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#883238 06/18/22 12:04 pm
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I have just acquired a pair of conrods 67-270. I am led to believe these can be used on Triumph 650, which is what I would like to do. Aside from the small end bush which could be made, are there big end shells suitable? This may have been covered previously I would guess....


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You can buy steel rods from MAP to suit your motor the exchange rate is not favorable but they do not fatigue and they are not expensive for their quality. A65's have a larger diameter crank pin and it's narrower. Rods are 6" same length as a T140 not a 650 which are longer. A65 rods work in neither.


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These rods are definately 6 1/2" centres, are they A10 not A65?


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A10 rods are 6.5 inch centres, the big end pin will either be too big if they are late a10 type
or too small if they are early a10 type. Both have 0.75 inch small ends.
I suppose you could have the early type sized for a trumpet but it's a questionable thing........

I've seen people do horrible things like putting two sets of shells in rods before.
Running one set inside the other, not very elegant or kosher really.

A65 rods are 6 inch centres.

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I spoke with a friend who says he did it with .30 oversize bearing shells and found a suitable small end bush locally. I haven't double checked the measurements as yet but if he says he has done it then I know he has.


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Far as I know, the A10 rod is .031 shorter than Triumph 650..


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A10 and the triumph 650 both use 6.5 inch centres.
A65 is 6 inch centres.

TC
If your mate has done it i don't dispute it.
I've put BMC Mini rods on a norton crank before now.
Loads of things are possible just time and agro sorting.

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Bearings in my A65 883 are .050" under mini. But acl stopped making them. And I cannot find any. Plus those rods are alloy. It's just complicated to run a bigger engine when smaller configurations deliver so well.


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Originally Posted by NickL
A10 and the triumph 650 both use 6.5 inch centres.
A65 is 6 inch .
Not to start an arguement with Nick who knows way more about this than me...but.this is one of the few things I think I know.
The photo shows my A10 crank fitted with R&R rods.. They are based on their 650 Triumph rod with the ends resized for the A10. They are advertised as 6.5 centers and were by my measurements... Pushing a piston pin through the both a R&R and stock larger journal A10 rod, the .031 length difference was obivious.. Thunder Engineering told me their rods were the stock length not Triumph length..

8E31AA90-DBCA-49AD-B3C2-F41F62C0BAB2.jpeg

61 hot rod A10, 89 Honda 650NT .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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Originally Posted by Hillbilly bike
Originally Posted by NickL
A10 and the triumph 650 both use 6.5 inch centres.
A65 is 6 inch .
Not to start an arguement with Nick who knows way more about this than me...but.this is one of the few things I think I know.
The photo shows my A10 crank fitted with R&R rods.. They are based on their 650 Triumph rod with the ends resized for the A10. They are advertised as 6.5 centers and were by my measurements... Pushing a piston pin through the both a R&R and stock larger journal A10 rod, the .031 length difference was obivious.. Thunder Engineering told me their rods were the stock length not Triumph length..


Just measured my A10 Thunder rods and I can confirm what you say. Should anyone want a pair of new, unused rods... I doubt I will be using these any time soon.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

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Blimey, MAP better get some new rods then, so had Harris.
They both spec 6.5 centres for t120 and a10

750 trumpets used 6 inch as did a65/70 (they were the same forging.)

The likelihood of me ever needing to know t120 rod centres again is pretty low,
so i don't want to argue about this trivia.

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Originally Posted by Mark Parker
Bearings in my A65 883 are .050" under mini. But acl stopped making them. And I cannot find any. Plus those rods are alloy. It's just complicated to run a bigger engine when smaller configurations deliver so well.

You could just have the crank ground to a65 sizes then you could use 6 inch centre rods.
You are close to that anyway with -050 big ends. If the grinder is friendly, have him grind it
so you have a 90mm stroke. I did that with a standrad norton crank and carrillo's, just made
it +0.030 under.

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My neighbour runs an engineering shop and he is going to bore them to fit the Triumph shells. Just need to find the big end diameter. UPDATE - its 1.77"


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Originally Posted by tiger_cub
My neighbour runs an engineering shop and he is going to bore them to fit the Triumph shells. Just need to find the big end diameter. UPDATE - its 1.77"

I was under the belief that triumph big ends were smaller than late A10/A65 motors. Is this 1.77” the big end size in the rod or the journal itself?

