Pro X pistons??

Anybody heard of Pro X pistons? How accurate are their stated sizes? Just had my 98 xr600 rebored and engine shop supplied new 0.5mm oversized pro x piston and rings. It ran fine for 2 hours before starting to smoke badly. It appears that the rebore was done to the stated size on the box not to how the new piston measured up with the micrometer. Any ideas? :)

You should always mike the piston before boring and bore about .003 back from the finished size you need, You hone to the finished size...

From there you give what clearance is recomended by the manufacture specs for the piston, always follow there specs..........

I been doing engine and building engines for over 26 years.....

I have just discovered why my bike is not only smoking(understatement), but using the piston as an oil pump(its pumping about a spoonful a minute into the exhaust ports...question is: why and how is it still running? Well done NGK plugs). It appears to be the oil rings not sealing properly, obvious comment I know...

So has anyone ever had problems with that wavy middle oil ring and if so, any advice on installation or even a better type of ring?? Just bear in mind that I'm in the UK and parts/ advice/ help is strictly limited to having a big bank balance!!(or a nice blonde girlfriend to ask for :D:):)

Were the rings installed correctly........ :)

Just an update to the story....

Yet again the engine has been stripped, cleaned, measured, checked, new valve stem seals fitted and a new head gasket. Guess what?? No difference at all...dammit...Everything checked out ok and the bike is running with no loss of power. The big question is....why is it still smoking? After a few hours of head scratching and thrashing the bike on the track, I decided to change the oil from semi-synth 10w/40 to the cheapest 20w/50(probably recycled) to see if it changed anything. The thinking behind this move was the lack of any signs that the new bore was bedding in, even to the point where the honing marks are still visible! The only thing I can say about this new oil is that the compression is now so good that I'm struggling to kick it up(and I'm not a small guy). So once again I'm asking all my fellow TT'ers for absolutely any thoughts/ ideas before I dig a very big hole for this :)

When I pulled the valves out of the head there was a burr on each valve stem. That burr scored the valve guide and ruined the valve seals. The valve stems need to be "dressed" with a dremel tool and buffing wheels before they are removed from the head. If you have good compression, the leak maybe coming from the head not the rings. Just a guess!


the valve stem seals could be the culprit if they weren't replaced ... the new rings would give much more vacuum, sucking oil past bad stem seals ... or at least, thats been my experience with V8 engines ...

Here are some possible causes for your engine smoking-

*When the barrel was rebored they may have allowed too much piston/cylinder clearance, meaning that the ring gaps will be too big, causing the engine to smoke.

*The rings may have been installed upside down (this is very important)

*do NOT use fully synthetic oil while running the bike in, with the quality of the fully synthetic oils these days, most wont let rings bed into the cylinder, resulting in a less than perfect seal, causing the engine to smoke.

*Overly rich jetting can cause the cylinder finish to "glaze", resulting in the above.

Hope this gives you some ideas, cheers


By the way, a few months ago I rebuilt an XR400 engine and ended up using a Pro-X 1mm oversize 10:1 compression piston, it has proved to give the bike a handy performance boost right through the rev range and the bike has used zero oil along the way.

Cheers guys! Out of all the suggestions, I'm opting for the cylinder/piston clearance being too big. This would suggest why there is next to no compression when cold and why you can stand on the kickstart when hot! I am taking the cylinder/piston & rings back to the machining shop this week for a check up...I'll keep you all posted. ps..I did fit new valve stem seals after deburring the stems and it didn't make any difference at all..but hey, thats life.... :)

If memory serves me right, I don't believe you should have more than 0.004" of clearance on a new engine when cold and even that would be too much for my taste. Depending on the piston, I believe you can go down to 0.002" or possibly even 0.0015", but if you're going to be doing a lot of higher RPM stuff across lake beds and beaches, then you'll likely want 0.0025" of clearance at the tightest to minimize the chance of heat seizure. These are the numbers I recall from memory, but things are a little fuzzy right now as I'm stuck lying down on ice all weekend after having some spinal fluid removed and other things done, so please consult an experienced 650r engine builder for hard numbers if you want guaranteed accuracy, but these numbers should get you thinking.

Hey Gadsan! Thanks for the figures and get well soon...... :)

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