Sherpa 190 Piston Markings And Wiring

[d250b]

Could someone please enlighten me to the markings on the top of the piston please? was intending fitting a new set of piston rings before reassembly seeing as how I'd come this far anyway; so need to know what to order.

I'd like to re-wire the ignition and join the wires better than they are at the moment, any idea what sort of wire I should use, i.e thickness/rating etc??

Thanks

D

[john collins]

Hi

Hope I can help

In theory your Model 190 would be a 238 Sherpa about 1977 / 78

So - 238 cc BUT many people bored them out to 250cc , not least because there were a lot of 250 pattern pistons available at less cost etc, or just wanted to turn them into 250, or have been bored several times. A 238 in theory can be bored out about 8 times ( although we usually missed a size out to allow for ovality) Even after that Sammy Miller etc used to sell extra size pistons ie 73mm up

Ah - the good ole days!!!

The 238 had a 71mm bore and earlier ( up to 1975/76 ) all 250cc were 72 mm bore

The markings you show are not that important. If you look underneath te piston, on inside skirt, it will tell you 72mm if 250 or 71 if 238

These are standard bore sizes - if there are no markings underneath - just put vernier across and it will be 71 or 71 plus - or 72 or 72 plus - The " plus" I refer to is when it is fitted with an oversize piston. I

If you clean the top of piston with some wire wool etc , you will find out if it is " oversize" as it wil be stamped, usually toward front ie + 25, or + 50 or + 75. This means if you had the original 238 piston and +50 on it - size will be 71.50. If you have a 250 piston fitted and it states + 50 it will be 72.50

If not oversize mark ie the + is shown it will be a standard.

Wiring - easier. From your engine you should have

Black - Connect to your HT coil

Green - Connect to a good earth point - I use one of coil mounting bolts ( clean paint off frame)

Thats your ignition sorted

The yellow only connected if you want lights

The red - again rarely connected for brake lights

[d250b]

Thanks very much John. I will get the piston out and look for the marks and rub off the top properly.

I've got black, red, yellow and orange wire and I don't have any lights!!! I have two condensers with a spare up behind the steering head of the frame and I assume the wiring is normal but its just be redone at some point with odd bits of wire. What I was wondering was does the wire need to be of a certain rating for current/amps/volts etc, I want to get the right thickness for the job. I have some auto wire in the shed but it looks thinner than some of the wire on the bike.

D

[dadof2]

The standard wire looks to be about 0.75mm, using 1.0 or 1.5mm will minimise voltage and current loss, leaving more energy for the spark.

[stevem75]

On your piston it says T2+. As others have mentioned if it was an oversized piston you would see .25 or .50 etc etc depending on what oversize it is. Since yours has no oversize markings it is still on the standard bore. The + mark is important. This denotes the finished bore size for that piston. In this case the bore size should be between 71.050- 71.060 mm. Let's say for example that the piston says T2-. This would indicate the finished bore size would have to be 71.040- 71.050mm. Having said this if you decide to get new rings the markings on the piston don't make a difference to ring size. Piston clearance can be a hotly contested debate amongst owners. Bultaco did extensive research and testing with their engines. If you decide to rebuild your top end and use a stock mahle piston I highly advise that you follow their instructions for finished clearance. With modern two cycle oils and proper break in you'll be fine.

Steve

[d250b]

Thanks Steve. I was hoping to stick with the piston I have, my neighbour who is a mechanic reminded me that there's a chap in the village who builds custom rally car engines and he would have the kit that could measure the piston and bore accurately.

Cheers

D

EDIT: I ordered an exchange coil from In. Motion and it comes pre-wired so the wiring problem is taken care of.

Edited May 31, 2014 by d250b