Pistons get holed due to chamber temp. and pressure exceeding aluminum alloy's strength needed to contain such pressure.

Pre-ignition holes them 90% right in the center(under spark plug). Detonation nibbles and overheats the exhaust side of the piston crown.

Deto can lead to pre-ign.



cliff

I really appreciate the response MarkT. If I do get a CHT meter, I'll get the Trail Tech. I've seen some other cheap ones that go to higher temps, but the reviews mostly say they're junk. BTW, I saw a video of a guy installing the TT sensor on a scooter, and he cut the washer off the spark plug. It doesn't seem to me that the TT sensor ring is designed to act as a crush washer, but his logic was to keep the plug from being shimmed upward. I have a two-plug head, and was thinking I'd put the ring sensor under the plug I'm not using, and not removing the crush washer. Make sense?

I always remove (I never cut one, I just spin it off) the original washer and just use the trail tech.. Can be a little of a PITA to change plugs sometimes so installing on spare plug hole is not a bad idea.  (I don't run a spare spark plug, I use the stock blanking plug.)

Spacing the plug up could possibly result in an issue with the exposed thread carboning up over time...  and if you think about it, you're still using the trail tech washer to seal. 

I don't think I ever had the stock blanking plug, but if I did, it seems like that would be a good place to put the sensor. Wouldn't it read the same as it would if placed under the spark plug? I know I put a compression release in the CT-1 I had in HS, but don't remember if it had the blanking plug, or an extra spark plug.

Good luck riding while detonating and watching CHT or EGT's.

cliff

Is there really and truly a need for EGT or CHT on old Yamaha trail bikes?

Don't look at me. I bowed out a long time ago.

I really like the trail tech CHT on my 175. You can see trends, even on hard single track it's not too hard to watch, and I'll start easing off over 380f and stop just before it hits 400f. Cheap insurance, the trail tech costs less than filling up my truck (and there are cheaper generic ones out there now). My 175 developed a base gasket leak over time and I would have never seen it without the CHT, but I did see the temps higher than normal, took breaks as needed while headed home and found the issue before it caused damage.

A good EGT is expensive, and in my experience, you have to pay way too much attention to them, I have a snowmobile that came with an EGT and it's just too much for me, the temps go up and down so fast I can't mentally process what is happening while riding. More modern snow machines will retard timing and cut power as temps get too high, that's really nice.

Well said DarinM. 

Cheap and easy way to gain some piece of mind and potentially head off a disaster as you did. 

(last Trail Tech CHT I bought was right at $30 shipped on ebay...  so not so cheap anymore, but what is?)







 

I guess it's very difficult to know for sure if the holed piston was a result of not using a CHT/EGT?

If that was the cause, it would be useful to know, as obviously if a full engine strip, clean, rebuild, with new parts, can be avoided, by using a CHT/EGT, then it's well worth going down that route..........

With the almost universal use of ethanol type fuels now, jetting is certainly an issue, and spark plug colour, is not even vaguely accurate in getting the settings correct.

Perhaps something to outline exactly how to set a bike up using CHT/EGT, would be really useful, and help to avoid issues related to things like holed pistons and seized cylinders?