A quick piston question

fiatfan · Dec 21, 2015

I have an 850, engine number 305010. Looks like I´m bound for a rebore, can squeeze a feeler gauge .45 mm :shock: down beside the piston, about 1/2" down from the TDC. The pistons are +0.20, but there´s no big "edge" at the top of the bore, can just about feel it with a finger nail. (I´m borrowing a correct measuring tool from a friend tonight) Now to the pistons: the ones in my engine and many for sale are flat, no grooves for the valves. Then there are other pistons, also flat top, but with grooves for valves. And then there´s the high top versions, which of course are high compression pistons, although they are sold as standard 8.5 CR in some places, which must be wrong, right?
Have seen a pair of +.40 pistons at a great price, they are "EMGO" (EMCO?). Any good? Info on this is appreciated!

Tommy

L.A.B. · Dec 21, 2015

fiatfan said:
Now to the pistons: the ones in my engine and many for sale are flat, no grooves for the valves. Then there are other pistons, also flat top, but with grooves for valves. And then there´s the high top versions, which of course are high compression pistons, although they are sold as standard 8.5 CR in some places, which must be wrong, right?

The photo may not always show the exact pistons so unless you can be more specific-for instance, the seller below (The Green Spark Plug Co.) uses the same image for all their Hepolite BSA, Triumph and Norton piston kits.

http://stores.ebay.co.uk/The-Green-Spar ... w=hepolite

fiatfan said:
Have seen a pair of +.40 pistons at a great price, they are "EMGO" (EMCO?). Any good? Info on this is appreciated!

As far as I'm aware "Emgo" pistons are made by JCC, the same company that makes the modern "Hepolite"-boxed pistons.

fiatfan · Dec 21, 2015

Ok, thanks, seems they´re a bit lazy with the pics.... :lol: But there´s still the flat tops with or without the cut-outs for the valves?
Tommy

L.A.B. · Dec 21, 2015

fiatfan said:
But there´s still the flat tops with or without the cut-outs for the valves?

Well, 750 Commando pistons have valve cut-outs. Standard comp. 850 pistons don't.

jseng1 · Dec 21, 2015

Emgo sells JCC pistons which is Jieh Chueng Industrial - Taiwan, Asia,
There has been some discussion on this forum about the wire clips being too loose.

fiatfan · Dec 21, 2015

L.A.B. said:
fiatfan said:

But there´s still the flat tops with or without the cut-outs for the valves?

Click to expand...

Well, 750 Commando pistons have valve cut-outs. Standard comp. 850 pistons don't.

But in the manual, and yes I know the pics are drawn, but still, every pic of a piston has cut-outs for the valves. It´s a Mark III since there´s an electric starter. Do they have a different CR? Or have they just been lazy and copied the pics from the 750?
Tommy

L.A.B. · Dec 21, 2015

fiatfan said:
L.A.B. said:

fiatfan said:

But there´s still the flat tops with or without the cut-outs for the valves?

Click to expand...

Well, 750 Commando pistons have valve cut-outs. Standard comp. 850 pistons don't.

Click to expand...

But in the manual, and yes I know the pics are drawn, but still, every pic of a piston has cut-outs for the valves. It´s a Mark III since there´s an electric starter. Do they have a different CR?

No. The 850 piston assembly (063838) part number remained the same throughout 850 production.

fiatfan said:
Or have they just been lazy and copied the pics from the 750?

Regardless of how they may be drawn in various diagrams (the '73 and '75 parts book drawings show 850 pistons without cut-outs, (see links, below) all standard compression 850 pistons had flat tops and no cut-outs, also unlike 750 pistons which are "left" and "right" handed (due to the cut-outs) so there is one of each, the 850 pistons are a matching pair.

850 pistons [39]
http://www.oldbritts.com/1973_g1.html
Mk.III
http://www.oldbritts.com/1975_g1.html

Commando drawings-even the 850 Mk.III show scrolled rocker spindles which were never fitted to any Commando so the drawings are not always entirely accurate!

http://www.oldbritts.com/1975_g3.html

ILLF8ED · Dec 21, 2015

And if you used Pistons with domes like these they would knock the head off your Commando. Listen to L.A.B. He is wise. No valve cut outs and flat tops for 850 Pistons.

