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pre unit vs unit pistons 650, same???? #220180
10/22/08 10:56 pm
10/22/08 10:56 pm
Joined: Aug 2001
Posts: 770
Central Virginia
T120C Offline OP
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T120C  Offline OP
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Joined: Aug 2001
Posts: 770
Central Virginia
Having discussion with friends about this, so pre unit triumph 650 pistons different than unit pistons 650?? specifically 1959 bonnie vs 1966 bonnie??


Jack
1967 MGB Roadster..
1959 Bonnie
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Re: pre unit vs unit pistons 650, same???? [Re: T120C] #220218
10/23/08 3:01 am
10/23/08 3:01 am
Joined: Aug 2001
Posts: 6,734
Vermont
Jon W. Whitley Online content

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Jon W. Whitley  Online Content

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Posts: 6,734
Vermont
I don't see why they would be or that you couldn't use the 1966 pistons. They both have the same rods, aluminum head and pushrods. I would think that comparable CR would be the same piston.

bigt


Jon W.


1957 6T Thunderbird 650
1968 T100R Daytona 500
1971 TR6R Tiger 650
1970 BSA A65F 650
1955 Tiger 100 - Project

Re: pre unit vs unit pistons 650, same???? [Re: Jon W. Whitley] #220275
10/23/08 1:34 pm
10/23/08 1:34 pm
Joined: Aug 2001
Posts: 770
Central Virginia
T120C Offline OP
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T120C  Offline OP
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Central Virginia
That was my thought Jon... They had stated that the pistons were heavier and the piston skirts were longer on the pre unit.. and I did not remember that in my pass projects... Thanks for confirmation...

Jack


Jack
1967 MGB Roadster..
1959 Bonnie
Re: pre unit vs unit pistons 650, same???? [Re: T120C] #220278
10/23/08 2:10 pm
10/23/08 2:10 pm
Joined: Mar 2005
Posts: 10,022
Boston, Massachusetts
J
John Healy Online content

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John Healy  Online Content

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J
Joined: Mar 2005
Posts: 10,022
Boston, Massachusetts
Before about 1966 Triumph made their own pistons. They were made from a hypereutectic aluminum allow and can be identified by the "Lo-Ex" (low expansion) on the casting next to the wrist pin hole on the side of the piston.

http://kb-silvolite.com/article.php%3Faction%3Dread%26A_id%3D2

This is an important detail, as the later pistons, made by AE/Hepolite and supplied by Triumph as o.e.m. and available now in the after market, are not made from the same alloy and are NOT low expansion.

The early Triumph o.e.m. pistons, being low expansion required less piston clearance than the later equivalents. Fitting a late low silicone Hepolite, JCC, etc with the piston clearances for the earlier high silicone - low expansion pistons can lead to piston seizure!

Recommended clearance for Triumph 650 "Lo-Ex" pistons: .0035"
Recommended clearance for AE/Hepolite 650 pistons fitted as o.e.m and available today in the after market: .0046" to .0061"
John


Re: pre unit vs unit pistons 650, same???? [Re: John Healy] #220293
10/23/08 3:30 pm
10/23/08 3:30 pm
Joined: Aug 2001
Posts: 770
Central Virginia
T120C Offline OP
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T120C  Offline OP
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Central Virginia
Thanks John, that is a good thing to know.... The other discussion we had is boring cylinders more than 40 over... I had read that triumph did not recommend going over 40 over or over 20 over for alloy cylinders... does everyone agree with this?? then why do they make 60 over pistons???

Thanks for the help

Jack


Jack
1967 MGB Roadster..
1959 Bonnie
Re: pre unit vs unit pistons 650, same???? [Re: T120C] #220294
10/23/08 3:42 pm
10/23/08 3:42 pm
Joined: Mar 2005
Posts: 10,022
Boston, Massachusetts
J
John Healy Online content

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John Healy  Online Content

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J
Joined: Mar 2005
Posts: 10,022
Boston, Massachusetts
The Triumph distributor in Baltimore MD supplied piston up to .100" over for the 650 with a disclaimer: "If the cylinder broke at the flange it was not their fault."

You can find the quote in one of the TriCor Service Bulletins.
John


Re: pre unit vs unit pistons 650, same???? [Re: John Healy] #220304
10/23/08 4:13 pm
10/23/08 4:13 pm
Joined: Aug 2001
Posts: 6,734
Vermont
Jon W. Whitley Online content

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Jon W. Whitley  Online Content

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Very interesting. I did not know that eek

beerchug


Originally Posted By: John Healy
Before about 1966 Triumph made their own pistons. They were made from a hypereutectic aluminum allow and can be identified by the "Lo-Ex" (low expansion) on the casting next to the wrist pin hole on the side of the piston.

http://kb-silvolite.com/article.php%3Faction%3Dread%26A_id%3D2

This is an important detail, as the later pistons, made by AE/Hepolite and supplied by Triumph as o.e.m. and available now in the after market, are not made from the same alloy and are NOT low expansion.

The early Triumph o.e.m. pistons, being low expansion required less piston clearance than the later equivalents. Fitting a late low silicone Hepolite, JCC, etc with the piston clearances for the earlier high silicone - low expansion pistons can lead to piston seizure!

Recommended clearance for Triumph 650 "Lo-Ex" pistons: .0035"
Recommended clearance for AE/Hepolite 650 pistons fitted as o.e.m and available today in the after market: .0046" to .0061"
John


Jon W.


1957 6T Thunderbird 650
1968 T100R Daytona 500
1971 TR6R Tiger 650
1970 BSA A65F 650
1955 Tiger 100 - Project

Re: pre unit vs unit pistons 650, same???? [Re: T120C] #220327
10/23/08 5:16 pm
10/23/08 5:16 pm
Joined: Oct 2003
Posts: 11,138
North Georgia, USA
RF Whatley Offline
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RF Whatley  Offline
BritBike Forum member
Joined: Oct 2003
Posts: 11,138
North Georgia, USA
Originally Posted By: T120C
Having discussion with friends about this, so pre unit triumph 650 pistons different than unit pistons 650?? specifically 1959 bonnie vs 1966 bonnie??


Like John said.... but there is also another big difference if you are discussing generic pre-unit vs. unit construction. Generic pre-unit could also include iron head engines, like the Thunderbird. The iron head has a much deeper "dish" and no squish band, therefore the crown of the piston rises much higher up into the head than on the alloy sport heads.

So you can run alloy head pistons in an iron head, but you cannot run iron head pistons in an alloy head (without some tricky piston crown machining).

You'll want to remember this next time you see new pistons on sale for ridiculously cheap prices... they're probably iron head pistons.

bigt


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes

Moderated by  John Healy 


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