The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: TimK on 09.03. 2016 07:56
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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.
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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
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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
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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?
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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
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I think that split skirt pistons have to be installed in the correct direction but solid generally not.
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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.
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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?
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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
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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
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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*
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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
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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
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Hi,
Vintage Motorcycle Pistons seems to have +0,080 pistons:
http://vintagemotorcyclepistons.co.uk/epages/1f6dc7d5-163a-44dd-ab8f-26d48061f44f.sf/en_GB/?ObjectPath=/Shops/1f6dc7d5-163a-44dd-ab8f-26d48061f44f/Products/287
Markku
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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.
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Wow, 7.25 = high compression :o
Might be if your using kero as fuel. LOL
Cheers
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Perhaps they mean that if you're going to +80 this should be the highest compression you should risk!