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  1. #1
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    Default replacing 750 kit pistons on t120r 650, having major issues

    so i have a 1970 t120r 650 that someone put a 750 kit in with a 10 bolt jug, i trashed one of the pistons and scored up the jug pretty good, the pistons are 76mm+.020, i have another 10 bolt jug that is 75+.020 that i want to use because i dont want to bore the 76mm jugs out to get rid of scored marks and make them too thin, the problem im runnning into is that i have now purchased 2 different hepolite piston sets that are 75mm+.020 but they are taller than the 76mm pistons i am trying to replace. im tired of spinning my wheels and not getting anywhere.

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    I think youll find what you need here,
    http://www.morgo.co.uk/bigbore.html
    contact them and make sure there correct for your cylinders

  3. #3
    dazegoneby
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    taller ?more compression?or just wrong?if they are taller, they may have sent you 650 stuff instead of750,750 is shorter stroke eng,,,,
    Last edited by dazegoneby; 08-25-2012 at 7:37 AM.

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    there at least 3 different piston/cylinder configurations the can be called a 750 Triumph
    .
    #1..... Is the 650 to 750 big bore kits available available everywhere, (known as a long rod 750 kit) most of these are ether Morgo or Aerco kits and take propitiatory pistons that can only come from the kit manufacture, many of these kits come in the box with the 76 mm piston clearance set up too tight (@ .002) and they seize up shortly after you start riding them.
    As with everything in engine building you need to measure things before you assemble an engine, these kit's don't always come ready to go, most times you need to take these aftermarket kits to a machine shop and get them honed to the correct piston clearance of .0045" clearance.
    The bad news, the replacement pistons are hard to get and expensive at about $400.

    #2. .... is a factory 10 bolt head Triumph T140 750 (76 m/m) piston/cylinder that is completely different piston, rod and wrist pin size and location because of the shorter rods (same stroke as a 650, just a shorter rod). you can easily find + .020, .040 and .060 pistons anywhere.

    #3. ....is the weird motor that was built in mid year 1972 that has a 650 rod and cylinder that uses the 75 m/m BSA piston (actually 725 c/c engine) and these are hard to find replacement pistons and refered to as a 9 1/2 bolt motor because of the weird head that has 10 bolt castings with only 9 drilled out for bolts.

    The easy way to tell what set up you have is to count the cylinder fins, if you have 8 fins it's a long rod 750 motor, if you have 7 fins it's a short rod 750 motor.


    I hope this helps, post some pictures of all your stuff you and I'll see if I can identify it.
    Last edited by Torch; 08-25-2012 at 8:41 AM.

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    the jugs have 7 fins, im just going to buy (76mm)19255+.060 pistons, the pistons i fried are (76mm)19255+.020. i figure this way the piston number is the same they have to be the same height, im not going to use my spare jugs, i talked to the guy at the machine shop and we are just going to bore the jugs that i had on the bike out to .060 + proper clearance, to fit the new pistons.

    am i correct in thinking that as long as the piston number is the same the pistons should be the same height.

    You would think the longer the rod the smaller the height of the piston and the shorter the rod the taller the piston, but since my jug has 7 fins i obviously have short rod pistons but the pistons themselves are very short, and all the wrong ones that i have ordered have been much taller.

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    heres pics of the pistons im trying to replace
    Attached Thumbnails Attached Thumbnails IMAG0133.jpg   IMAG0131.jpg  

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    Ae makes pistons that fit the short rod cylinders but you can run the stock 650 rod . Basicaly they are 750 pistons using the 650 pins .

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    Quote Originally Posted by TRUMPNUT View Post
    Ae makes pistons that fit the short rod cylinders but you can run the stock 650 rod . Basicaly they are 750 pistons using the 650 pins .
    that's something new, but it kinda defeats the whole reason they went to a short rod motor, it was to slow down the piston speed to allow the air intake speed to balance with the ability of the intake air volume based on cam lift and valve size.

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    I know this is an old post, but guys still read these for information, and some misinformation (no disrespect intended) has been given here.
    1) Fin counting only works on stock cylinders, not on aftermarket as a rule. I can cite at least 2 examples I personally own and have ran. The A.R.E./Dunstall alloy big bore kit has 7 fins, but is a long rod cylinder. The MORGO big bore has 7 as well. The Routt and Chantland cylinders have 8, as does the Wellworthy I believe. The only say you can be certain is to measure the height of the cylinder from mounting flange to head gasket flange. If anyone needs that info I can post it, as well as any specs or clearances needed. I have a wealth of info from a triumph
    dealer /racer I bought out in the early 90s, and I am crazy enough to use it!��
    2) The 750 short rod engine pistons differ in diameter between 75 and 76 mm standard bore as mentioned. The pin size is also larger. However the pin height is compatible, I had a set of JRC alloy rods I drilled and honed to fit 750 76mm pistons. Those rods have extra meat on the small end and can take this mod.
    The total height of a piston takes into account the compression height and skirt length if placed side by side. Skirt length does vary a bit. You can use one pin and join the 2 pistons to get an idea of the comparable sizes. The most important dimension, assuming diameter is the same is the pin height. That is the measurement from the pin center to the edge of the piston. If that is correct and the skirt clears the rod, all you have is a difference in compression ratio, and I have ran 11:1 compression with Megacycle road race cams and cleared the valves.
    The comment about the long rod short cylinder affecting piston speed is incorrect. The length of the cylinder is not the factor here. The length of rod/stroke, called rod/stroke ratio is. On this configuration the only change to make it work is the pin height, it has been moved closer to the rings, and the rings were probably moved closer together.
    I know this was a long post, but I made mistakes before the internet was available to inform (or misinform) me and I only want to help with the info I have learned over my 46 year affair with motorcycles. Please take this as I mean it.

