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Is there a downside to going with a Big Bore kit? #577211
12/20/14 2:04 pm
12/20/14 2:04 pm
Joined: Dec 2006
Posts: 536
Folsom, CA
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yellow_cad Offline OP
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My 70 bonny has the older style cylinders and needs a bit of freshening up so I wondered if a big bore kit is a good option. The bottom end is reasonably tight and I will rebuild the head. By the time I buy a used correct cylinder and bore it, I am only a couple of hundred away from the big bore kit. An thoughts and/or experiences would be greatly appreciated. Jim


Jim

1970 Triumph Bonneville
1973 Commando Interstate
1971 OSSA MAR
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Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #577212
12/20/14 2:11 pm
12/20/14 2:11 pm
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devon
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kevin large Offline
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only downside I can think of is a little more vibration

Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #577215
12/20/14 2:18 pm
12/20/14 2:18 pm
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triton thrasher Online content
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What's stopping you boring the barrel you already have?

What are the "older style cylinders?"

Last edited by triton thrasher; 12/20/14 2:18 pm.

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Re: Is there a downside to going with a Big Bore kit? [Re: triton thrasher] #577220
12/20/14 3:25 pm
12/20/14 3:25 pm
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Folsom, CA
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yellow_cad Offline OP
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Originally Posted By: triton thrasher
What's stopping you boring the barrel you already have?

What are the "older style cylinders?"


Apparently the older cylinders (pre 1968) don't allow for more precise and efficient lubrication of the exhaust valve to cam works. The later model cylinders are around for $100 to $150 so that would seem like a reasonable investment for the 68 and later upgrade especially since my bike is a 70.


Jim

1970 Triumph Bonneville
1973 Commando Interstate
1971 OSSA MAR
Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #577225
12/20/14 4:13 pm
12/20/14 4:13 pm
Joined: Dec 2004
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kommando Online content
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If you bike is a 70 it has nitrided cams so no need for cam oiling through the cylinder casting as the nitriding cured the cam wear issue and the oiling route can be blocked. So you can keep the cylinder.

Re: Is there a downside to going with a Big Bore kit? [Re: kommando] #577238
12/20/14 5:01 pm
12/20/14 5:01 pm
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yellow_cad Offline OP
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Originally Posted By: kommando
If you bike is a 70 it has nitrided cams so no need for cam oiling through the cylinder casting as the nitriding cured the cam wear issue and the oiling route can be blocked. So you can keep the cylinder.


Right now the cam lobe on the exhaust valves shows slight wear (flaking) but tappets look good so it seemed that the extra oil might pay off. I don't know how many total miles are on any part as the PO didn't know much.


Jim

1970 Triumph Bonneville
1973 Commando Interstate
1971 OSSA MAR
Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #577242
12/20/14 5:32 pm
12/20/14 5:32 pm
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triton thrasher Online content
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If "flaking" is visible, I think the time for extra oil has passed.


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Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #577267
12/20/14 9:11 pm
12/20/14 9:11 pm
Joined: Dec 2013
Posts: 3,528
ohio, usa
kevin roberts Online content

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the morgo big bore kit doesn't have a drillway for the cam oiling anyway, so if you get new hard-faced cams they will match nicely with it.


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die only once.
Re: Is there a downside to going with a Big Bore kit? [Re: kevin roberts] #579000
01/01/15 4:23 pm
01/01/15 4:23 pm
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malla1962 Offline
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im sure the EX guide block oiling was as far back as 66

Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579063
01/02/15 11:20 am
01/02/15 11:20 am
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bon Offline
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One thing i have heard from people who have fitted 750 kits to 650's, is the engine seems to lose that eager to rev, free spinning characteristic that triumphs are known for.

Re: Is there a downside to going with a Big Bore kit? [Re: bon] #579087
01/02/15 3:19 pm
01/02/15 3:19 pm
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Stein Roger Online content
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Hi Bon, in my experience that's not always the case, in fact if properly set up they can rev up quicker. The port sizes (1 1/8" on most Bonnies, 1 1/16" on TR6/7) are better suited to bigger bores. The 750s responds very well to bigger cams though.
The quickest, but not ultimately fastest top end twin I ever built was a TR7 with a Hyde 1/2 race exhaust cam. Revved like an air drill. Sold it to my brother in law and I couldn't stay with him on a curvy road to save my life, riding my very fast Rocket 3.

Re: Is there a downside to going with a Big Bore kit? [Re: Stein Roger] #579095
01/02/15 3:43 pm
01/02/15 3:43 pm
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Running from demons in WNY
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If you build a 750 and get ring and valve seal as it should be with the engine tuned properly it'll be a thing of beauty.
If you just bolt on the 750 kit and do nothing else, there might be some ugly...


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579361
01/04/15 9:13 am
01/04/15 9:13 am
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Naarfuk, UK
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Tigernuts Online content
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What is the advantage of the longer 650 rods? I'd expect the shorter rods to be stronger, and the shorter pushrods less flexible?


