Simple valve spring length question

[Guest]

I searched a lot of valve and valve spring related threads but didn't find the answer to this:

Do valve springs get shorter or longer?? The manual has a service limit spec of 38mm but no standard or new spec and the first two springs I pulled out of the head I'm practicing on measure 36.84mm. I assume springs get shorter in free length with age but I also know this is a pretty low mileage head so I didn't expect the springs to be shot. In fact I was thinking of replacing the ones in my bike with these because I thought they would be fresher.

So... are these undersize and shot or do springs somehow relax and grow in free length?

Thanks!
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[GregT]

The springs lose tension over time and "sink" slightly - ie the free standing length will be shorter than new.
If the spec for minimum length is 38mm then it's time those were replaced.

Greg T

[Macmatic]

The springs lose tension over time and "sink" slightly - ie the free standing length will be shorter than new.
If the spec for minimum length is 38mm then it's time those were replaced.

Greg T

Yeah, thats what I figured. I'm surprised though, this engine only had about 10,000mi on it so I thought they would be pretty good. I hope the ones in my engine are still good... going to have to be good enough for now at least.

I wish the manual said Minimum Length! All it says is Service Limit: 38mm and while I assumed it was a minimum it wasn't exactly clear. Can't see how springs would stretch in this application but I'm not exactly a materials engineer.

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[GregT]

If you've got two heads worth of springs to choose from, pick the best/longest set from all you have.
The absolute best should go on the exhaust side - if they float at high RPM they will hit pistons while the inlets usually won't.

Greg T

[Nessism]

Service limit is minimum length when it comes to valve springs. Time to replace those I'd say although I hope you are using a Suzuki manual and not an aftermarket to get the specs from.

[Macmatic]

Service limit is minimum length when it comes to valve springs. Time to replace those I'd say although I hope you are using a Suzuki manual and not an aftermarket to get the specs from.

Yep. Factory manual for the 700/750. Is it common for springs to be out of spec on a such a low mileage engine?

If you've got two heads worth of springs to choose from, pick the best/longest set from all you have.
The absolute best should go on the exhaust side - if they float at high RPM they will hit pistons while the inlets usually won't.

Thanks for the tip. I wonder if the intake side of both heads will be the best. I've actually got three to choose from if it comes down to it but the third head is on a complete 700 engine I have for a project bike.

Live and learn! Every time I look I find another difference between the 750ES and the 700. Now I see that the 700 uses a double valve spring where the 750 uses a single. The 750 spring is no longer available but the 700 double spring is. It looks like everything else (per valve spring assy) is the same except for the lower seat so maybe I'll pick up a full set of springs when I can spare the cash. Right now I'm hoping I can get away with just stem seals but I plan to re-ring the engine over the winter or maybe build an 816 engine.

I guess the double springs are to reduce float? Something about different harmonic frequencies in the two springs or something?

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[Nessism]

Yep. Factory manual for the 700/750. Is it common for springs to be out of spec on a such a low mileage engine?

Sorry, but can't answer that question. I do know that spring tension is more important than free length, so if you took the springs to a hot rod shop in your area with a spring tension gauge you should be able to judge the springs that way. The factory manual has a tension spec and as long as they fall into the proper range you are safe to reuse the springs.

[GregT]

Low Mileage engine ?? What you know for sure you have is a low mileage speedo....

My guess is that the 700 was seen as being more likely to be revved hard than the 750 so got double springs.
Yes the poundage at installed height is a better indication than free length but if you haven't got access to a spring tester, free length is good enough for a road motor.
Is the installed height the same 700/750 ? easy enough to mock up & check with a vernier caliper. If yes, then the double springs would be my choice.

Greg T

[Macmatic]

Low Mileage engine ?? What you know for sure you have is a low mileage speedo....

My guess is that the 700 was seen as being more likely to be revved hard than the 750 so got double springs.
Yes the poundage at installed height is a better indication than free length but if you haven't got access to a spring tester, free length is good enough for a road motor.
Is the installed height the same 700/750 ? easy enough to mock up & check with a vernier caliper. If yes, then the double springs would be my choice.

Greg T

Well this is true, I know the SPEEDO was low mileage but I believe it is probably correct for the engine. Near enough anyway. The whole bike seemed low mileage, clean rims, very little damage to the finish on the engine, even down on the leading edge of the oil pan. Overall it just seemed like it was a lot mileage bike at the time.

I'm going to try and find a local shop that will measure my springs for tension at installed height. I've measured eight springs out of three heads so far and they ALL come in at 36.8Xmm. Weird, eh?

/\/\ac