Scooterotica

Some months back on Facebook I asked if anyone had drilled & tapped their cylinder heads to accept the SIP CHT probe (as I was fed up with those red sensor rings falling apart and causing loose plugs). I've now got around to doing it myself & thought I'd share some experience. My setup is a GT240 (unaltered, non-bridged), Li150S2 gearbox, FranRace and 46/16 sprockets (at 18 stone 46/17 was hard to get on the pipe in 4th).

The first bit of pain is that the SIP CHT probe is M4.5 (0.5 pitch) and this isn’t something you’ll find typically in your local tool shop, but easy to get hold of from Amazon or eBay. The other painful thing to note is that the probes are very delicate and easy to snap, so be careful on your fitment.

The GT240 head has an unused flat space in the centre where you could fit a centre plug, which I calculate to be approx. 18mm thick, so I drilled 10mm deep and tapped to M4.5 and fitted the probe. As mentioned, be careful when screwing in the probe, as it’s easy to snap them with over-tightening, just lock it off with the nut. The centre of the cowl will need a hole in and I fitted a rubber grommet to protect the cable.

I did around 120 miles yesterday and it was my first longer run to see how the CHT probe performed. As expected, temp readings are much higher, which can be a little scary. At 50mph on the flat I would see around 170-180c, but on a longer stretch of 60-65mph riding I read 205-230c, depending on hills etc.

What’s very nice is the almost real-time changes you see in the temp reading. With changes of throttle position or going up/down hills you will see a +/- change in CHT within 1-or-2 seconds. I could watch temps creep up and then roll off the throttle a little and quickly see drops. Everyone will have different running temperatures & you get used your ‘normal’, but what I want to see is temps drop when taking load off the engine, if temps are continuously rising I think that indicates a problem. Like I said, seeing temps over 200c is a bit alarming, as I would rarely see above 180c with the under-the-plug sensor, but it’s just a new ‘normal’. The SIP sensor tops out at 250c, so whilst I’m close to that figure I’m a way off & am now not expecting to hit that, which was a previous thought.

(Also posted on Facebook 'Lambretta Street Racers (with an open mind)')

Cheers,
George.

Timely, I was thinking about this the other day for my Avanti.

Do you think that it matters how close to the centre of the head it is the better?

Hmm, not sure. I'm thinking the combustion chamber recess in the head should give a reasonably constant temperature, but I think you might get more material between the probe and the chamber by moving away from the centre? If so, I suppose that could mean that you'd get the same readings but not so instant?

I know it's not Lambretta but the Vespa BGM kit has a threaded hole right in the centre of the head over the combustion chamber for the sensor to fit.
I've seen temperatures up to 340°f.

I tapped the dummy centre plug on my GT kit for the red ring heat sensor. It's reading is below actual, but I'm okay with that. I just concerned with differential. I've got a feel now for max normal running temp and periodically check to see it's in this range. I've experimented by trying hotter plugs and it shows up in the temp.

Dd

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Shaunbond wrote: ↑Mon Jun 03, 2019 5:34 pm I know it's not Lambretta but the Vespa BGM kit has a threaded hole right in the centre of the head over the combustion chamber for the sensor to fit.
I've seen temperatures up to 340°f.

I saw 360°c this week on a good thrash about...