MS850 · Dec 22, 2015

This is what I put in my MK3. JCC Emgo .040 with hastings rings. Ive got about 1k miles, doing fine. The top ring end gap is normal and the 2nd ring end gap is wider by about twice as the top ring, stops ring flutter.
http://www.ebay.com/itm/NORTON-850-...348488?hash=item542739be08:g:0R8AAOSw8-tWU4AE

Frankie17 · Dec 22, 2015

The 850 Hepolite pistons are 8.5 CR flat top with no valve pockets

The pistons in Green sparks listing are Triumph T100 unit pre 1967

they must use a generic image

fiatfan · Dec 22, 2015

Thanks everyone, useful info. I kinda figured those high rise pistons wouldn´t fit that well....
Mark: I´ve been warned about those pistons, but yours seem fine? :?
Just to show that I´m not making things up, this is a pic from the Norton Villiers workshop manual for the 850:

But as I can see on my own engine, they´re wrong :shock:
Tommy

fiatfan · Dec 22, 2015

Ok, moving on. Here´s some pics of my pistons, just removed the cylinder, and there they were :shock:

. They don´t look like they have many miles on them, very clean.
First; does anyone recognize this logo, not sure it´s the right way up.

This shows how clean they are.

This is the writings on top of the right piston.

Here´s something really weird, at least I think so; when I saw the H on the right piston, I figured maybe that stands for Hepolite? I mean it can´t be for right, which in Swedish begin with a H, that would mean that the markings on the left piston would be a V as left in Swedish begins with. But the marking on the left piston begins with a V! What´s going on here? Why is there an H and a V? Help someone :?
Tommy

L.A.B. · Dec 22, 2015

AE = Hepolite.

http://www.totalbikebits.com/wassell_history.html

Originally Hepolite was a brand name of Hepworth & Grandage, who later merged with Wellworthy Pistons Ltd, to form A E Piston Products Ltd.

I think it is the first time I've seen any "H" or "V" markings?

Could it be that the previous (presumably Swedish) owner or the person who did the re-bore (as they are oversize pistons) marked them so they wouldn't get swapped over?

fiatfan · Dec 22, 2015

L.A.B. said:
AE = Hepolite.

http://www.totalbikebits.com/wassell_history.html

Originally Hepolite was a brand name of Hepworth & Grandage, who later merged with Wellworthy Pistons Ltd, to form A E Piston Products Ltd.

Click to expand...

I think it is the first time I've seen any "H" or "V" markings?

Could it be that the previous (presumably Swedish) owner or the person who did the re-bore (as they are oversize pistons) marked them so they wouldn't get swapped over?

Thanks, then it´s the "real thing". About the markings I guess that´s the only explanation, even though it seems a bit "over-worked". I mean it would have been enough with some careful scratching, but this is stamped. And the pistons are not designated left/right as I´ve understood it, so... Strange, but it doesn´t matter now, the pistons are ok, and that´s what´s important.
Tommy

B+Bogus · Dec 23, 2015

fiatfan said:
Mark: I´ve been warned about those pistons, but yours seem fine? :?
Tommy

I've used the 'new Hepolite' pistons in a couple of bikes, and know quite a few others who have too - no bad reports here.
They use Hastings rings, but they come in a plain packet - the oil rings are 3-piece which caused some consternation to a forum member who was expecting one-piece oil rings.

I'm happy to keep using them and recently fitted their rings to my 850 which runs totally smoke-free

kommando · Dec 23, 2015

After seeing your pistons I have my doubts about you needing a rebore, the top ring area is recessed until the small oil holes and so if you are only measuring the gap at this point it does not tell you a lot. You need to take a piston and measure the clearance on the front of the piston inside the bore about 1/3 of the way down. Newly fitted pistons have a 4.5 to 5 thou clearance and you get another 5 thou of wear on top of that before you need to rebore.

fiatfan · Dec 23, 2015

kommando said:
After seeing your pistons I have my doubts about you needing a rebore, the top ring area is recessed until the small oil holes and so if you are only measuring the gap at this point it does not tell you a lot. You need to take a piston and measure the clearance on the front of the piston inside the bore about 1/3 of the way down. Newly fitted pistons have a 4.5 to 5 thou clearance and you get another 5 thou of wear on top of that before you need to rebore.

