Square 4 pistons

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malcolm.johnson
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Square 4 pistons

Postby malcolm.johnson » Fri Jul 06, 2012 3:38 pm

Hello, this is my first post. Last August I took over a VERY long term project, being a 1947 rigid/teles 4G. This is my first Ariel and now looks like a motor bike again. A good deal of the engineering work such as crank regrinding and reboring had originally been done, but unfortunately the cylinder block and head had not been stored very well in the 10 +years since the project stalled due to bereavement. The bore was standard and had been taken out to approximately +.020 Ariel size, to fit BMC A series pistons pattern AE 16179A at +.040, with new little end bushes fitted to the rods and reamed accordingly . The bores are sadly beyond use now, due to pitting with rust and a further rebore is now required. I should add that the "new" pistons that came with the project are minus rings. Do I take the easier option of reboring to suit +.060 A series pistons, or bore to suit +.040 "Ariel" pistons, with the need then to replace the little end bushes? Would appreciate opinions here, and especialy recommended piston clearances, piston patterns, sources etc. Any help is appreciated. I have questions about the cylinder head, but that will keep.
Thanks, Malcolm.

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brenton.roy
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Postby brenton.roy » Fri Jul 06, 2012 4:35 pm

Hi Malcolm, welcome to our group. It sounds like you have made great progress.
My two cents is that material taken out of the bore is permanent, and at +60, you will have nowhere to go but resleeving.
Are you sure you can't get away with a hone? Sometimes it looks worse than it really is.
Bushes are relatively cheap and easy to replace, so of your options, I would definitely go with +40.
I have the remains of a '46 Rigid / Tele , in ten million pieces, so will be interested in what you discover.
If you can, post a photo or two.
regards, Brenton
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

malcolm.johnson
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Re: Square 4 pistons

Postby malcolm.johnson » Fri Jul 06, 2012 5:08 pm

Thanks Roy. Standard bore size as you know for the square 4 is 65mm. Some BMC A series engines are 64.5mm, the 0.5mm difference equating to approximately 20 thou. As I see it, even if I use +.060 BMC pistons, I am still at + .040 on the "Ariel" size, leaving another rebore to +.060 "Ariel" size a possibility for someone in the future. I was led to believe that using BMC pistons was not uncommon amongst square 4 owners - am I badly informed? I did not know whether this was due to availability or the extra oil rings on the BMC pistons? Initially I thought the bores would clean up until I honed them, they are BAD! Regards, Malcolm.

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Postby john.nash » Fri Jul 06, 2012 5:29 pm

resleeve and then bore to take the BMC A series pistons (+20 ariel size) that you have already have ( I assume) ?

Depends on how much you think you are going to use the bike ?
If it's a keeper (and you have the money) they you wil regret not resleeving and going back to standard in x number of years .....
John "Josh" Nash
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2009 royal Enfield,1978 t140 bonneville, '67 CJ750, 196-ish Ural M62 outfit, 1960 k750, ''51 kH500, ''49 soon to be Ariel bobber, 47 VH twinport, '44 Ariel WNG, '43 Ariel WNG, '41 Ariel WNG and piles of rusty scrap ....

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Postby nevhunter » Sat Jul 07, 2012 2:45 am

I'm not a fan of resleeving unless you have no option. The engine is not as reliable and should run a bit more piston clearance as the heat doesn't get out of the pistons as easily. The sleeve cannot be considered as a load bearing component either, and it is not cheap to sleeve 4 cylinders., I used to do a lot because the metal in a sleeve is centrifugally cast, but I have seen too many cylinders separate from the flange and sleeves move and seizures happen. The size you have to bore to to fit a sleeve is up to 3 mm more than the original bore size and often weekens the casting unacceptably. If the interference fit is a bit much the cylinder can split. Nev
Last edited by nevhunter on Sun Jul 08, 2012 9:12 am, edited 1 time in total.

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Re: Square 4 pistons

Postby david.anderson » Sat Jul 07, 2012 10:04 am

Malcolm
The use of BMC pistons was fairly common 30 years ago when Ariel pistons were almost impossible to get. Likewise big end slippers could not be got for the square 4 but there was a BSA stationary engine that used the same size slippers. Most spares are now available through Draganfly so the substituting of engine parts is now not as common.
Like Nev I have seen problems with liners moving and I have seen the shoulder at the top of an alloy cylinder liner crack around the line of the shoulder. (cast iron cylinder liners are not shouldered at the top)The liner dropped a little until the rings caught over the top of the liner and pulled it down into the bottom end where it was broken into 100 pieces. As such I now always ensure that any liner that I fit is pegged at the bottom to stop it from dropping. My Mk2 square 4 has been pegged under the barrel bottom flange. I drilled and tapped 4 allen key head screws through the cylinder and liner below the ring line prior to the cylinder being bored. For cast iron cylinders there are tapered pegs available that can be fitted. If possible these pegs should be fitted below the cylinder base flange to avoid weakening the wall. Many machine shops fit them between the base flange and the bottom fins which forms a weak point.
Draganfly or Westwood should be able to supply liners for you. Going this way you will have to purchase the liners, be charged to initially bore the cylinder to fit the liner, for pressing in the liner and for boring the liner to size to fit the pistons, so oversize pistons may end up cheaper.
It is common in Australia at the moment to use 350 Honda pistons in the Square4 as they are always much better priced. The gudgeon is however smaller so the little end must be bushed to suit.
David

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Re: Square 4 pistons

Postby admin » Sat Jul 07, 2012 10:25 am

david.anderson wrote:Malcolm

Like Nev I have seen problems with liners moving and I have seen the shoulder at the top of an alloy cylinder liner crack around the line of the shoulder. (cast iron cylinder liners are not shouldered at the top)The liner dropped a little until the rings caught over the top of the liner and pulled it down into the bottom end where it was broken into 100 pieces. As such I now always ensure that any liner that I fit is pegged at the bottom to stop it from dropping.


My rebore bloke (in his 70's) pegs the liners.
You never need tell him the clearances either ...
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