer bore gauge.
I got the following measurements with the bore gauge:
Perpendicular (to crankshaft) Parallel Top (of bore) +0.0035" +0.0030" Middle +0.0030" +0.0025" Bottom -0.0000" -0.0005"
I considered setting the vernier calipers using the top land of the piston for a relative measurement, instead of using the nominal 80 mm, but didn't try it because I was interested in the distribution of the wear. I was careful to avoid ridges and chamfers. I repeated the measurements, including setting the 80 mm using the vernier calipers (oddly, I found the vernier scale easier to read with the lines horizontal - I wonder if it's related to this?
I also measured the piston diameter using the calipers, with the following results:
Perpendicular (to crankshaft) Parallel Top land 3.126" 3.131" Middle (or close) 3.138" 3.135" Skirt 3.140" 3.136"
That looks like a fairly consistent 0.003" wear on the bore in the area swept by the rings. I don't have an official diameter for the piston, but Don Nichols suggested finding the recommended clearance between the piston and bore. The engine's manual states 'RECOMMENDED CLEARANCES ON RECONDITIONING Piston - On diameter of piston: 0.009" skirt, 0.018" top land'. Presumably that means the piston is slightly conical?
The gap around the piston really bugged me at first, but given those figures, maybe the wear isn't so bad. The clearances sound big, but I wonder if I'm making the error of expecting an aluminium piston to fit a cast iron bore as closely as a cast iron piston at room temperature? Looking at the coefficients of expansion, a temperature rise of a few hundred centigrade is going to close that gap considerably.
Almost all parts are available for this engine, so I have a lot of options, ranging from free to expensive. But I want to get this rebuild right first time.
I could:
- Do nothing.
- Replace the piston rings only.
- Replace the piston with a new standard piston and new rings.
- Get the cylinder bored oversize, fit an oversize piston and rings.
- Fit a brand new cylinder, new piston and rings.
My priorities are (in order):
- To have a reliable engine that starts easily and develops rated power.
- To maximise the life of the engine (i.e., not rebore if unnecessary).
- To save money.
Based on the evidence, my gut reaction is to go for a new set of piston rings only. But I'm unsure how to inspect rings for wear. Can anyone advise me here?
Sorry for the length of this message! I'd be very interested to hear what people think.
Best wishes,
Chris