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#391020 - 08/22/11 6:24 pm Using a Triumph 750 Top end for Big Bore 650  
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RyanF Offline
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RyanF  Offline
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KY, USA
Hey Guys,

Did some searching and didn't find anything on the subject so wanted to ask.

I'm getting ready for another motor build. I love the 750 Big Bore kits on 650 motors, but they've gotten rather expensive over the years. I have motors laying around so why not keep it all Triumph??

What about just taking the Triumph 750 Top End and bolting it up?

I'm assuming the following work involved. Is there more to it?

1. Opening up the cases to fit the 750 Barrel.
2. Using 750 Rods & Pistons & Cylinder Head.


Is there any drawback/benefit to this all 750 setup over the "Big Bore" kits that utilize the stock 650 head with special head gasket?

Thanks, any info appreciated as it seems most just go with the big bore kits vs. using Triumph 750 parts for this upgrade.

- Farducci


1950 BSA A10 Chopper 1951 BSA Golden Flash
1960 Triumph 5T
1962 BSA A65 Royal Star
1968 BSA A65 Bobber - In Progress
1969 Triumph T120 Bobber "The Rabbit"
1972 Triumph T120RV
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#391026 - 08/22/11 7:07 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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Pete R - R.I.P. Offline
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Pete R - R.I.P.  Offline
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Vic. Australia
You'll also need the shorter pushrod tubes and pushrods.Engine height will be reduced about 3/8",which affects anything connecting the rocker boxes to the frame (head steady).

#391030 - 08/22/11 7:46 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: Pete R - R.I.P.]  
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Dale Hoover Offline
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Phoenix, AZ
Cool idea. According to JR Nelson's book, "Bonnie, the development and history of the Triumph Bonneville," the crankshaft stroke is the same (82mm) for the 750s and 650s, but they have different part numbers...

'63 - '71 E4643
'72 E13097
'73 E13298
'74 on 71-3298 (same as '73 but new numbering system)

With the same stroke however, it seems like it could work if you could make sure the big end on the rod fit the journal on the crank??

#391038 - 08/22/11 8:10 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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RyanF Offline
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RyanF  Offline
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KY, USA
I'll probably be starting with a 72' T120V Five speed motor and retrofitting '73 TR7/T140 components.


1950 BSA A10 Chopper 1951 BSA Golden Flash
1960 Triumph 5T
1962 BSA A65 Royal Star
1968 BSA A65 Bobber - In Progress
1969 Triumph T120 Bobber "The Rabbit"
1972 Triumph T120RV
#391107 - 08/23/11 2:25 am Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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HawaiianTiger Online content
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Maui Hawaii
I've ridden both short rod 750 and long rod(kitted 650)and I don't know why no one else mentioned it, but here it is.

The long rod 750 is WAY more fun to ride than the short rod 750.

Well, it is all about the fun isn't it?

Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#391111 - 08/23/11 2:52 am Re: Using a Triumph 750 Top end for Big Bore 650 [Re: HawaiianTiger]  
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desco Online content
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Pardon my ignorance about long rod/short rod but, why is one more fun to drive?


1968 T120R
1972 T120RV
Any advice given is without a warranty expressed or implied.
#391119 - 08/23/11 4:33 am Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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Torch Offline
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Torch  Offline
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so.cal
if you are gonna do it you'll need the whole top end and the con rods. but the question is.. the balance factor changed on the 750's to 74% and the T140 cams make power at the right balanced RPM so even if you have a metric bearing 71/72 650 crank and cases you might need to get a 750 flywheel. the only real advantage is you still have a right shit bike but it would be cheaper just to buy a 1973 TR7 or T140 engine.
In my opinion a properly balanced long rod 750 is a great engine. and you have a better clutch with the separate primary in ATF.


"Statesman" is the term often used to describe a deceased politician, we need more Statesmen.
#391123 - 08/23/11 5:54 am Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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HawaiianTiger Online content
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HawaiianTiger  Online Content

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Maui Hawaii
It's all about the power characteristics.
I like torquey motors and the Routt kitted bike I used to own and the others I built or rode had the kind of power that seems to be instantly on when you twist the throttle.
The OIF 750's I have ridden seem to need more revs to get the power out of them. I've never liked revving a Triumph motor real high for very long.
I hope that explains it. I realize that its a personal prefference type of thing.
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#391133 - 08/23/11 8:17 am Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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Stein Roger Online content
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Stein Roger  Online Content
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Skudeneshavn Norway
Tiger, the different character is mostly due to different camming. A short rod will in fact tend to shift the torque curve downwards. Still, like you, I prefer the long rods, partly due to less vibration (different rod angles and lighter pistons), partly because the 3/8" taller engine looks better...

#391156 - 08/23/11 12:00 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: Torch]  
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Pete R - R.I.P. Offline
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Pete R - R.I.P.  Offline
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Originally Posted By: Torch

the balance factor changed on the 750's to 74% and the T140 cams make power at the right balanced RPM so even if you have a metric bearing 71/72 650 crank and cases you might need to get a 750 flywheel.


No difference in the crank counterweight,both 689 gms for each cylinder.
The T140 conrod has about 15 gms more weight at the big-end,about 20 gms less at the small-end.The T140 piston is about 30 gms heavier than the T120.The weight of the rods and pistons changed the balance-factor.It put a little less imbalance force on the crank (the vertical force increased slightly,but the high horizontal force was reduced),and the engines were happy in the oily frame.

