Engine bore measurements

Got a ring compressor, expander, deglazing hone and now a bore gauge. I don't have a ridge reamer or compression tester yet.

Chris

The piston came out from below, though given the size of the ridge, I suspect it would have come out the top too.

I just found my lost feeler gauge, so I'm going to check the ring grooves tomorrow.

Chris

Hi folks,

I found my missing feeler gauge, so I've been able to check the clearances between the piston rings and grooves on my JAP engine. According to the manual, the clearances should be between 0.0035" and 0.005" 'on reconditioning'. The measured clearances are:

The manual doesn't give figures for the ring end gaps. The 1st compression ring has a gap of about 0.040", and the gaps on the lower rings are larger, much larger for the oil ring.

My gut reaction is that the figures aren't bad for a worn engine. I'm inclined to replace the rings only. Does that sound reasonable?

Best wishes,

Chris

Might be OK! You just may have more oil consumption. Are there still parts available for those old J.A.P. engines? As far as I was aware, their last product was the Seagull outboard motor. For those who do not know, they were J.A. Prestwich Industries, in the UK.

Steve R.

In my younger days, when I thought a motorcycle was the best means of transportation available, a JAP engine was probably in as a speedway or grasstrack racer or maybe a Rudge, or some other exotic breed of motorcycle.... now this guy says he's got one in a LAWNMOWER???? My goodness, an alcohol fed, 50 HP lawnmower???

I had to google around for a while before I discovered that J.A.P. made all kinds of engines.

John B.

I had one of those Seagulls -- a Silver Century. I had no idea they were made by the same company as J.A.P.

There was a famous dirt track racer here in my township, many decades ago, who rode a J.A.:P. He was one of my uncle's former students. My uncle was kind of an engine connoisseur, and thought they were pretty exciting motorcycles.

I've only seen them in museums.

Ring end gap is not as important as face gap. Your top ring gap is out of spec, but might work ok in a demo engine. There might be other problems, but you could try to just break the bore glaze with a light hone. That will allow the rings to reset with load. Some people might just put in new rings and hope for the best, but a good engine man with experience should give you a second opinion. I'd take the engine to a shop and get is properly measured to see if new rings would seat properly. Then you need to consider an oversize piston and re-bore

I got a 7hp single cylinder JAP engine from a chap I knew about 15 years ago. He had worked for Villiers and he ran it through their reconditioning program and it had sat in his workshop for 25 years or more. It came from a small stationary crane IIRC and was governed and had a huge heavy flywheel. I checked the oil and other bits, turned it over for awhile to get the oil thrown about, added a spark plug as all that was fitted was a cork, after cleaning the magneto points it started on about the second turn of the starting handle. I used to run it about once a year and the points always needed cleaning with that little use but it always seemed to start and run well on the second turn of the starting handle. The stop c*ck on the petrol tank leaked a bit but a chap I knew said they always did that. I realised in the end that I didn't need it and sold it to a chap that was going to use it for a portable compressor for diving use. He seemed impressed at how well it started on the 2nd crank, not heard from him since, I don't know if he still works for the same company a mate does.

My neighbour is more familiar with the higher spec JAP engines as he had a grand prix Morgan 3 wheeler, amongst other Morgans, with the overhead valves and that is still in existance. He tells a few tales of the troublesome acetylene lamps it originally had and fitting a bicycle back light as the acetylene generator was too feeble to run the front lights and the rear one at the same time.

Parts availability is pretty good. You can still get cylinders, pistons, oversize pistons, rings and most other parts, all new. If you need suppliers, let me know.

Best wishes,

Chris

The engine is one of these:

They appear to be highly regarded engines. I don't need one in perfect condition, just a reliable one. I've never seen a JAP 7 hp. What model was that? Mine is a 4.5 hp Model 5 Type 53. I'm pretty sure the starting handle is wrong as it wobbles around (it's an S-shaped piece of bent round bar), whereas in pictures the handle appears to be fabricated from flat bar. So I'm looking for a handle, if anyone knows of a source.

Best wishes,

Chris

Can anyone tell me what a typical end gap for 80 mm piston rings is, assuming the engine is in top condition?

Chris

Recomended .0045" per inch of bore diameter for an air cooled, or around .014" in your case.

My gut reaction with the .035" cylinder wear you mentioned in your first post is to bore it over sized. If you don't bore it at least put in new piston rings.

Aprox .007 to .010 inches.

I forgot about it being an air cooled engine when I said .007 - .010 inches. .010 to .020 would be more correct - so yes, .014 would be good.

But a 356 Porche specs .008 and that is 82.5mm bore - so the rule of thumb does not ALWAYS hold.

I hate to be snippy but Google IS your friend. Try

John B.

J.A.P., (James A. Prestwich), made the V-twin engine for the Brough Superior Motorcycle. That was the one T.S. Lawrence was riding when a crash killed him. Lawrence of Arabia. Don't know why I remember such useless information.

Garrett Fulton