Author Topic: Pistons the wrong way around?  (Read 1206 times)

Online TimK

  • A's Best Friend
  • ***
  • Join Date: Mar 2012
  • Posts: 97
  • Karma: 0
Pistons the wrong way around?
« on: 09.03. 2016 07:56 »
When I took the head off my Gold Flash I noticed that a previous owner had marked the pistons "L" and "R" and that one of the pistons had also been marked with an arrow. My problem is that the pistons seem to be transposed - the one marked L is on the right hand side and vice - versa, and the arrow is pointing to the rear (I've always installed pistons with any arrow pointing forward). Does it matter which way the pistons go in, and should I switch them around or leave well alone. The bore is +.060 and seems to be in good condition, but I haven't measured it yet. Any advice gratefully received.
Good Advice
0
No reactions
Tim Kerridge
Australia

Online bsa-bill

  • Wise & Enlightened
  • *
  • Join Date: Mar 2006
  • Posts: 5535
  • Karma: 65
Re: Pistons the wrong way around?
« Reply #1 on: 09.03. 2016 09:16 »
HI Tim
some thought as I like you would like some clarity on this one, it's probably something many of us have wondered about.

the bike having been run with the pistons being transposed I would think you would put it back together with them still transposed, don't think it matters for pistons but rings would probably like to stay in familiar territory, when you get barrels re bored they generally ask for a piston, one being suffice, so pistons are not regarded as pairs, rings on the other hand will bed into whichever cylinder they encounter first  (I think, other comments more than welcome.)

The arrow thing, I pondered this often, on the one hand pistons are not symmetrical in all dimensions, on the other hand how can they not be symmetrical front to back if the gudgeon hole is central, again more enlightened comments welcome.

If it was my bike I'd put a new set of rings on then put the pistons back anyway that makes sense to you providing the bore is OK
Good Advice
0
No reactions
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8474
  • Karma: 111
  • Lithgow NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: Pistons the wrong way around?
« Reply #2 on: 09.03. 2016 09:42 »
Std pistons have the pin central. So if the valve pockets are large enough (must be as it's been running) I'd leave it alone. Is there a manufacturers arrow on the crown? If there is they must go in with the arrow forward (offset pin). Bit hard to see if the cutouts are the same size. If different the smaller goes forward.
Cheers
Good Advice
0
No reactions
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online Butch (cb)

  • Wise & Enlightened
  • *
  • Join Date: Aug 2010
  • Posts: 1561
  • Karma: 14
Re: Pistons the wrong way around?
« Reply #3 on: 09.03. 2016 13:28 »
Is there a manufacturers arrow on the crown? If there is they must go in with the arrow forward (offset pin).

So - if so swap the pistons around to make them right at this point, but keep the rings running in the same bores as before?
Good Advice
0
No reactions
Warning - observations made by this member have a 93% unreliability rating.

Of Bikes; various, including ...
'58 Iron Head Flash Bitza


Online bsa-bill

  • Wise & Enlightened
  • *
  • Join Date: Mar 2006
  • Posts: 5535
  • Karma: 65
Re: Pistons the wrong way around?
« Reply #4 on: 09.03. 2016 17:03 »
Thankee Muskrat, one of those things I've pondered on occasion but never investigated.
were any A10/7 s fitted with offset pistons, and what's the reason for them
Good Advice
0
No reactions
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Online trevinoz

  • Newcastle, N.S.W. Australia.
  • Wise & Enlightened
  • *
  • Join Date: Jul 2006
  • Posts: 2922
  • Karma: 67
Re: Pistons the wrong way around?
« Reply #5 on: 09.03. 2016 20:32 »
I think that split skirt pistons have to be installed in the correct direction but solid generally not.
Good Advice
0
No reactions

Online TimK

  • A's Best Friend
  • ***
  • Join Date: Mar 2012
  • Posts: 97
  • Karma: 0
Re: Pistons the wrong way around?
« Reply #6 on: 09.03. 2016 21:05 »
Thanks for your thoughts Gents, I'll get the barrel off at the weekend and as long as there's no obvious reason to swap them I'll put them back in the same bore but with new rings.
Good Advice
0
No reactions
Tim Kerridge
Australia

Offline jachenbach

  • Resident Legend
  • *****
  • Join Date: Feb 2014
  • Posts: 563
  • Karma: 8
Re: Pistons the wrong way around?
« Reply #7 on: 10.03. 2016 01:25 »
Maybe I missed it, but why did you take the head off? If just decarbonizing, and it was running fine, I'd leave it as is. On the other hand, if it was running fine, why would you be taking the whole top end apart?
Good Advice
0
No reactions