The T140 intake cam likes higher rpm,but the T140 exhaust cam has run out of puff before the engine gets there.
The T120 cams work as a team in the mid-range,and when both cams are happy the engine pulls harder.

I don't think the rod/stroke ratio is the real issue ( 1.8585:1 Vs 2.0134:1 ). I reckon the T140 rod is a stronger rod.

Compression ratio could be more of an issue.A standard T140 could be 8.6:1,or 7.9:1.You can get low-compression big-bore kits at 8.5:1 .Some are still 8.9:1 or 9.5:1,and you could always get more than that.

If all other things were equal,I wouldn't be too concerned about using the shorter T140 rod.

#391196 - 08/23/11 3:35 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: Pete R - R.I.P.]  
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JubeePrince Online content
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Originally Posted By: Pete R
The T140 intake cam likes higher rpm,but the T140 exhaust cam has run out of puff before the engine gets there.


Hi Pete,

I've seen this metioned several times and people advocating for use of the stock ex. cam from the T120 (3134 profile). Would something like this work "out of the box" or would the timing gears need to be adjusted too?

Steve


'77 T140J
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
#391208 - 08/23/11 4:33 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: JubeePrince]  
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
Originally Posted By: JubeePrince
[quote=Pete R] would the timing gears need to be adjusted too?

Steve


If you can fit a camshaft into a Triumph, you can easily time it.


Amateur Loctite enthusiast.
#391385 - 08/24/11 11:56 am Re: Using a Triumph 750 Top end for Big Bore 650 [Re: JubeePrince]  
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Pete R - R.I.P. Offline
In Remembrance
Pete R - R.I.P.  Offline
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Vic. Australia
Originally Posted By: JubeePrince

I've seen this metioned several times and people advocating for use of the stock ex. cam from the T120 (3134 profile). Would something like this work "out of the box" or would the timing gears need to be adjusted too?
Steve

Yes,for the T120 (3134 grind) exhaust cam.There's a good chance you'll get 55/34 timing (or close) with the standard keyway and timing mark,using T120 or T140 camwheels.

Yes and no,for the T140 intake cam.You'd have a fair chance of getting good timing (like 47/67)with a T120 camwheel.
In a T140,you get around 40/74 with the standard timing mark. Advance it 5 degrees,and you get 45/69 (a bit better).
Advance it 10 degrees,and you get 50/64.That's still good timing,better below 5500 rpm or so.Hawaiian Tiger would be happier with this.

It still pays to check the timing,and not just rely on the marks.Trust no-one.Trust your own measurements.

#393742 - 09/09/11 3:36 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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RyanF Offline
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RyanF  Offline
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Okay, just tore down the '73 750cc Motor and it was very clean inside and judging by these posts, there's no reason I can't do this.

Since everything is in such great shape, I will be taking the '73 750 Crank, Rods, new 9.5:1 Pistons, Cylinders, pushrod tubes, pushrods & Cylinder Head with rockers and retrofitting to the 650 bottom end. Only change is that I will be doing this to a 1969 Triumph 650 Bonneville bottom end now instead of the later 5 speed cases.

I will also be using a MAP Primary Belt drive, so I'm not worried about the extra cc's on the primary system.

Rf


1950 BSA A10 Chopper 1951 BSA Golden Flash
1960 Triumph 5T
1962 BSA A65 Royal Star
1968 BSA A65 Bobber - In Progress
1969 Triumph T120 Bobber "The Rabbit"
1972 Triumph T120RV
#393752 - 09/09/11 4:27 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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John Healy Online content
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Posts: 9,970
Boston, Massachusetts
As touched on above you will either have to use a late 71 or 72 metric crankcase, adapt the T140 Metric flywheel to fit the inch case or rebalance your 650 crankshaft an put your 750 rods on it.


#393754 - 09/09/11 4:52 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: John Healy]  
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RyanF Offline
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RyanF  Offline
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KY, USA
That's right...

Was originally going to do it with a '72 bottom end, but I've decided I wanted to keep the bike a four speed and use the matching ##'s '69 bottom end. So I will just swap the rods on the 650 crank and re-balance it.

... or could I not just use the stock 650 bottom end & rods and use "Morgo" style big bore pistons with the adjusted wrist pin location?

Last edited by RyanF; 09/09/11 5:04 pm.

1950 BSA A10 Chopper 1951 BSA Golden Flash
1960 Triumph 5T
1962 BSA A65 Royal Star
1968 BSA A65 Bobber - In Progress
1969 Triumph T120 Bobber "The Rabbit"
1972 Triumph T120RV
#393802 - 09/09/11 8:40 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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bon Online content
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bon  Online Content
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I have been told that the 750's don't have the free revving nature of the 650's which lets face it is one of the good attributes of the 650's. Its something to do with the bore to stroke ratio but i have never looked into that subject.

#394006 - 09/11/11 3:42 pm Re: Using a Triumph 750 Top end for Big Bore 650 [Re: RyanF]  
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Stein Roger Online content
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Stein Roger  Online Content
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Skudeneshavn Norway
I've had 750s that revved like air-grinders, and the quickest one was a TR7 with standard but cleaned up ports, almost 9:1 CR and a Hyde half race exh cam. Even a friends very fast 850 Commando couldn't keep up. But by 6500 rpm it was all over, unlike some 650 Bonnies that will pull 8000 if you let them.
But I've also tried 750 that were slugs, it's all down to preparation.


Moderated by  John Healy 


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