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Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579366
01/04/15 9:37 am
01/04/15 9:37 am
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kommando Online content
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A few years ago modified longer rods coupled with lighter pistons were introduced for Commandos, the Triumph longer rods do not have lighter pistons in the 750 kits but the changes due to the longer rods will carry over.

http://www.jsmotorsport.com/technical_rodspistons.asp

Quote:
The advantages of a longer rod are:
A) More power: The increase in power is a result of more dwell time at the top of the compression stroke, allowing more time for the fuel charge to burn completely and exert maximum force on the piston throughout the power stroke, This translates to more power for the amount fuel you burn.

B) Less Vibration: More dwell time at the top and bottom of the piston strokes means less "snap" due to the pistons not having to stop and reverse direction as quickly and abruptly as they would with a shorter rod.

For example: When a Norton crank is at 30 BTDC the distance from piston crown to TDC with our longer rod is 6.8mm compared to 9.2 mm with standard rods. So both sets of pistons are going from 6.8 and 9.2 mm BTDC to TDC in the same amount time. You can see that the piston with the longer rod spends more time closer to TDC (more efficient burning & power) and is moving slower when it reverses direction (less vibration).

This reduces stress on the crank, cases, big and small rod ends, etc. This is very important for Nortons as they have a tendency break cranks & cases and vibrate everything to pieces. Our longer rod also reduces the side pressure that the piston exerts on the cylinder, causing power-robbing friction, and because of this helps the engine to run cooler and more efficiently.

Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579391
01/04/15 12:47 pm
01/04/15 12:47 pm
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Tigernuts Online content
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Thanks. I'm sure these people know what they're talking about, but they are employing Carillo steel rods which must be a great deal stronger than stock. When comparing standard T120 with standard T140 conrods, is there no strength advantage with the shorter rods?


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Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579412
01/04/15 2:58 pm
01/04/15 2:58 pm
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If you want an answer in engineering terms, I can't help you. But if you want to know what the bikes are like and how they are different, I might.

When I first rode a Triumph short rod 750 I have to say that my major impression was that of disappointment.

I thought Triumph had made a huge mistake. Poor low end performance and a powerband that came on smack in the middle of the worst of the vibration. Even so, it wasn't any faster than a good 650. The five speed was nice....

I had something to compare it to. I had a long rod 750. They were made by Routt in those days.
That was a strong running bike. It came close to a good running Commando, the king of twins at the time. But it did not surpass.

That was then. Now there are folks like PeteR who knows how to get the best out these machines.

For my money though, I would always get the long rod 750.

Cheers,
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.
Re: Is there a downside to going with a Big Bore kit? [Re: Tigernuts] #579422
01/04/15 4:41 pm
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Originally Posted By: Tigernuts
Thanks. I'm sure these people know what they're talking about, but they are employing Carillo steel rods which must be a great deal stronger than stock. When comparing standard T120 with standard T140 conrods, is there no strength advantage with the shorter rods?


A short bar is more resistant to bending strain than a long one, all other things being the same, but when are all things ever the same?


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Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579423
01/04/15 4:43 pm
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I expect cams make more difference to performance and flexibility than rod length does.


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Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579424
01/04/15 4:50 pm
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Stein Roger Online content
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I agree that the long rod is the sweeter engine all round and the TR7 I mentioned was a shaker. The pistons are quite a bit heavier and accelerated harder as stated above, which means the forces involved are a lot stronger.
For anything other than racing, Carillo rods aren't needed or desirable. On the road, a broken Triumph twin rod is a rare thing.
I don't see many, or perhaps any, downsides to the 750 kits as regards the engine, though the high torque gives the transmission a beating, especially the clutch shock absorber. You want to avoid increasing spring pressure to mitigate clutch slip. The best you can do to combat this is to modify the clutch a la Pete R or a 7x7 plate kit, and use the least spring pressure possible.
It's the spring pressure that forces the SA back plate against the spider, inducing wear as they rub against each other.

Re: Is there a downside to going with a Big Bore kit? [Re: yellow_cad] #579429
01/04/15 5:24 pm
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I run a 750 Morgo kit on my pre unit Tribsa combined with E3134 cams ex & inlet and late 1971 unit crank & conrods with the 71 bonneville D type head as well I am impressed at how this bike accelerates out of the corners pulls like a train and yes it reminds me of my old Commando! but as its a lighter bike it is easier to throw around.I built the engine without lightining the crank/big valves as I wanted torque and have now geared the bike to suit, but this engine will also rev hard if you hang on and yes I do get clutch slip but only for the first 1/2 mile of my ride.I also have fitted the duplex chain & sprockets of the 71 unit donor engine on the primary.Dave


1941 BSA WM20
1958 TRI-BSA 750 PRE UNIT
1957 THUNDERBIRD
1932 R E MODEL GS SPECIAL
1947 BSA YM21

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