Author Topic: 72mm bore Piston +0.060...?  (Read 767 times)

Offline sixpak

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72mm bore Piston +0.060...?
« on: 03.05. 2017 22:27 »
Hello

i hava found 2 Cylinder Barrels (Think Flange) for my A10 Project. The Cylinder will be bored to 72mm. What i wanna go for is 9.0 :1 compression relation... (would love to go higher) ;) Which Piston can you recommend me?

I have also buyed a Crankshaft (large Journal) which is 0.010 under. On top will be probably a 67-1106 Cylinderhead.

what i found!

http://vintagemotorcyclepistons.co.uk/epages/1f6dc7d5-163a-44dd-ab8f-26d48061f44f.sf/en_GB/?ObjectPath=/Shops/1f6dc7d5-163a-44dd-ab8f-26d48061f44f/Products/287 but this is simple 7.25 compression...

any suggestion... not a fan of sleeving...

greets

David




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Re: 72mm bore Piston +0.060...?
« Reply #1 on: 03.05. 2017 22:34 »
9:1 or higher compression on present-day petrol. Are you quite sure you want to do that?
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Online ellis

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Re: 72mm bore Piston +0.060...?
« Reply #2 on: 03.05. 2017 23:04 »
I run Hepolite 9.0:1 pistons without any issues, but I would not go any higher for a road bike.

ELLIS

Offline sixpak

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Re: 72mm bore Piston +0.060...?
« Reply #3 on: 03.05. 2017 23:33 »
happy i live in switzerland...

;)

Online chaterlea25

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Re: 72mm bore Piston +0.060...?
« Reply #4 on: 03.05. 2017 23:44 »
Hi Sixpak,
Thin flange cylinders and 9:1 pistons = disaster *problem*  *warn*
If you want to go for performance you need the thick flange cylinders

John
1961 Super Rocket
1963 RGS (ongoing)

Offline sixpak

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Re: 72mm bore Piston +0.060...?
« Reply #5 on: 03.05. 2017 23:56 »
They are thick flange

A picture attached and a Sheet of measurement

Oben means = upper measurement
mitte = middle
unten = lower

Fahrtrichtung = Driving direction
Quer = 90 Degrees offset

Online KiwiGF

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Re: 72mm bore Piston +0.060...?
« Reply #6 on: 04.05. 2017 04:36 »
happy i live in switzerland...

;)

Then you will be able to afford billet rods to go with the 9:1 pistons  *smile*
New Zealand

1956 A10 Golden Flash (1st finished project)

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Offline sixpak

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Re: 72mm bore Piston +0.060...?
« Reply #7 on: 04.05. 2017 06:19 »
we have at least not so bad Gas here, thats a good point to Switzerland... but New Zealand.. is a dream!!!

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Re: 72mm bore Piston +0.060...?
« Reply #8 on: 04.05. 2017 06:37 »
Just get the 7.25:1 pistons. You won't notice any performance difference and you will have a flexible, reliable, easy-starting bike. Trust me.
2 twins, 2 singles, lots of sheep

Offline Tomcat

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Re: 72mm bore Piston +0.060...?
« Reply #9 on: 04.05. 2017 08:29 »
Just get the 7.25:1 pistons. You won't notice any performance difference and you will have a flexible, reliable, easy-starting bike. Trust me.

Yep, it's an old motorcycle. Keep it standard and enjoy it for what it is. Buy a modern bike if you want to lose your licence.  *eek* *eek* *eek*
'48 A7 '59 SR '74 850 Commando TDM900

Offline duTch

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Re: 72mm bore Piston +0.060...?
« Reply #10 on: 04.05. 2017 08:39 »

 I'm running 7.25's with L/J  crank, B/F barrels & 357 cam and it's a great ride- no need for too much bang, and JP pills are reputed for being on the heavy side, so consider balance
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 worntorn

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Re: 72mm bore Piston +0.060...?
« Reply #11 on: 04.05. 2017 16:03 »
Going to upset the masses here.
Nine to ones in my 63 West coast SR. After some mag work, it starts easily, doesn't ping and is very tractable. It's quite a smooth runner. At 70 mph the mirrors are clear. I marked the throttle in increments for carb tuning. 70 mph GPS is just over 1/4 throttle in calm air. That's with an oversized engine sprocket, 23 teeth instead of 21.
I would like to test ride a lower tuned A10 to see if it is smoother and similar in power output. This bike of mine is no screamer but it does OK. I have wondered if there really is much difference in on road performance between the stock iron head GF and an RGS or late West coast SR.(same tune as RGS)
 My friend has a plunger Gold Flash he has just completed, might get a ride on that.

Glen

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Re: 72mm bore Piston +0.060...?
« Reply #12 on: 04.05. 2017 20:36 »
It's worth bearing in mind that as originally designed, the A10 had dished pistons (as did the Norton Atlas) and a cr of about 6.5:1. The high compression option was 7.5:1. Admittedly they had to cope with low octane petrol but modern petrol isn't anything like the 4* and 5* we used to be able to get. If you look at road tests of Gold Flashes against Super Rockets, the top speed are not that different. Is there any real point in trying to coax an extra horsepower or two from an engine which is a good 55 years old? In real life, how often do you ride at full throttle and 6,500 rpm? As you have a choice of what pistons you are going to fit, be kind to the engine, you won't regret it.     
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Offline JulianS

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Re: 72mm bore Piston +0.060...?
« Reply #13 on: 04.05. 2017 20:58 »
All a matter of personal choice.

My A10. alloy head, uses 8.75;1 GPMs and 357 cam and pulls goes very well and does lots of miles. Does not pink. No issues. But  would not want to go any higher, memories of a mates alloy head A10, years ago which was slower and a pig to start and ride with 10;1 when compared with 8.25;1.

Also tried these GPMs with iron head, that was a bad idea, too high, pinked a lot, got too hot.

Have used 7.25;1  which were OK with alloy and iron, but I preferred the GPMs  with alloy head.




Offline worntorn

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Re: 72mm bore Piston +0.060...?
« Reply #14 on: 04.05. 2017 21:44 »
I can vouch that a little bit higher is ok in that the 9 to one 357 cam SR is a sweet running engine. This was how they were delivered to Hap Alzina from 61 on. I haven't read that this version was particularly problematic for longevity .
One positive for increased compression is that it generally gives an increase in power from idle to red line. Hot cams and big carbs often rob the bottom and middle to boost the top.

It's usually a small hp gain per point of Cr. though, so I wonder about the validity of the late SR/RGS  BHP or brochure HP as Phil Irving called it.

Glen