Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board A65 Rods For Triumph?

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....

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.

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

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.

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.

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

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.

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.

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.

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.

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.

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)

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.

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?

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?

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.

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

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

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?

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.

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.

in my understanding when saying "small journal crank" that refers to the diameter of the TS Main journal. I have the small journal crank in my A10 and the TS journal is smaller than the so called large journal cranks on latter A10s ( which were the same diameter as the A65 )


I recall figuring this out when i had to rebuild my A10 after a CATASTROPIC failure ! ( Yep blew one case to bits etc) , ... I was pissed off because the TS bush i needed was smaller diameter than the A65 ones i have And the BE's are different as well

But from an engineering perspective good practice would dictate denominating the cranks by the most obvious and easiest to measure part.

I believe that the alleged breakage problems on the Small J s are more likely due to young guys ringing the tripe out of them whilst unable to afford oil changes.

They , the Small Js , are sound design and excellent material.

The early a10 cranks had smaller big end journals than the late ones as well.
They were not as strong.The timing side main was of little consequence.

Another example was to bore t140 rods to use in a65's as they were a steel capped rod
and were in fact the same forging as the very late and unobtainable a65/a70 rods. They
were a very strong rod and allowed a10 cranks to be run without fouling on the cases.

Here's a link to chapter 22 of BSA development engineer Roland Pikes memoirs where he talks about the early A10 cranks with the small rod journal and how they routinely broke out on the track, and a solution. It starts with the 4th paragraph down but the rest of it is worth reading as well.

https://beezagent.blogspot.com/2009/01/roland-pike-autobiography-chapter-22.html

Sorry Nick, probably not my best choice of words and no offence meant.

My thoughts we could the poster no find the correct rods for the bike? There are a lot of people who buy parts for tuppence and quite literally bodge them together, resizing the rod big ends to a new size altogether is what crossed my mind.

A lot of late A10 parts have similar/identical dimensions to those used on the A65, so using A10 rods with A70 pistons on an A65 crank would work, as would B44 pistons on A65 rods on an A10 crank in A65 crank cases also work. But with all those things the key dimensions are the same, it just takes someone with better engineering skills than mine to do all the measurements and machining. Clever things have been done with Norton cranks too but a lot of this is outside my skill set.

There’s nothing wrong with aftermarket parts. I always use aftermarket rods, whether from MAP or Thunder engineering (both good rod makers)

But as I said earlier, there are people like yourself, Mark Parker and others that can cross match parts from other brands and models and make something bloody fantastic. There are others that create a time bomb from a similar selection of parts and probably waste more money doing it.

As this topic is asking about Triumph Rods, it might be worth the OP contacting thunder engineering. He’s a big triumph guy and will likely make the OP what ever he wants if the dimensions are not standard. If the crank has been ground too far then it’ll probably be worth looking for a new crank.

When I bought the A10 rods from Thunder for my long rod A65, (and this rings true with HillBilly’s comment above, I asked Steve for some 6.5” rods, the A10 rods were a fraction smaller and I ended up settling for those because I figured I could shave that few thou off the barrel instead.


I’m also one who has a good collection of spares with the intention of turning 90% of them into something a bit unique. I think a 90° 500cc twin carb will be fun!


I digress…

the key words are "out on the track," ...

you ring anything hard enough for long enough and it will let go

Late comer, why not just have the crank welded and ground for the larger big end A65 rods?

Oh and as an afterthought regarding the "weak" small journal crank.

When i dropped a rod at 60mph and blew one half of the cases to bits ( fortunately not the side with the numbers) bending the valves, piston one side , obliterating the "returns" where the barrel goes inside the case etc ( i was real quick on the clutch so didnt take a dive)

I pulled it down and the crank was FINE ! I was able to burnish off the alloy which had "cold welded" to one of the BE journals and it was still in spec, I didnt even have to get it ground.

That would have been a huge shock load on the crank ya know but it survived.... so i dont consider that indicates weakness.

Since the big bang i have put around 5k miles on it .

Apparently, according to Pike, 3 hours of track time was enough.

Why not just buy the proper rods for your Triumph?