Hi guys,
Can anyone help me here please?
I'm trying to identify the piston that's fitted to the bike, with a view to replacing it & it's rings.
Trouble is, I can't quite make head not tail of the numbers etc...

I tried to read what was inscribed and came up with the following:-
(On the crown)

435 P6 IV + 1/2mm OT

66-?024 (??) WELLWORTHY

Cast into the base inside is: LO-EX WP 435B


I took some pictures:-

Can anyone tell me what I need please?

Many thanks
Ian

email (option): fozzie001@hotmail.com

I haven't worked out what OT is? But 1/2mm O/S is the equivalent of 20 thou oversize. Ron

email (option): ronpier@talk21.com

LO-EX is the name for the low expansion alloy that was used for pistons at the time...The other number inside is more than likely the Wellworthy casting number for the piston blank...

66 (etc) will be the BSA part number...+.020" is the size as detailed by Ron...I'm unsure what OT means either....The numbers before the size might refer to the ring set but they could just as likely be something like a Wellworthy manufacturing code used to identify the piston application during and after manufacture.

Essentially all you need is the make of bike, model (M20) and piston size so that you know what to order when you have to get one before a rebore...BSA original pistons only went to +.040" oversize...Pattern ones such as Wellworthy, Hepolite etc. went to +.060" oversize...Ian

email (option): ian@wright52.plus.com

So would a standard piston such as this one do the job??

"New Genuine BSA Piston, M20 SV WD, 82mm STD, 66-1024, NOS 66-1190, P41"

Not too sure about this stuff, with oversize pistons/rings/bores etc... :-(

Other thing is, where would be the best place to source a replacement set of rings for my current piston?

Thanks for your replies so far guys

email (option): fozzie001@hotmail.com

NO! Absolutely you can't replace your piston with a 'Standard' piston. Your current piston is 1/2mm (020" or as we say 20 thou) oversize which means your barrel has been re-bored in the past.

You are really going about this the wrong way though. I guess you've take it apart for a reason? Smoking - poor compression - lack of power? You would do better to take your barrel and piston to an engineer for measuring properly to determine what wear has taken place to your bore and piston since it's last re-bore. It's possible that it might need another re-bore to the next size up ie. 1mm (40 thou). In which case that would be the piston you would have to track down 'Before' your engineer performs the work. Or it might be within tolerance to just require a new set of rings.

Also, whilst you have the whole thing apart, (and you say you don't really understand them). It would be a good idea to get the valve guide wear inspected along with valve faces and seats etc by an Engine engineer.

Just my thoughts, Ron

email (option): ronpier@talk21.com

Ron is right, the piston and the cilinder should be carefully measured + checked by a (preferably m20) specialist.
Isn't there also a possibility that this piston is from another bike alltogether, then 0.20 means nothing.

I had a NOS 40 thou piston and rings from Les about 6 months ago, might be worth checking out.

Scott

email (option): larkesATgmail.com

I bought a + 40 thou piston and rings from Draganfly a few years back.....

email (option): stinkypete80@hotmail.com

Thanks for the updates folks.

The only reason I took it apart was to locate & cure the oil leak. The engine was (up to that point) running good, with plenty of power and no smoke. On opening it up, it was plain to see the carbon deposits on the crown and the cylinder head, so, common sense dictates get rid of the carbon deposits. While I'm doing that, I might as well find out (for later reference) what size the piston is, in case I need to make some alterations further down the line.

At present, the bore is smooth to the touch and the rings seem to be okay too, although the piston, as you can all see, is certainly not new.

Nothing rattles or bumps inside the engine & the oil leak turned out to be the gasket (or lack of!) between the head and crank case, which I now have a replacement for, along with a replacement head gasket.

So the opening up was only really a precautionary measure, although, I wouldn't object to having a new piston and sleeve available "Just In Case".

email (option): fozzie001@hotmail.com

Well then all you need to do is put it all back together with new gaskets. But as you admit you are not expierenced in these things maybe an expert (preferably M20) engine builder may be the way to go.

I'd say it's best to have a go...Just get as much information as you can about anything you are unsure of before starting...

You'll never learn anything by giving it to someone else to do, unless they are prepared to show you the job as they do it and explain it all....

Even the chap who might do the job for you didn't know anything once. Everyone has to start somewhere...Ian

email (option): ian@wright52.plus.com

I've stripped and rebuilt a few engines in the past (modern Japanese ones)... BUT, the only thing I'm not too 'familiar' with is the size differences with pistons, bores and rings... I can do the rest :-) (I hope!) lol

email (option): fozzie001@hotmail.com

Funny thing is that the partnumber 66-1024 is the partnumber for a standard (none oversized) piston according to the BSA spare parts list. The 66-1025 number is for the 1/2mm O.S. (over size) piston according the same partslist. The partnumber on this Wellworthy piston is a bit confusing, I would definitely measure it before I started to look for a new piston and/or rings.

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

OT stands for "Oberer Totpunkt", that's Top Dead Center (TDC) in german.
You probably are issued with a german piston.

email (option): michasteinmann@gmail.com

But what has that to do with piston size? Wellworthy is distinctly English! Ron

email (option): ronpier@talk21.com

Even more confusing now!?!? As the barrel is off the crank case, and the piston is also out, I guess it wouldn't do any harm to get the dammned things physically measured by someone with the relevant tools.
Next question is, what internal diameter should a "standard" bore be?

Next question after that is, who sells piston rings? (just to reiterate, I'm only looking at options right now)

email (option): fozzie001@hotmail.com

Standard bore size is 82mm (3.228") Yours of course is plus 1/2mm (.020") Any rebore specialist will have the required internal micrometer to measure the bore accurately.

The piston skirt at the bottom is ground slightly oval and the widest part is found towards the front and back of the piston. The bore has to be such that it gives this wide part a .003"-.005" (3 to 5 thou) clearance to allow for piston expansion. When your bore is measured against your piston skirt, 7 or even 8 thou is acceptable as long as new rings are fitted and gaped appropriately (8-12 thou)

Anything over 8 thou clearance to your piston would require a rebore and start again with the next size piston.

If you are dealing with an engine machine shop who is more used to working on modern stuff, you would need to take these figures along with your barrel and piston for appraisal. Maybe ask around your area, who is the go to guy who is OK with old bike engines.

I'd start with old Les at Russell's first for piston and or rings. Ron

email (option): ronpier@talk21.com