Online TimK

  • A's Best Friend
  • ***
  • Join Date: Mar 2012
  • Posts: 97
  • Karma: 0
Re: Pistons the wrong way around?
« Reply #8 on: 10.03. 2016 04:00 »
jachenbach, it's a recent purchase and the total strip down is to find out if the engine is as ratty internally as the rest of the bike is externally. I'd prefer to find out what's wrong in the shed rather than on the road. So far it's not too bad, with most of the rattiness being cosmetic rather than mechanical.
Cheers Tim
Good Advice
0
No reactions
Tim Kerridge
Australia

Online TimK

  • A's Best Friend
  • ***
  • Join Date: Mar 2012
  • Posts: 97
  • Karma: 0
Re: Pistons the wrong way around?
« Reply #9 on: 22.03. 2016 00:24 »
I finally got around to lifting the barrel and having a closer look at the pistons and bores. The pistons are scuffed and the bores are marked (see pics). Any input on whether I'll get away with honing the bore or if I should re-bore to +80 will be appreciated.
Cheers
Good Advice
0
No reactions
Tim Kerridge
Australia

Offline duTch

  • Ricketty Rocketty Golden Flashback
  • Wise & Enlightened
  • *
  • Join Date: Oct 2011
  • Posts: 4540
  • Karma: 41
Re: Pistons the wrong way around?
« Reply #10 on: 22.03. 2016 08:36 »

  Hola Tim, No comment on your last query, I'll leave that to more qualified folk, but with regard to Jachenbachs query, when I bought my Gutzzi (850T) in  '92; 18 years old 21K miles on clock, which came from Michigan and looked as it'd been in a chook shed for a while (last rego '87-), I debated, but gave it a whirl.
       Did 4 years and about another 35-40K MILES, and the day before stripping it down for a 'LOOK', did a 200 mile back road ride, no probs or smoke or any reason to strip down except for peace of mind. The chrome bore was worn out, replaced the big end shells, but I'm still twenty years later still running the original main (plain Alloy) bearings.

 So it's worth the look *beer*
Good Advice
0
No reactions
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia

Offline Topdad

  • bob hebdon
  • Wise & Enlightened
  • *
  • Join Date: May 2011
  • Posts: 2253
  • Karma: 32
  • l
Re: Pistons the wrong way around?
« Reply #11 on: 22.03. 2016 12:23 »
Hi Tim, sure someone was looking for +80 pistons a little while ago but I think  without any luck! What about resleeving and going back to std ? Also are you taking it all apart to check the big ends and main bearing ? ,if so don't forget the sludge trap in the crank , that can save some grief , best of luck Bob
Good Advice
0
No reactions
" rules are made for the guidance of wise men and the blind obediance of fools"
United Kingdom

Online muskrat

  • Global Moderator
  • Wise & Enlightened
  • **
  • Join Date: Jul 2009
  • Posts: 8474
  • Karma: 111
  • Lithgow NSW Oz
    • Shoalhaven Classic Motorcycle Club Inc
Re: Pistons the wrong way around?
« Reply #12 on: 22.03. 2016 19:14 »
G'day Tim.
The pistons don't look tooo bad but the bores might take a bit to remove the scratches. Measure the piston/bore clearance at top and bottom of the bores. If you can fit a 5 thou" feeler anywhere it will be too big once cleaned up and need a rebore. As stated +80 is like finding rocking horse poo, so a sleeve back to std is the way to go.
Cheers
Good Advice
0
No reactions
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline BVSR

  • A's Best Friend
  • ***
  • Join Date: Nov 2011
  • Posts: 78
  • Karma: 2
Good Advice
0
No reactions
Markku
A7 1961    A10GF 1954    A10GF 1960    A10GF 1961 sidecar    A10SR 1958    A10SR 1959 
A10SR 1960   A10SR -61  A10SR 1963    A10SR 1963   A65T 1968    A65L 1968   B25SS 1971    B31 1952  
B31 1952   B33 1947  B33 1951   DBD34 GS 1958    NSU Supermax 1962    Husqvarna 510TE 1984
Finland[/iu

Online TimK

  • A's Best Friend
  • ***
  • Join Date: Mar 2012
  • Posts: 97
  • Karma: 0
Re: Pistons the wrong way around?
« Reply #14 on: 22.03. 2016 21:07 »
Thanks for the advice gents. I'm planning to take it right down to check out everything before reassembly Bob. I'll check the piston/bore clearance as you recommend Muskrat and make a decision about a re-bore then. Thanks for the piston link BVSR.
Good Advice
0
No reactions
Tim Kerridge
Australia