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  1. #1
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    Default Triumph Unit 750 Big Bore Kit

    I found a 650 to 750 big bore kit on eBay for $599. The listing says it's a straight bolt on applicaton. Is it that easy, do you just bolt everything down and fire it up? Does anyone here have any experience with these types of kits? Is it worth it performance wise?

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    when triumph went from a 650 engine to 750, they did it with bore and stroke.

    that kit is an over-bore only. i heard things about lots of vibration. not that that precludes its use by itself.

    but it's new pistons, rings, and cylinders. replace those, bolt your head and rocker boxes on and break in properly.

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    I've had 750 and 650 Triumphs ,To be honest my 70 runs alot stronger than my 73 did ...Granted my 70 has some secret shit in it that the older guys I got it from did ...but the 750 seemed to just vibrate more and make no more if not less power ....take that with a grain of salt though ...I am sure there is more to it than that...

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    With alot of the inexpensive big bore kits, they are not honed to size. They are close, but too tight. we have seen alot of them seize up because of too close of tolerences. They should be ok, but before you put them on, mic them out to see if they are within spec, usually about 4 1/2 thou. Also you are bolting on more cc's but to really perform to the fullest, head work and cams will make a huge differance also.

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    send your head to gepetto to get that thing laced out.

    if you want cams, you might as well NOT buy the big bore kit because you have to split the cases. if you're splitting the cases, get a 750 crank and cylinders.

    then that bike will rip panties off.

    i had a '73 750 that classic cycles did the engine on and it pulled hard.

  6. #6
    fozz
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    if its a MORGO kit its ok they been doing em fa 30 yrs an ive never heard of a problem , theres a bit more torque some say they vibrate a bit more outhers say not , whilst your abouit it they also do a morgo big bore oil pump well worth the money , the newer rotory oil pump is a bonus to but costs more

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    Quote Originally Posted by CRFyou View Post
    when triumph went from a 650 engine to 750, they did it with bore and stroke.

    that kit is an over-bore only. i heard things about lots of vibration. not that that precludes its use by itself.

    but it's new pistons, rings, and cylinders. replace those, bolt your head and rocker boxes on and break in properly.
    The stroke on a 10 bolt Triumph 750 is 82 m/m and the stroke on a 9 bolt Triumph 650 is also 82 m/m the bore on a 750 is 75 m/m and the bore on 650's is 71 m/m. the difference on a Triumph 750 (short rod ) motor is the location of the wrist pin, it is an effort to slow the piston speed.
    the kits for sale as a drop in 750 kit are what is refered to as a long rod 750. it is all just a bore difference.
    Those kits work great and make good power but one word of caution is make sure you take the kit down to a good machine shop and have the pistons sized to the cylinders at 4.5 thousands like Ryan said, other wise you will seize the pistons if you run them right out of the box.
    Last edited by Torch; 07-17-2010 at 10:12 AM.

  8. #8
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    So there is some conflicting information/opinions on this. Will these kits make a difference in power (after matching the cylinders/pistons as mentioned)? Would I need to change the cam as well? I'm just trying to debate whether or not it's worth it vs. freshening the top end/head this winter.

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    the bolt on 750 kits makes good power , you don't need to replace the cams if you don't want to, obviously if you do cams and carbs, along with porting work on the head and doing some cam timing it will make a huge difference but it is fine without them. I don't think the increase in piston size it a major cause of additional vibration, the crank balance factor is only 85% on most Triumph engines and on some years it was as low as 55%.
    if you need to replace the pistons anyway you might as well go 750.
    call Lowbrow or Mapp and order them now, I am sure an operator is standing by.

    Here is some info about the Triumph 750 top end, it is not a drop in kit , the cylinders are wider apart because of the 10 bolt pattern and it takes some machining on the cases to get them in. the rods are different the head and the pistons are too. the crank shaft is interchangeable with a 650 crank but the end of the crank is different because of the tripple row primary chain so don't get into that, anyway if you want 750 c/c's just get a aftermarket kit., one of the best one out there is the Map cycle kit but it is a bit more money. the other kits work well if you set them up right.
    And I my opinion you do not need a Morgo rotary oil pump, we build full race 850 c/c 75 horse power motors that run the regular Triumph oil pump.
    Last edited by Torch; 07-17-2010 at 5:39 PM.

