Checking the free length of VF500 valve springs

Hello there,
First post here. I'm a new VF500F2F owner (fully faired Euro model). I bought my 1986 bike in the UK a few weeks back after doing plenty of research on here so I knew what I was letting myself in for. I had a VF400FD way back in 1984 but I always wanted a 500 so now I have one 27 years later. The VF500 is in really good condition, 18500 miles, no nasty top end noises, carbs are fine. Everything works. I intend to use it quite a bit. The only question I have for you experts is can the valve springs be swapped/checked without removing the cyclinder heads?

I am keeping the bike at a 9K redline as recommended on here but I have no knowledge of how the bike was treated in the past. I intend to give the top end a good inspection in the near future and I would like to measure the free length of the valve springs but I don't want to take the heads off if at all possible. Anyone managed this? Use one of the Snap on type Metric Valve Keeper tools and fill the bore with string is one suggestion I read somewhere. Any ideas?

Regards
Jeremy

 

I fill the cylinder with air, like doing a leak down test, and then remove springs. You can then put a piece of hose on the valve stem to keep from falling in.

Don't know why you want to check the free length. It won't tell you anything useful. You need to check the pressure at installed height and at full open. Need to use a valve spring checker tool. Automotive/mc engine build shops/machinists have them. You have to find the values for both first.

 

Thanks for that. THe free length is the only thing that can affect the closing force of the spring(s). It's not like anything else can change. The elastic modulus of the steel is a constant. I can measure those other parameters too if I can get them out.

 

Interesting post. Look forward to seeing what you find.

For reference, from the '84 FSM:

 

And that may be the problem with the 500's eating valves. The elastic modulus of the steel can change over time from fatigue. Aka weak springs. Yes the free length may change.

A valve needs X amount of pressure closed to keep the valve from bouncing on the seat and X amount at full lift to keep from floating. When the spring pressure is less, those things happen and when more pressure other things happen. If the springs are made according to spec all is fine. If for some reason they do not meet those needed pressures you won't be able to tell by free length. Only by the pressure the spring makes at installed height at both close and full open. If the springs are on the weak side you can shim several thousands as long as you don't go in to or close to coil bind. I'm sure Honda has figured this out to work correctly with their specs.

If the maker of the springs has a batch of not so in spec springs how would they/you know if pressures are not checked. Now trying to find those specs from Honda may be impossible. I've been trying to find out and haven't any luck so far. It would be interesting to check a new set against a long mile used set.

What do you if you change cams? Spring pressures most likely will have to change also. I have seen what the wrong spring pressures do to heads and valves many times. I just don't trust what some mfg's claim about their products. I try to check everything going in to a motor.

Wanna hear about 3 new $$$ race motors that got wasted because of a broken ARP rod bolt on each. They were spec'd for more than what was needed for a margin of safety. Centers of each bolt was crystallized.

 

The elastic modulus most certainly doesn't change with time. If it did the world would be a very different place. Fatigue is a very different mechanism involving the initiation and propagation of cracks. I don't think cracks and fracture is what happens with the VF valve springs. I suspect it's just a series of microplastic events accumulating to give the spring a permanent set, which in turn cause the valves to float. These events are probably linked to high RPM excursions.

The free length will be a good indicator of whether this has happened or not. No idea what you mean by "Centers of each bolt was crystallized". All bolts are made of millions of metal crystals called grains. If you mean it looked shiny and facetted that is different.

 

Good info here.

Thanks for the post jeremyr62. It is nice to have a fresh pair of eyes looking at this ongoing conundrum, for VF500F owners.

If I could ask, have you given any thought to aftermarket replacement inner and outer valve springs, as the OEMs are NLA?

 

I have no more knowledge of the real failure mechanism than you guys, I'm just going on what I've read on here. I'm a professional engineer so this kind of stuff floats my boat so to speak. It's the main reason I bought the VF to be honest. When I have the bike (it's still in the UK atm) I intend to inspect the top end. If my springs have a set I'll have to think about what to do then. It's a PITA that Honda stopped making them. They are not the easiest thing to get duplicated unfortunately.

 

Maybe there is a problem in semantics here. I always thought that "spring fatigue" is a result of the spring's tendency to lose tension over time due to the heat generated by constant cycling.

 

Could well be. I'm not a practical engineer, more a theoretician. However fatigue to me is about cracks. What you describe is more likely to be creep but is probably inaccurate anyway as steel doesn't creep (appreciably) at the temps encountered in an engine. Heat isn't generated by cycling either at least not if the spring is cycled in the elastic regime. If it goes plastic then OK but if that happens then it will go boom very quickly from low cycle fatigue.

I read the service manual the other day. It was quite interesting how they actually state not to over over compress the springs when putting them back. That might be good practice for all engines tho for all I know.

 

At an engine speed of 6000 RPM the spring is cycling 50 times per second. This generates a considerable amount of heat which is bound to affect spring tension over time.

 

That is news to me. I'll believe you if you can explain where this excess energy comes from that can raise the temperature of the spring. I would have thought the spring temperature is dominated by the surrounding oil. (or not in the case of VF engines....). Anyways metals start to creep appreciably at about 0.4 of their melting temperature (in K) so the spring is nowhere near this, assuming the steel melts at about 1800K. That's not to say it doens't happen tho..

 

Yes it seems like semantics maybe. I don't see fatigue as cracks as much as weakness. I understand when you say cracks from fatigue. My point being is the pressure is more important to me then free length measurements. I've come not to trust manufactures.

Jeremy you might find this vid interesting. This shows what I'm pointing at. Without knowing what pressures the spring is producing when needed it turns in to a crap shoot.

YouTube - ‪Valve Float‬‏

 

The valve spring is basically a torsion bar wound into a coil. If a torsion bar is twisted (flexed), heat is generated by internal friction, lots of heat when the bar is twisted back and forth at a high oscillation rate. Adding to this is heat from the engine oil, heat generated by friction against the seat in the head and the spring retainer and heat from the head generated by combustion. All this heat results in the spring losing tension over a period of time and the spring can no longer properly do its' job. The standard rule of thumb for determining if the spring is not servicable is to measure its' free length. If the specs for seat pressure, installed and compressed length were available it would definitely facilitate an accurate evaluation of the spring's condition.

 

OK, well we will just have to agree to differ on that one. Not that it really matters anyway, the VF500 springs don't seem to be up to the job long term whatever the mechanism of relaxation. The general consensus on another motorcycle usenet group I read is also that compressed air pumped into the cylinder will hold the valves closed sufficiently to change valve springs and stem seals. I found this site which illustrates it pretty well for a Lexus engine. How to replace valve seals without taking off the heads

 

Please take photos along the way. Looking forward to seeing what you find.

 

Will do. Here's a picture of the bike.

 

That's the cat's pajamas!

 

That is a fine looking ride, good score!! BTW, I have used the air pressure method of holding the valves closed on auto engines, works just fine. As grey points out, a length of snug fitting rubber hose slipped over the valve stem will keep it from falling in if it is necessary to remove the air supply.

 

Nice looking bike. The full fairing looks oem. Who made it, do you know?