Yeah, it seems like I don´t need to re-bore

. I´ve done some measuring of both bores and pistons, and all seems within the tolerances. What I find a bit strange is the difference in width when comparing top and bottom of the pistons. At the bottom of the skirt perpendicular to the piston pin it´s just over 77.4 mm (about 3.047"), but the top above the top ring is just 76.9 mm ( 3,027"). To me that seems very much, but the pistons aren´t almost worn at all so I guess it´s ok? :?
The bores are round and the correct diameter, so they will just get a careful grounding (honing?).
Why can´t these measures be either in metric or inches, instead of both??? The bike is British with all that comes with it, all the different threads used and what not, but pistons and bores are in mm! :? UNTIL you need to rebore; then the pistons are marked for example +.020. Suddenly it´s inches! And it would be so easy to use the metric system all the way because .020" is almost exactly .5 of a mm, so 77.5! (It is a difference of 8 thousands of a mm which is .000315 of an inch) Easy :wink:
Tommy

Fast Eddie · Dec 23, 2015

fiatfan said:
kommando said:

After seeing your pistons I have my doubts about you needing a rebore, the top ring area is recessed until the small oil holes and so if you are only measuring the gap at this point it does not tell you a lot. You need to take a piston and measure the clearance on the front of the piston inside the bore about 1/3 of the way down. Newly fitted pistons have a 4.5 to 5 thou clearance and you get another 5 thou of wear on top of that before you need to rebore.

Click to expand...

Yeah, it seems like I don´t need to re-bore . I´ve done some measuring of both bores and pistons, and all seems within the tolerances. What I find a bit strange is the difference in width when comparing top and bottom of the pistons. At the bottom of the skirt perpendicular to the piston pin it´s just over 77.4 mm (about 3.047"), but the top above the top ring is just 76.9 mm ( 3,027"). To me that seems very much, but the pistons aren´t almost worn at all so I guess it´s ok? :?
The bores are round and the correct diameter, so they will just get a careful grounding (honing?).
Why can´t these measures be either in metric or inches, instead of both??? The bike is British with all that comes with it, all the different threads used and what not, but pistons and bores are in mm! :? UNTIL you need to rebore; then the pistons are marked for example +.020. Suddenly it´s inches! And it would be so easy to use the metric system all the way because .020" is almost exactly .5 of a mm, so 77.5! (It is a difference of 8 thousands of a mm which is .000315 of an inch) Easy :wink:
Tommy

Tommy, the fact that your pistons do not measure the same at the bottom as they do above the rings is a very good thing!

Pistons are always a smaller diameter around the 'ring lands' (ie, the area where the pistons rings are).

And they are not cylindrical. Pistons are tapered from top to bottom and oval front to back.

Definitely a shame to rebore barrels that don't need it...

kommando · Dec 23, 2015

The last piston I saw without the recess machined around the ring area seized within 20 ft of setting off, it was a JP piston and they had forgotten to machine the area and left it the same as the bottom of the piston. The top is where all the heat is so it expands a lot more than the bottom hence the weird piston shape.

kommando · Dec 23, 2015

Why is the engine apart ?, you only need to hone if it needs new rings, if the current set of rings gaps are within wear limits (not first fit limits which are 2 different limits) and there was no blue smoke in the exhaust and the oil consumption is normal for a Norton then put it back together as it is as long as you have marked the old parts so they go back in the right place. With an oil filter in place you can reckon on 2 to 3 sets of rings per rebore and maybe a set of pistons.

A quick piston question

fiatfan

L.A.B.

Moderator

fiatfan

L.A.B.

Moderator

jseng1

fiatfan

L.A.B.

Moderator

ILLF8ED

MS850

Frankie17

fiatfan

fiatfan

L.A.B.

Moderator

fiatfan

B+Bogus

kommando

fiatfan

Fast Eddie

kommando

kommando

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