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    ALL the kits I have seen have never been a true bolt on . You have to remove your old tappet blocks and drive them into the new cyliners and make sure the are straight . Always make sure the cylinders are sized correct I have seen tapper and tight tolorances in them . You also have to gap the rings and such .

  11. #11
    fozz
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    Quote Originally Posted by Torch View Post
    the bolt on 750 kits makes good power , you don't need to replace the cams if you don't want to, obviously if you do cams and carbs, along with porting work on the head and doing some cam timing it will make a huge difference but it is fine without them. I don't think the increase in piston size it a major cause of additional vibration, the crank balance factor is only 85% on most Triumph engines and on some years it was as low as 55%.
    if you need to replace the pistons anyway you might as well go 750.
    call Lowbrow or Mapp and order them now, I am sure an operator is standing by.

    Here is some info about the Triumph 750 top end, it is not a drop in kit , the cylinders are wider apart because of the 10 bolt pattern and it takes some machining on the cases to get them in. the rods are different the head and the pistons are too. the crank shaft is interchangeable with a 650 crank but if you want 750 c/c's just get a aftermarket kit., one of the best one out there is the Map cycle kit but it is a bit more money. the other kits work well if you set them up right.
    And I my opinion you do not need a Morgo rotary oil pump, we build full race 850 c/c 75 horse power motors that run the regular Triumph oil pump.
    Id agree you dont need a morgo rotory oil pump , but id definatley go for the big bore plunger when the beast gets hot an the oil thins its deliering about 30 percent more oil to the bottom end that can only be a good thing

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    < oil getting thinner when it's hot.>...the advantage to using a multi weight 20/50 oil is it is supposed to be ...20 weight when it is cold and 50 weight when it's hot.

  13. #13
    TigerChris
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    Dude there's no point in going 1/2 assed, your gonna be disappointed. You got an already bad breathing 650, now your gonna make it a bad breathing 750. Not knockin your big bore kit idea, Its awesome, but you would be way better off tearing the whole motor apart and doing the big bore, dual KEIHIN carbs, head work, remove some rotating mass, (cam gears, clutch basket, pushrods, R tappets) Jomo 15's and dynamic balance.
    Your gonna hate yourself for puttin in different pistons without a balance.

  14. #14
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    Quote Originally Posted by TigerChris View Post
    Dude there's no point in going 1/2 assed, your gonna be disappointed. You got an already bad breathing 650, now your gonna make it a bad breathing 750. Not knockin your big bore kit idea, Its awesome, but you would be way better off tearing the whole motor apart and doing the big bore, dual KEIHIN carbs, head work, remove some rotating mass, (cam gears, clutch basket, pushrods, R tappets) Jomo 15's and dynamic balance.
    Your gonna hate yourself for puttin in different pistons without a balance.
    by Keihin carbs, are you refering to the PWK Keihin carb or are you talking about the FCR Keihin carbs
    Last edited by Torch; 07-18-2010 at 12:16 AM.

  15. #15
    TigerChris
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    Pwk

  16. #16
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    one more question, what ballance factor did you set it at?

  17. #17
    TigerChris
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    78%

  18. #18
    fozz
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    Quote Originally Posted by Torch View Post
    < oil getting thinner when it's hot.>...the advantage to using a multi weight 20/50 oil is it is supposed to be ...20 weight when it is cold and 50 weight when it's hot.
    TRue to a point , as viscopsity additives break down due to use., as oil heats up it doesnt thicken like it did thereby viscosity rateing lowers under presure so pumping more oil to crank can only be a good thing , road bikes dont change oil every 800 miles or so where the additives work correctly

  19. #19
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    Quote Originally Posted by TigerChris View Post
    78%
    later model Triumphs were balanced at 85% , what was the reason you went with a less balanced set up?
    Last edited by Torch; 07-18-2010 at 1:16 AM.

  20. #20
    TigerChris
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    Quote Originally Posted by Torch View Post
    later model Triumphs were balanced at 85% , what was the reason you went with a less balanced set up?
    and on the PWK carbs are they the 28 M/M ones?
    I talked with a few engine builders (machinist forums) and asked what they were using on there motors. It seems that if the crank is dynamic balanced, and the pistons/pins/rods are weight/height matched you can push the useable (relatively) vibration free R.P.M's up toward 8000 by using a lower factor. I've had my motor to 7800 so far, It's shaky, It will rattle your fillings out. with the factory balance my motor was happy at 4k, but would kill you at 6k. Now it seems happy at 5-6k.

    Yeah, 28mm #138 mains.

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