Standard A65 big end is 1.687” the big end size of the rod should be 1.844” (without shells fitted)


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

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I should clarify that these are small journal A10 rods, not A65 as I originally thought..... 1.77" is the diameter of the Triumph rod big end, not the journal.


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1.77" is as close to 45mm as you can get. Looking at the rods, is there enough metal to bore them out to that without interfering with the big end bolts?


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Hi All, rod big ends s have not been resized as there isn't enough metal. I am now looking at regrinding the Triumph crank to suit. Could someone give me a definitive answer on the small journal crankpin diameter? There is a lot of info out there which is conflicting / confusing. Its either 1.46 or 1.374?


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Looks like John Healy was wrong as well eh?

http://www.tioc.org/britbike/crankshaftart.pdf

Around it goes
by John Healy



Other Useful Information:
Connecting rod centers for the Triumph 650 is 6.500 (6.499” to 6.501”).
Connecting rod centers for the Triumph 750 twin is 6.000” (5.999” to 6.001”)
torque for Triumph rod bolts/nuts 5/16” CEI - 26 tpi threads 28 foot pounds
torque for Triumph rod bolts/nuts 5/16” UNF 24 tpi threads 22 foot pounds
When checking a rod for bending or twist the wrist pin bushing should be parallel/square to the big end to .001”
over 6”.
The finished inside diameter of the 650 and 750 big end of the rod is 1.770”. A word of warning!!! I give this
dimension only to allow checking a used rod. Aluminum connecting rods are not servicable if either stretched,
twisted or the big end is out of round. If you find your rod has stretched, twisted or the big end is oval cut it in
half and get a new, or donor rod from another motor.

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Thanks, but its the BSA small journal details I am after, the Triumph data is easy to locate


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Since there are more than one crankshaft available for the early BSA's I have attached a copy of the data from the BSA dealer book. Hope this helps. Lloyd L

crankshaft regind info.jpg

1951 ZB GS
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1963 RGS
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BSA had breakage problems with that small journal crank your rods went with. Their cure at the time was to roll harden the radii. They then went to a larger journal. I'm wondering how good of an idea a smaller journal crank really is?

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Originally Posted by Stuart Kirk
BSA had breakage problems with that small journal crank your rods went with. Their cure at the time was to roll harden the radii. They then went to a larger journal. I'm wondering how good of an idea a smaller journal crank really is?


Im wondering if the correct rods are not available for the crank in question?

You can spend a lot of money bodging other parts to fit a something else.. If its done right then it can work, but if its being done in a way that "saves money" then your more likely to end up with a crankcase with a newly installed breather window.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

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Originally Posted by Allan G
Originally Posted by Stuart Kirk
BSA had breakage problems with that small journal crank your rods went with. Their cure at the time was to roll harden the radii. They then went to a larger journal. I'm wondering how good of an idea a smaller journal crank really is?


Im wondering if the correct rods are not available for the crank in question?

You can spend a lot of money bodging other parts to fit a something else.. If its done right then it can work, but if its being done in a way that "saves money" then your more likely to end up with a crankcase with a newly installed breather window.


I totally disagree, i am one of the blokes who puts a limited worth on these old clunkers and therefore
will use what's available to sort them out. It doesn't mean bodging stuff but different people have different
points of view. Many race motors are prepared using stuff from different makers, does that make them
all bodge ups? Blokes have used different cranks and rods in different cases for donkey's years with
very successful results both on the track and on the road. It's one of the attractions of running these old crates.

Haven't you just put an A10 crank into an A65 Allan? Using aftermarket rods etc?

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Originally Posted by NickL
...... Many race motors are prepared using stuff from different makers, does that make them all bodge ups?.....
You make several good points.
My only concern in this particular situation was that the small journal A10 crank was known to be weak. But I may be going out on a limb to infer that a Triumph twin crank ground undersize so those BSA rods fit would likely be weak too.

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Stuart, i tend to agree that in this instance it may not be a wise move but in
many situations it can and does improve the motor. My most successful race
engine had a norton crank in an a65 bottom end. Tell the guys on their 1200 Honda
four's that i used to beat that was a bodge up!.

I did it as i could never put the worth into a custom crankshaft, i could buy norton cranks
for reasonable money rather than thousands on a nourish or similar crank.

Last edited by NickL; 03/11/23 12:18 am.
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