Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board A65 Big End Size

Hello, I have half a dozen BSA A65 rods in a box that I acquired. I measured the big end and all them range from 1.840", to 1.842", and I have one at 1.844". I looked through the book I have and I am not seeing a big end size to reference. None of them look particularly damaged. What would be standard size for a big end? Was sizing different by year?

Big end/ crankpin diameter was constant through all A65 years, Cant help with std size, did you check for ovality?

Checked for out of round they are all out of round half to a thousanth. Does anyone have a factory size spec?

1.840
Ovality is more important.

BSA's manual is clear: if ovality exceeds 0.0004" the rod should be renewed. That might specifically mean replace, but it could be interpreted as resized (at least in Texas we might read it that way).

A few years ago i bought 4 NOS rods that were sized, from the factory, too large. I had them resized to 1.8435" and they fit perfectly. This number came from JB Nicholson's Modern Motorcycle Mechanics for late A10/A65. I've also resized a pair of late A65 rods that had ovality close to 0.001". They were never going to be flogged hard. Same experience: perfect fit.

It is not that the aluminum rod's big end (or overall length) has changed, but why! Because the aluminum has been stretched beyond its Modulus of Elasticity. This event permently weakens the aluminum. It is the only clue you can garner that the aluminum rod has reached the end of its useful life.


The fact that the rod has not sprung back to its original dimensions is that clue. Just like you would not reuse a connecting rod bolt that has streched IN USE, and surpassed its Modulus of Elasticity, the same goes for the aluminum rod.

For a performance build "in the day" I used donor rods for years only if they present dimensionally. Until JRC started to make Triumph forged alloy rods in the 1970's all we had commercially was new factory, or donor rods. "Knock on Wood" I have never had a used rod fail if it was straight, showed no signs of heat around the small end or that the big end was oval. (I also helps to see the condition of the piston for signs of detonation). I also would check for signs of distress and write down the length, of the rod and the rod bolts, and diameter of the big end, and check these during the next rebuild. Then destroy the rod if it showed any signs of exceding the material's Modulus of Elasticity. A rod where the big end has gone oval by one thousandths of an inch is a "dope slap" that shouldn't be ignored.

Steel rods 29,000,000 psi and aluminum 10,000,000 psi Modulus of elasticity are different animals.
HTH

I think the term you meant for permanent deformation is the metal in the rod has exceeded its proof load or its yield point. The modulus of elasticity (Youngs modulus) is a constant for a given material - defines the slope in the elastic region of the stress against strain graph.
Disregarding the terminology, I agree with your advice. If there has been permanent deformation of the component, it is trash.

I have no plans to reuse any of these rods. I am looking to have rods made. Anyone else have a reasonable measurement or should I shoot for 1.840” and resize the crank as needed?

This is the info, if you get another number you do not resize the crank but get some more rods. There is a shell bearing between the bore of the con rod big end and the crank journal so any resizing of the crank journal will give some interesting effects on clearances, as the shells are not bored true but oval for good reasons, no doubt ending in a bang.

I thought I was slick once when playing junior machinist in my shop and thought I'd repair oval'd rods.

Thought me: I'll mill down the rod cap joint a bit and rebore the journals on the mill to get back to perfectly round at original I.D.

Everything measured out perfectly! I won!

. . . soon learned I had not won.

Important lesson for me on my journey of learning: there is more to consider than just dimensions and materials.

I've been pretty shy about using aluminum BSA rods since. Perhaps over cautious, but the cost of rod failure is high.

Jason

I have the same sentiments about used rods as you do Jason. MAP and Thunder Engineering make fantastic rods, replacing second hand ones with them is cheap insurance against a blown engine… or at least a long way towards preventing creating a new breather hole in the crank cases.

You can also get A70 rods, with steel end caps and no little end bushes (the thunder rods don’t have little end bushes either), I think LF Harris has something to do with them, but as I haven’t used them I won’t comment on their quality… but I’m sure they are still better than used ones.

The thunder engineering rods in my Lightning have regularly seen 8k, though I don’t hold it at that speed for long.