Hey guys I've made a few postst (man this site is huge)but I though I'd intoduce myself;
I bought a 1979 xt500 last month and I am very green behind the ears. The bike is in great condidtion for its age and starts on first kick. I was origianlly looking for an xt350 from the early 90's but somehow I found this 30yr old bike and really liked the look and lower seat height but after a few rides I was starting to think a 500 was too much for a such a newb rider as myself...

The bike had a low rpm sumble under load in the higher gears but it seemed to get smoother after swapping to a K&N drop-in eventually I'll learn how to fix that with the jetting... I'll also eventually figure out how to to a FULL 12v conversion so I can run better lighting and possibly an MSD box.

The biggest thing right now is that I thought I had bought a stock xt500 with a supertrapp and a top-end re-build @ 12k (now has14k) well I talked to the PO the other day trying to figure out why it had a shorter Sparkplug in it and he said @ 12k the origainal owner (I'm the 3rd) rebuilt the top end and put in a Higer compression piston. So I have no idea weather the top end was rebuilt stock or improved.
As far as the piston goes I am guessing a 10:1 because an 11:1 would ping with midgrade wouldn't it? Or am I totally off?
Would it be safe to assume this is a 540cc or do they make those pistons for the stock bore?
And oddly with all this work done It still has the stock carb.
This thing is quick! I have not had it much past 1/2 throttle yet and that was kinda scary lol.

I will be using it mostly as a gas saver (40MPG last tank)

Ok sorry for the long post but thanks for reading.:ride:

Dropping in a 10 or 10.5 to 1 piston was a common thing to do and I'm sure the cylinder had no wear, so stock size could be a save bet...Those engines were made for off idle power and don't like to rev out...You can spend a sh*t load of $$$ and get some HP on top...Different carb/cam/exhaust system helps a lot, but destroys the idea of the bike...Have fun with the bike and save your pennies for that KTM 530 plated dirt bike...

its power fells pretty constant from 2500-6000. I from what I've learned on here I guess make the 1100-2500 range smooth out more by messing with the pilot jet.

Did the 10 or 10.5 piston require a shorter plug? Is there a way to find out if its safe to put the stock plug back in?

ghkj21 said:

the stock plug (BP7ES) is 3/32 longer than the plug it came with(B7ES).
I'm going to attempt an engine off clearance test. starting @ TDC I'm going to poke arround with some feelers and then I will slowly cycle the piston and see if it contacts the plug...
I will post results in case somone else with a tt500, xt500 or sr500 encounters the same situation.

RESULTS;

Ok I could actually see the piston @ TDC. I sild a small clean metal ruler into the cly to check piston depth. Do not leave the ruler in there while finding tdc becuase the valves will hit it and possibly score something or get stuck. I used hotter pulgs because of my short distance low rmp riding style but measurmens are the same through the ranges of heat. Plugs were gapped to max of .032 and gaskets were pre-crushed before measuring;

gasket seat to piston; 1 8/32
BP6ES gasket to tip; 28/32 clearance 12/32
B6ES gasket to tip; 25/32 clearance 15/32

that leaves (3/8) when using the stock plug with my high compression piston.

the plug nolonger comes out black and fuel covered after idleing and power feels smoother.