Is there a way one can check it? They are still the original ones...
Thanks a lot – Michael.

email (option): michasteinmann@gmail.com

The only DIY way to check them is to have a new one and put both a new and old one in the vice back to back separated with a piece of sheet steel, compress them in the vice and measure them, if your spring is tired it will compress more than the new one. The only downside to this is that you have to have a new one handy! I'm not sure howm much would be allowable on the compression, but I wouldn't have thought it would be much, maybe 1mm difference?

email (option): davmax@ntlworld.com

The BSA standards book states. Free length 2" +or- 1/64". There are pressure loadings which indicate to be condemned at 1 15/32" at 45lbs or 1 9/64" at 76lbs. Pressure testing can only be carried out with the necessary equipment. So you could re-use the springs if the free length is OK. Or just fit new ones anyway. Ron

email (option): ronpier@talk21.com

Length of valve springs should be 2"+/-1/64" when new.
As a rough guide, if the spring has 'settled' more than a 1/16" it best to replace it..
A poundage test is the preferred test though.
There are two test poundages...
To compress the spring to 1 15/32 should require a load of 50-54lbs. A used spring should take no less than 48lbs to achieve the same degree of compression.
For the second test...compressedlength 1 9/64"...weight range..84-88lbs....minimum..82lbs... Valve spring condition is not quite as critical in a sidevalve motor, the valves can't hit each other or hit the piston as they can in an OHV unit and there are no pushrods, rocker arms to keep under control....Ian

email (option): ian@wright52.plus.com

put a bar across the bottom of the spring a wire/small chain through the spring put the spring against the side edge of a vice then use a spring ballance which are easy to get hold of to pull the spring will need someone to measure the difference be careful the spring doesnt bounce out try not to pull upwards or the spring will come out and a thick piece of cloth over the vice so if it springs out it doesnt go far and you spend the rest of the day looking for it

email (option): roger.beck@node6.com

Wouldn't you just renew valves springs as a matter of course?

If you don't know the history of the engine and have the opportunity to renew, surely you'd just take the old springs out and stick 'em on the shelf.

With the British Army during WW2, I suspect that it was more important not to change parts unless it was really necessary. But this is peacetime. Replace (with quality parts), and put your mind at rest. Isn't the question of free length and poundage academic? Or am I missing something here?

email (option): dannydefazio@sumpmagazine.com

It all depends how you come at the bike and what your budget is.
On a limited budget there is no point throwing away seviceable parts...even if you have spare cash it seems a bit pointless.
If however, you are looking for a completely 'in spec' bike as it was when first built a lot of parts are going into the scrap pile irrespective.It is important though, that you determine the parts you are going to use ARE within an acceptable spec. In the case of materials used to make parts that can be nearly impossible to determine. So, if you have any doubts that the part meets all the criteria it's best to replace it....Ian

email (option): ian@wright52.plus.com

That is why I have hardly ever cleaned a spark plug in my life. If I take the effort to remove a plug, I will normally replace it with a new one....After all they only cost 2/6d. Ron

email (option): ronpier@talk21.com

I agree about using/re-using serviceable parts. That's why I've still got all the original rear mudguard stays on my M20 following my brake torque arm disaster (all the stays look either perfectly straight now, or 90-95 percent straight, which is good enough for me).

I've got a few other "issues" with worn parts that could be replaced. But I like the slightly beaten up look.

But with stuff like valve springs, and with an engine of unknown mileage, usage, etc, it sounds like it's better to replace. Aside from anything else, if you've gone to the trouble of taking out the springs, and have gone to the trouble of buying a new one (or a pair) for comparison, doesn't it make sense to fit and forget rather than replace a vital component that's old and might be marginal?

I'm betting that the "free length" test is pretty crude and doesn't take account of the more subtle behaviour of a spring in real world use (not that we're exactly talking high-tech here).

I'm broadly in favour of not dismantling an engine (especially an old engine) any more than absolutely necessary.

email (option): dannydefazio@sumpmagazine.com

If you have a drill press (pillar drill) and a bathroom scale they can easily be pressed into service testing springs of all types. Be wise and buy an old scale rather than pinch the missus's. Not easy or necessary for our sidevalve engines, you can make a simple wooden jig to hold OHV heads at the proper angle and test the seat pressure of the installed valve. This is critical for high-RPM heads like the Gold Star. But avoiding excessive seat pressure on softly-tuned engines also improves longevity of the cam and valve train and minimizes internal power loss.

Valve spring and piston ring technology have improved immensely in the last 6 decades. Broken exhaust springs were a common malady in race bikes back in the day. Now, car engines run hundreds of thousand miles without spring fracture. The testing rig I describe allows you to use cheap, high-quality modern valve springs with measured parameters as installed.

When in doubt, discard parts subject to metal fatigue, such as con rods and springs and bearings. Failure is expensive!

Jeff Bandola

email (option): jjbandoo@aol.com

A 'free length' test is a crude measure...but better than nothing if measurements are all you have.
With easily replaced, relatively low cost parts I tend to just replace them. I like Jeffs idea as well of sourcing modern alternatives if original NOS springs are not available and pattern items are from a dubious source, such as India.
I have done this myself with OHV BSA engines but not, so far, with the M20/21...any recommendations Jeff?
When it comes to conrods (in engines with steel rods)I am pretty careful with anything that is going into an engine that was a)highly tuned as manufactured or b) is being asked to do more than was originally intended from it..then I will go to the likes of Carillo for a 'peace of mind' solution. If the engine originally had alloy rods (such as an A10) and still has them I will only rebuild with new replacements...fatigue here is far more of an issue.
With most 'cooking', standard, single cylinder engines rod/crank and other replacements are not practical..either from a cost point of view and/or because replacements are not available. In the case of BSA forged steel rods they rarely give any trouble in standard motors that are driven sensibly and generally will stand a bit of tuning as well in most cases.
The fact is you can't replace everything and even the most carefully rebuilt road bike will still contain a fair percentage of 40-70 year old parts with the risk that entails..it's the nature of the game.
All you can do is take advantage of any test procedures that are available..such as crack testing..to improve the odds in your favour...Ian

email (option): ian@wright52.plus.com

this free length measurement is that on a hot day or cold day

email (option): roger.beck@node6.com