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A65 Cylinder measures small #785929 09/30/19 10:11 pm
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Nick H Online Content OP
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I finally have the time and funds to get back to my '67 Lightning project and have been doing some measuring of the engine parts.
Stock standard cylinder barrels are 2.95276" (75 mm) according to my sources.
Using my Starrett inside micrometer I'm getting 2.94".
Can anyone explain what may be going on here? They don't appear to be sleeved or altered in any way that I can see.
[Linked Image from i.imgur.com]

Last edited by Nick H; 09/30/19 10:15 pm.

1966 BSA Lightning
1967 Triumph "Choppa"
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Re: A65 Cylinder measures small [Re: Nick H] #785931 09/30/19 10:18 pm
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NickL Offline
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Maybe it's an A64-3/4.............................
How often do you use an inside mike? A bore gauge may be the answer.

Re: A65 Cylinder measures small [Re: Nick H] #785943 09/30/19 11:46 pm
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gavin eisler Offline
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Does your inside mic have rounded anvils? Maybe try a piston if theres one handy.


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Re: A65 Cylinder measures small [Re: Nick H] #785946 10/01/19 12:04 am
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Nick H Online Content OP
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I somewhat confirmed the measurement with my dial caliper. I'm getting nearly 13 thousandths of an inch less than 75 mm. Wouldn't think my tools are that far off.
I'll buy standard pistons since I need them anyway and report back.


1966 BSA Lightning
1967 Triumph "Choppa"
Re: A65 Cylinder measures small [Re: Nick H] #785987 10/01/19 4:44 pm
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Interesting

Does this barrel appears to be "used"?

Could it be an A50 barrel someone punch-out?

I would agree with NickL, get a bore gauge and a micrometer and set up @75MM to get more of an accurate reading

or perhaps it left the factory on the tight end of the tolerance?

I suppose I would go to +.020 rather than standard, take into account wear, taper, etc if there is any


[Linked Image]
Re: A65 Cylinder measures small [Re: Nick H] #785994 10/01/19 5:40 pm
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gavin eisler Offline
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If its an A50 barrel the fin count will be less, its quite possible its been overbored, 7 on A50 , 8 on A65 , IIRC


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Re: A65 Cylinder measures small [Re: gavin eisler] #786030 10/01/19 10:31 pm
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NickL Offline
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Originally Posted by gavin eisler
If its an A50 barrel the fin count will be less, its quite possible its been overbored, 7 on A50 , 8 on A65 , IIRC


A50 wont fit the head stud pattern.

Re: A65 Cylinder measures small [Re: Nick H] #786035 10/01/19 10:57 pm
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gavin eisler Offline
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Well its a weird one then, maybe a 650 rather than the usual 654 cc. For a certain race class maybe.?


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Re: A65 Cylinder measures small [Re: Nick H] #786044 10/02/19 12:21 am
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Nick H Online Content OP
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Ok, I'm not that experienced with mic's but on looking at it today, it's maybe a bit larger than I thought but if I set my Starrett to 2.95276" (75mm) - it definitely is less.
This is not a dial gauge I am using. It is an inside micrometer kit made by Starrett and considered highly accurate.
I can check my inside mic with my outside mic and they jive.
I can't swear that the cylinder was ever used but the rest of the motor (crank etc) shows some wear. The bike sat in a collection for some years.

Last edited by Nick H; 10/02/19 12:28 am.

1966 BSA Lightning
1967 Triumph "Choppa"
Re: A65 Cylinder measures small [Re: Nick H] #786064 10/02/19 8:36 am
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kommando Offline
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The picture is low res but if I blow it up there appear to be thin liners installed, maybe as Gavin suggests it was to enter a 650 class and the std barrel would have disqualified.


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