Jp3 Js3 differnces

[Jake65mm]

I was doing some research On an engine I will be picking up at a show next month, it appears to be a Jp3 but the tag is missing. I have heard that there is also a Js3 that has a troublesome center bearing. Is there any was to decipher a difference?
Thanks
Jake

[listard-jp2]

It is easy to differentiate between a JS3 and JP3. As only the JS3 will have oil feed lubrication to the rocker shafts, also the JS3 will not have compression changeover valves.

The JS3 engine is a high speed version of the JP3 engine, and is supposed to run at 1500 RPM, compared to the JP at 1200 RPM. This increase in speed was what caused crankshaft problems you are eluding to.

If you do have a JS3 engine, that will be a rare engine, as very few were made compared to JP engine.

[Jake65mm]

Thanks for the info that makes since now!

[richardhula]

It is easy to differentiate between a JS3 and JP3. As only the JS3 will have oil feed lubrication to the rocker shafts, also the JS3 will not have compression changeover valves.

The JS3 engine is a high speed version of the JP3 engine, and is supposed to run at 1500 RPM, compared to the JP at 1200 RPM. This increase in speed was what caused crankshaft problems you are eluding to.

If you do have a JS3 engine, that will be a rare engine, as very few were made compared to JP engine.

Sorry beg to differ. The short lived JS3 did have the pressure fed rockers and higher rpm, but being an interim model retained the compression change over valves of the JP series. The JS was only made in three cylinder configuration and with the higher rpm, did suffer from the centre main bearing failure you mention.

It was with the introduction of the slightly later JK series with improved crank bearings, and which covered the full cylinder configuration range of the JP series, that head modifications fixed the compression ratio at 17.5:1. This was essentially achieved by a blanking plug with square head replacing the COV wheels. It was found that this fixed compression ratio was an adequate compromise between reliable cold starting and engine longevity.

Rebuilt even rarer marine JS3M

[listard-jp2]

Thanks for correcting me, you are quite correct that the blanked off compression ratio changeover valves only made an appearance on the later JK [and of course 8/1, and 16/2] engines. My apologies for providing miss leading information, my memory is not what it used to be.

[listard-jp2]

Since this is a post concerning Lister JP3 engines, here is one that recently sold on ebay UK (complete with generator) for surprisingly little money.

http://www.ebay.co.uk/itm/162135259658