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Servodyne
Servodyne
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T140 pistons in A65 barrels?
#794091 12/28/19 4:24 pm
Joined: Sep 2008
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A friend needs a set of +.040" pistons for an A65 project. I have a set of standard pistons from a T140. The 750 Triumphs were based on the A70 conrod and, I believe, pistons.
The first run of bikes were actually made with 75 mm bores. I was wondering if the wrist pin and other dimensions are as the A65, and if my friend can use my pistons.
Someone out there may know.

Cheers
SR

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Re: T140 pistons in A65 barrels?
Stein Roger #794092 12/28/19 4:37 pm
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They will fit , if the core was central when cast there is just enough meat to bore out, in my notes it has T140 std as 76.5 mm. But, the pin to crown height is shorter , which means machining down the barrels ( in my notes 2mm shorter, but this was with an A10 crank, the stock crank will need more taken off , 84mm stroke A10 , a65 74 mm, 84- 74 = 10. half that , = 5mm, so around 7 mm shorter ) and shortening the push rods. And the piston dome is not a good match to the A65 Chamber. AFAIR the gudgeon pins are the same diameter


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: T140 pistons in A65 barrels?
gavin eisler #794095 12/28/19 5:08 pm
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Thanks Gavin. Std T140 is actually 76 mm, and my friend needs to bore + .040", but with the other differences it's way too much hassle.
It was just a thought, and I am interested in some triple parts he has...

SR

Re: T140 pistons in A65 barrels?
Stein Roger #794096 12/28/19 5:45 pm
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Originally Posted by Stein Roger
A friend needs a set of +.040" pistons for an A65 project. I have a set of standard pistons from a T140. The 750 Triumphs were based on the A70 conrod and, I believe, pistons.
The first run of bikes were actually made with 75 mm bores. I was wondering if the wrist pin and other dimensions are as the A65, and if my friend can use my pistons.
Someone out there may know.

Just to give you an idea of what a T140 piston looks like compared to an A65. The wrist pin diameter is the same but the T140 is shorter to accommodate the longer stroke. Curiously the early batch T140s were 75mm bore standard as you stated and the picture shows both T140 and A65 standard 75mm bore pistons on the same wrist pin.
Jim

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1957 BSA A10 Spitfire
1971 BSA A65 Firebird
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1975 Norton Commando
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Re: T140 pistons in A65 barrels?
Stein Roger #794103 12/28/19 6:22 pm
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Servodyne, that seems to confirm that the pistons may have been developed for the A70 with it's 84 mm stroke.
The Triumph 650 pistons are even shorter from the wrist pin and up, and the pins are smaller.
Anyway, using T140 pistons in an A65 is a definitive no-go! In an A70 replica with an A10 crank, possibly.

Thanks
SR

Re: T140 pistons in A65 barrels?
Stein Roger #794229 12/29/19 7:25 pm
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I´m using T140 pistons in my A65 motor with 85 mm stroke and 2mm longer than std conrods. The valve angles are different so valve pockets needs to be grinded to suit. The cylinder barrel height and pushrods can be as original A65 this way. So, as you said, a no go with a 74mm crank.


Arnstein

BSA Spitfire MK3.800cc (also engine 850cc 90degree)
Honda CB450T -71
Laverda RGS 1130cc -85
Ducati 1098 -08
Re: T140 pistons in A65 barrels?
Stein Roger #794385 12/30/19 11:16 pm
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Arnstein, it would be worth your while finding some A70 pistons.
Even after machining the t140 ones are not very good amd a lot
of meat has to be removed from the valve pockets to get the
compression up.

Cutting down an a 65 barrel or getting another set of rods would be
the only way to run t140 8.5-1 pistons with the standard crank.
If you can find some 11-1 t140 pistons you stand a better chance but
even then you'll have to skim the barrel qa few mill.

Re: T140 pistons in A65 barrels?
Stein Roger #794447 12/31/19 9:41 am
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https://1drv.ms/u/s!AsKcYdC1qRK7jltmuWi6J4QeFevD?e=F0rGWk
https://1drv.ms/u/s!AsKcYdC1qRK7jltmuWi6J4QeFevD?e=DpDwQU

NickL. Thanks for advise, but when building this 800cc motor in 2005 I used the TRW Routts T140 high comp pistons pictured. These are easier to machine for to suit the A65 chamber. Works verywell and has done high mileage except had to be polished a couple of times because of bad Nikasil in alu cylinders. This last summer I have used my "new" motor 850cc 90 degree, so the 800 motor is sitting in my workshop waiting for something to happen..


Arnstein

BSA Spitfire MK3.800cc (also engine 850cc 90degree)
Honda CB450T -71
Laverda RGS 1130cc -85
Ducati 1098 -08
Re: T140 pistons in A65 barrels?
Arnstein #794517 01/01/20 12:52 am
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Ahh, you managed to find some t140 high comp pistons.

They are very thin on the ground now.

Re: T140 pistons in A65 barrels?
Arnstein #794565 01/01/20 5:05 pm
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Originally Posted by Arnstein

I´m using T140 pistons in my A65 motor with 85 mm stroke and 2mm longer than std conrods. The valve angles are different so valve pockets needs to be grinded to suit. The cylinder barrel height and pushrods can be as original A65 this way. So, as you said, a no go with a 74mm crank.

Thanks Arnstein, my question was largely academic, as my friend has no problem buying new pistons. Unlike you I don't know too much about A65s and it's been ages since I last worked on one.
For those who have never seen, or heard, Arnstein's bikes I can tell you they go as well as they look and more, and stay together too.

I do have more A65 questions though, but will start a new thread.

SR


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