Valve Spring Length?

[chrisryder]

Howdy, been working on a 1275 head to put onto my 1098. Distances between valve and head face are: 7.2, 7.7, 7.8 and 7.5mm. So just to be on the safe side i'll be getting them all sunk to the magic 7.8.

Supposedly it's good practice to shim up the springs so that they apply the correct force. It's been said that you shim up by as much as you sunk down, but as they're all at different heights, they've obviously all sank at different rates over their lifetime, so if i was to shim up, i'd do best to shim up to put the spring at the correct (fitted) length that it would have been when it was born.

So the question is... how long?

I can't see much point in worrying about it. If i were to shim by however much i sink, it'd be a bit of a drop in the ocean to shim one of those springs by 0.1mm, not to mention the difficulty in making a shim that thin!

With the springs being longer than they were intended, they'll be applying less force, so will effectively be softer. I can't see softness being a problem, as it'll marginally reduce the amount of work the engine has to put into opening the valves. The other thing that soft springs will contribute to is earlier valve bounce, which could be beneficial to act as a rev limiter with my fragile 1098 crank!

Thoughts?

[Roni]

You might find the info in this article useful.
http://www.minispares.com/Articles.aspx?ty=ad&aid=149

[bmcecosse]

You CERTAINLY don't want double springs on a 1098! The magic clearance is 8mm (320 thou) - don't risk any less. And yes - you are correct - the weaker springs will simply absorb less power and promote earlier valve bounce. For the likes of a 998 Mini engine which is likely to get revved - the correct shim to fit would be 40 thou (1 mm) - fit to all 4 exhaust valves because as you correctly guess - some have already recessed by them selves. If you are happy with the reduced revs/weaker springs - then just leave them as they finish up after the 'sinking' . It is a good idea to run around the 'sunk' seats with a pointy stone to unpocket the seats as best you can - but don't take too much metal away. Also - stick a finger down the exhaust throat and feel how little space there is for the gases to turn the corner.... this can be GREATLY improved with a few minutes work with the grindstone taking metal off the corner beak and the guide boss in pretty much equal amounts - well worth doing.

[chrisryder]

It's only single sprung, and has had the exhaust port opened up in a past life.

i went around it with a carbide ball-tipped air grinder yesterday and got a few more rough spots out of it, including an actually sharp bit in an end exhaust port!

I had noticed that the valve which was at 7.8mm was starting to look rather shrouded by the head, so i'll look at relieving them when they're sunk.

My other worry is that the previous owner of this head told me 'number 8 valve spring broke, so i replaced it with one off a 1098 head i had lying around' would the 1098 spring be softer than a 1275 single spring? Should i get measure all of the springs with and without a known mass on to make sure they're all the same spring rate, or would i get away with fitting all 8 off my 1098 head?

I thought i'd give Declan Burns a run for his money on the measuring front:
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[bmcecosse]

Hahahaha - well done on the measuring.... a 1098 spring is MUCH shorter - no use at all on this head..... have to wonder though why ONE spring broke - maybe they are all now past their best. What rockers do you have for this head?

[chrisryder]

hmm... all the springs look the same length overall (all sat in a line on the workbench, none are higher than the others...) but i will go and measure them just to be sure.

the rockers are sintered solid ones.

i'm not sure now whether he said he got the spring of a 1098 head, or just another head...