LCGB Forums

Anyone know the thread size the SIP speedo heat sensor is, please? It's between 4 and 5mm.

I think it is 4.5mm.
Ben at Rayspeeds tapped a thread into my cylinder head for my sensor, so if nobody else can clarify the size, give him a shout to ask.

It's definately 4.5. Just had mine done recently....

Just for info, here's where my sensor is fitted

Put mine, or will when I get the tap, on the other side but more or less in the same place.

I had my engine built by mike davis of mesmoto.co.uk and he fitted it to my cylinder, but the barrel and head was the work of darrell taylor who said I would get a better heat reading if the sensor was under the plug.

Train driver wrote:I had my engine built by mike davis of mesmoto.co.uk and he fitted it to my cylinder, but the barrel and head was the work of darrell taylor who said I would get a better heat reading if the sensor was under the plug.

+1

Or on top of it!

Having ran CHT sensors in various places over the years I have settled with running under the plug, the reading is more inline with standard quoted temp ranges, it also responds to changes much quicker than anywhere else.

From my experience I can confirm the info given here in this technical paper "ANALYSIS OF THERMAL LOADS IN AIR COOLED ENGINE"

The only difference I have noted is that on a Lambretta the temps under the plug at average engine load are around 175 degrees C and on the edge of the fin on the right side (chaincase side) of the barrel is around 75 degrees C.

On a Lambretta if you are getting close to 200 degrees under the plug your overheating and need to ease off, with the sensor on the fin it very hard to determine what temperature reading is a sign of over heating because there are too many variables effecting he temperature of the fin, these include:

combustion chamber temp (which is what your really interested in)
the ambient air temperature
the speed of the air flow over the head (changes with RPM obviously)
the size of the fins (the end of an Avanti head fin will give a much lower temp than the end of a standard head fin)
Squish (tighter squish will transfer heat more efficiently to the head than a big squish)

there are other many other variables too like engine load, carburation etc but they effect readings from the fin and under the plug, its just more noticeable under the plug as the sensor is as close to the combustion chamber as its possible to be.


This image is a great illustration of the varying temparatures of different parts of the head on a RUNNiNG engine.

running is important because lets say you ran an engine up to temperature and then turned it off and then took thermal readings from various places around the head and barrel, it doesn't take long for the heat from the hotter area in the combustion chamber to soak out into the rest of the head and give a pretty even average across all of the head.







The trouble with CHT sensor rings under the plug on a Lambretta is that the ring, especially the flimsy ones you get with the sip speedo is hard to get under the plug due to the recess that the plug sits into in the head. To make them fit properly and not be eaten when tightening up the plug the ring needs to be loose on the plug so it doesn't turn as the plug is tightened and a small amount of material needs taking from the head so there is room for the part of the ring that the sensor screws or pushes into, depending which type of ring your using. This problem is the one reason people decide to put the sensor on a fin or on top of the plug, but doing so makes the CHT little more than just a gimmick as it wont tell you what you need it to tell you about how loaded and what temperature your running at. All you can use it for is to see if the temps are rising or falling but unless you learn to react to the rising quick enough its likely the engine is well into meltdown before you decide to ease off. I used to use my CHT in exactly this way and know its far too hit and miss doing so compared to using it as it should be used.

Scotty , by any chance could you put any images up of the sensor you put on the top of the plug , iknow it was in jetset but thats long gone

Interesting I'm trying to work out where my temperture should be. I'm running a sip sensors on top of the plug on an Avanti and seem to get readings between 145 and 180F... It would be great to know other peoples temperatures running this set up on TOP of plug in F

Still running in but depending on ambient temp I get 50-56c under the plug cap. Next time I take the head cowl off I will fit the sensor into the head.

This has been my preferred method of attaching sensors, long before the advent of more recent technical gadgets we used this method, which give good readings via the plug. Having fitted both beneath the plug and on top there is very little if any difference in the temp readings it also picks up change in temp very quickly. I have encountered issues with sensors fitted under the plug such as the rings splitting and causing air leaks, your not guaranteed a good seal imo.

Here's a pic as requested:

What pipe do you use to hold it as everything I have tried is either too big or too tight to fit over the plug?

If you take a plug, not that they will have plenty anyway, to a motorcycle shop you will find some petrol pipe that fits well. dscscotty you're right about it picking up temp changes quickly.

I went to halsfords got given some 8mm fuel pipe.. It's a pig to get on the 1st time just pop it in boiling water and use a screwdriver to get it on.. Once it been on there a while it's easy to get on and off. Better to be tight fit. If it's lose it can read 10-20f less than it should. I've just replaced my 1st attempt it was getting lose coming on and off. Cut mine abit slimmer than the picture so the cap fitted well.

The fun starts when you try and find out what a healthy temperture is. There would seem to be a lot of difference in readings people get. I think this is down to how well it's fitted and the kit.

I think between 130 to 180f on top of the plug seems about right pending on the kit. Eden gets 347f under the plug

Good luck they work well

Warrick

Be interesting to know what temps engines seize at but I suppose that would depended on piston clearance.

It generally said that a two stroke engine is in the red when the CHT reading from under the plug is at or around 210C but as you say it depends on other things like how much load the engines under and how much power the engine has as a high power engine review hard will heat the piston up faster than the piston crown can shed the heat to the head.

I've had nip up at 180C after less than a mile when suddenly going on to and holding wide open throttle yet I have also seen the CHT reading 195C on a motorway in Switzerland while climbing a hill at lower RPM and not nipped up.

You learn when your own engine is in the red while ridding and can judge different temps in different ridding conditions.

I try to keep my CHT under 185C

On a warmed up engine, if the temp reading continues to rise immediately after you have switched the ignition off, then you should assume that the sensor is not located in the best place.

Mal*1 wrote:It's definately 4.5. Just had mine done recently....

What is the thread pitch please? Need to get one to fit my sensor.

Cheers, Mark