Is the manual accurate?

[crashcourse]

I'm in the middle of a top-end rebuild. Measuring my valve-spring free length, they are a little short. They meet the pre-77 spec, but my bike is an '81 Special. Is the manual correct; am I due for new springs?

Clymer manual lists pre '77 lengths as 1.638 for the outer and 1.614 for the inner. 78 and later it gives 1.675 and 1.654. All measurements are inches.

 

[MaxPete]

I don't have a direct answer for you CC but I do have a question: are you certain that your engine is the same year as the bike itself?

These old gals are 33-47 years old and a lot of stuff has been swapped around over those decades. It may very well be that your engine is an earlier mill than the frame. Before buying a bunch pf new parts, I would check the VIN on the engine and the frame to see if they match.

There is a VIN guide on this site somewhere that can help you find all of this info.

Pete

 

[grizld1]

Crash, there's no question of "swapping around" here, and you were right not to trust Clymer. All 447 motors (that is, all motors from 1974 to end of production) show the same parts numbers for the valve springs. The "late" free length numbers are correct for your machine: 1.675" outer, 1.654" inner. Your springs are sacked and need to be replaced.

 

[MaxPete]

There ya go!

 

[TwoManyXS1Bs]

...The "late" free length numbers are correct for your machine: 1.675" outer, 1.654" inner...

I show the same spring free lengths for the 70-73 models.

I wonder if the other numbers are the "service limits", (1mm shorter).

Snippet from the 70-73 manual:

 

[5twins]

Yes, I'd think they are still fine. I have all the factory manuals. The only one I saw those lower numbers in was the 75B manual. That manual also mentions the 2mm wear range. All the others, both earlier and later, list the higher length values. I don't think I've ever found a spring, whether it be a fork spring, clutch spring, whatever, to measure exactly at the given spec. I don't expect it after that spring has been compressed for 30+ years, lol.

 

[retiredgentleman]

When I did a top end job a while back, I found my valve springs were less than the new spec, as expected.
Worse case outer spring was at 1.663", which was only a decrease of 0.012" ( new =1.675")
Worse case inner spring was at 1.630", which was only a decrease of 0.015" ( new = 1.645")

You're allowed to go down 0.080", so replacement would be at Outer = 1.595", Inner = 1.565"
That gives quite a lot of room for spring sag.

 

[crashcourse]

Thanks for all for your replies - I know I posted this a while ago. I finally got out to the garage and re-measured for sanity's sake. Inner lengths are 40.9, 41.0, 41.2, and 40.9. Outer lengths are 42.0, 42.2, 41.9, and 41.9. All well above the 1.565" (39.75 mm) and 1.595" (40.51 mm) limits. While I may get slightly higher performance out of new springs, these still have plenty of life left in them, at least for my needs.

 

[retiredgentleman]

I'm sure you will be fine to use those.