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    I know this is an old post, but guys still read these for information, and some misinformation (no disrespect intended) has been given here.
    1) Fin counting only works on stock cylinders, not on aftermarket as a rule. I can cite at least 2 examples I personally own and have ran. The A.R.E./Dunstall alloy big bore kit has 7 fins, but is a long rod cylinder. The MORGO big bore has 7 as well. The Routt and Chantland cylinders have 8, as does the Wellworthy I believe. The only say you can be certain is to measure the height of the cylinder from mounting flange to head gasket flange. If anyone needs that info I can post it, as well as any specs or clearances needed. I have a wealth of info from a triumph
    dealer /racer I bought out in the early 90s, and I am crazy enough to use it!🤕
    2) The 750 short rod engine pistons differ in diameter between 75 and 76 mm standard bore as mentioned. The pin size is also larger. However the pin height is compatible, I had a set of JRC alloy rods I drilled and honed to fit 750 76mm pistons. Those rods have extra meat on the small end and can take this mod.
    The total height of a piston takes into account the compression height and skirt length if placed side by side. Skirt length does vary a bit. You can use one pin and join the 2 pistons to get an idea of the comparable sizes. The most important dimension, assuming diameter is the same is the pin height. That is the measurement from the pin center to the edge of the piston. If that is correct and the skirt clears the rod, all you have is a difference in compression ratio, and I have ran 11:1 compression with Megacycle road race cams and cleared the valves.
    The comment about the long rod short cylinder affecting piston speed is incorrect. The length of the cylinder is not the factor here. The length of rod/stroke, called rod/stroke ratio is. On this configuration the only change to make it work is the pin height, it has been moved closer to the rings, and the rings were probably moved closer together.
    I know this was a long post, but I made mistakes before the internet was available to inform (or misinform) me and I only want to help with the info I have learned over my 46 year affair with motorcycles. Please take this as I mean it.

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    More info is always good and this is a way to save it. Thanks for posting!

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    Good stuff.
    Here is a Wellworthy for reference of number of fins - 8:

    Click image for larger version. 

Name:	Triumph 1958WellworthyCylinders.jpg 
Views:	0 
Size:	55.3 KB 
ID:	103189

    post the numbers for cylinder heights - will be a good reference.
    Last edited by TriNortchopz; 05-09-2020 at 9:20 AM.

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    Initially Triumph built some early 1957 TR6 engines with those Wellworthy alloy cyls, Bean counters reined them in for 2 reasons, Warranty costs and they were not as reliable as the cast iron cyls. They can be run,, but with gung ho dirt racers and speed demons the problems crop up.

    That is WHY 1957 TR6s are challenging to restore as there is a lot of minute details, BUT... most Triumphs at that time ran a black painted cyl, and because of the change from Alloy back to cast iron, And with publicity photos and Brochures already out there the factory painted all the TR6s of that time Silver. There is 3 models of TR6s in 57, TR6A, TR6B,TR6C and for the US some slight differences between a East Coast and West Coast. (JoMo vs TriCor).

    I restored 2 57 TR6s and its mind boggling the details, got another in the pipeline on the shelf. The brake plate for the front is one year only. HARD to find. I was missing mine and had to pay a small fortune on eBay, as the anally retentive bolt polishers know this and was a lot of bidding.

    Triumph ran all alloy motors on the 500 Preunits on certain motors, The early race engines (I have a 53' T100C with dual carbies) and variants of the T100R & RR as well as the GP bikes.

    This carried forward with the early Triumph 500 units and while a few race bikes used one off alloy cyls, they painted many of the early "Sports model" 500 units cast iron cyls silver to give that compy image.

    Im building up some 500s right now and the details are mind boggling. T100SS, T100SR, T100SC, T100C I got 2 SC variants and later versions were called the "Jack Pine Enduro" models because it won a prestigous east coast off road race. The ISDT team raced them alongside the 650s and yes,, Bud Ekins along with that McQueen guy represented the US half of the Triumph sponsored ISDT teams. Some cool websites about the bikes and the racing, as well as several books.

    I have seen a few of those Wellworthy cyls, the originals are rare as hell and if you have a servicable one, its worth a small fortune to certain restorers and fanboys. I am trying to talk a local guy out of his,, his is badly damaged, looks like a meltdown and the alloy separated from the cast iron liner and cracked, But it would make a COOL Cut away display motor as I have the cases and rest of a motor thats seen happier days but can live on for display and entertain the kiddies.

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    A few more comments in this 2010 Thread: Triumph Unit 750 Big Bore Kit:

    http://www.chopcult.com/forum/showthread.php?t=5262

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