Valve Clearances

[pmapp]

Morning y'all

Checked my valve clearences last night in my nice outdoor workshop (brrrrrr). All fine and within tolerance - but towads the bottom end.


My question is..... Are valve clearances likely to close up over time, or open up. Applying my way of thinking, they would open up - but is this right?


Thanking you all,
Phil

[startrek.steve]

Morning y'all

Checked my valve clearences last night in my nice outdoor workshop (brrrrrr). All fine and within tolerance - but towads the bottom end.


My question is..... Are valve clearances likely to close up over time, or open up. Applying my way of thinking, they would open up - but is this right?


Thanking you all,
Phil

yep, they used to on my GS400...
stopped replacing shims in the end....
Steve

[Ceri JC]

Ooooh, controversial. Last time this came up, the forum was divided into two camps, one of whom swore blind that on 90% of the bikes they'd done it one, they had closed up, the other claiming they opened up. To be on the safe side, I'd be inclined to set it to half way between the tolerances (in either direction).

[ejohnh]

Isn't it likely that wear on the valve and cylinder head sealing surfaces would cause the valve stem to retract more, causing the clearance to decrease? On the other hand I suppose that measurable wear on the cam end of things would contribute to the clearance. Maybe mix of both :o))

[jambo]

Generally there's a pretty sound theory that says inlets open up due to wear in the valve train being greater than wear in the valve seat. The reverse is true of exhaust valves, which have a much harder life for the valve seats.

In practice it depends on the engine, and the type of use it gets, for my ZX6R I've been in there periodically over the last 40,000 miles as well as working on a couple more, and doing a check on my SV, and I'd say I've had more clearances go tight on both inlet and exhaust sides than wide. But there really aren't any guarantees. Here's what I'd do:

Everything is currently in tolerance. That's a win, you don't need to change anything. Keep a note of exactly what every clearance is now. Next time you check you can see if your bike with your engine, with you riding it seems to be closing up or opening up for each valve and adjust as you see fit.

Jambo

[embee]

Not being specific to any particular engine, but typically a "compact" OHC valve train will more often lose clearance, a large flexible valve train like pushrod+rocker design will often increase clearances.

It's usually not difficult to minimise wear of direct action cams/buckets with proper design parameters (there have been a number of infamous exceptions to this, due to poor design/materials), pushrod systems have a lot more components and wear faces, plus pushrod ball ends and rocker/valve tips are tricky things to lubricate effectively due to reciprocating or scrubbing action so tend to be subject to steady wear.

Wear or recession of valve seating faces can be minimised by hard facings (e.g. stellite) or high grade insert materials, but the downsides are cost of the materials and more expensive manufacturing processes so usually an "acceptable" compromise is used. That's called engineering. ;)

[Alpinestarhero]

(there have been a number of infamous exceptions to this, due to poor design/materials),

my dad had to replace one cam bucket on my SV due to excessive wear; a google search told him that suzuki had problems with the hard-casing material used on the early SV's and the tl1000s's. Theres another cam bucket on the rear cylinder that needs replacing, but that'll get done when i replace the rear shock, as apparently the rear shock needs to come off.