piston pins

[dmite]

got a set off 040 oversize TR7 pistons to put in the sprint. I have had the motor rebalanced to suit. It wasn`t out very much at all. I have since come across a set of sprint piston pins, which are of lighter weight,which I would like to use. What will this do to the balance of the motor? Good or bad ?

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[Jon Tilson]

about using 7 pistons in a Sprint. Are the valve cutouts right? What about the CR and all that stuff.<br>
Jonners

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[dmite]

Thanks Jonners, The pistons I am using have a dish around 5.5cc +.6 for the extra valve cutouts, the combustion chambers messure around 42cc, the head has been mill down a bit. the pistons are 040' over. Gives a comp ratio of around 10.2 :1. With 050" head gasket.

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[speedracer]

dmite, <br>
<br>
how do you work out the CR based on what's been milled off the head, piston size etc?<br>
<br>
speedracer

<p><!--EZCODE LINK START--><a href="http://www.triumphowners.com/2">1975 Dolly Sprint</a><!--EZCODE LINK END--></p><i></i>

[dmite]

Thanks speedracer I measured the combustion chamber with a burette, same for piston dish and cutouts.Then use a compression calculator like this one,<!--EZCODE AUTOLINK START--><a href="http://www.torinocobra.com/Randys_tools ... <!--EZCODE AUTOLINK END--> Just fill in all the relevant data, and you get your answer. Static compression ratio of coarse.

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[george]

if i rember the tr7 pistons are the same as the sprint with the cut outs as well

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[John Skinner]

According to my Rimmer parts lists, the TR7 and Sprint piston has a different part number, but both use a 158071 Gudgeon pin and 158632 circlip.<br>
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My completely unqualified opinion is that if you have two types of pin and they have the same dimensions, but one is lighter and up to the job, go with the lighter ones.<br>
<br>
I would double check the parts you have to make sure you don't have 1850 pins or something. <br>
<br>
It is also possible that one has gone obsolete and my nice Rimmers parts lists have substituted the obsolete one!<br>
<br>
How was the assembly balanced? They may not have used the pins when balancing anyway.

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[george]

just checked in an old data book pins are the same <br>
pin diam 0.9734/0.9376in<br>
clearance pin to rod .0001/.0006in<br>
pin to piston .0/.0004in<br>
may be replacement pistons are different<br>
the ones i've done would have been original<br>
back then

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[dmite]

George, the valve cutouts are different, the pin sizes are the same, just the sprint pins have a taper at each end of the bore to lighten them a bit. Was woundering what it would do to the balance if I use the lighter pins, now that the rotating has been balanced to he heaver pins

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