Valve clearance tolerances

[yorkie_chris]

I'd say this will wear the alloy in the head round the sides of the bucket, but once this is slightly marked I think it will cause problems.

What size shims do you need. I will look through mine and see if I have any close to what you need.

Current ones in there were both marked 162 at just under 0.2mm clearance, so I assume I'd be looking for something between 155 and 158.

For the record, the post from Spannerman was quite informative, and I'll be changing the shims over for some of the spare ones tomorrow (162s as well). Works a lot better than yelling "OMGUNOOB!"

I always assumed the gap would increase rather than decrease. Can someone please explain what is making the gap get smaller, and if that is the norm?

Can do either. Clearance decreases as the valves wear into the seats and increases as cams/buckets etc. wear.

[yorkie_chris]

Exactly. Exhaust valves especially, as you've got very hot gas going past the valves every time they open. Then the very hot exhaust valve slams into its seat a few hundred times a second so gradually the seat and valve wear together taking up that clearance.

When the clearance gets to zero you get a big problem, the exhaust valve can only cool itself by heat conduction from the valve to the seat. So if they never touch the valve rapidly heats up. Cheap way is the valve then burns out. Expensive way is the valve starts to act as a hot-spot and initiates combustion instead of the spark plug causing preignition which can hole pistons.

[Spanner Man]

Originally Posted by Neeja

Current ones in there were both marked 162 at just under 0.2mm clearance, so I assume I'd be looking for something between 155 and 158.

For the record, the post from Spannerman was quite informative, and I'll be changing the shims over for some of the spare ones tomorrow (162s as well). Works a lot better than yelling "OMGUNOOB!"

Good morning.


As your clearances were .19mm (7+ thou in old money) I would replace the shims with ones that are somewhere between 1.52 & 1.55mm. This will give you a clearance of between .25 & .29mm, (10-11+ thou).
On engines with shim & bucket type valve systems, clearances decrease over time in the majority of cases. This is due to wear on the valve face, the valve seats, & the valve stem stretching.
Apart from an excess of carbon building up on the valve, instances where the clearances decrease are down to wear on the underside of the bucket, the shim, or the valve collar. Camshaft wear isn't a factor, as the clearance is measured on the base circle of the lobe, which never wears as it doesn't come into contact with anything, unless the clearances are under nil.
If the camshaft bearings were worn to the point where they had a significant effect on the valve clearances, your engine would be pretty 'tired' to say the least.

Some years ago, as an experiment, I attempted to reduce the thickness of a new shim, using my surface plate & some 1200 wet & dry.
I attached the shim to a magnet, & ran it across the wet & dry in a figure of eight motion, using lots of silicone lube. I was careful to keep rotating the shim to vary the leading edge.
After a 'pleasant' hour, I had reduced the thickness by a shade over 2 thou overall. However, there was a discrepancy of nearly half a thou at one point on the edge of the shim. In short I had tapered it slightly.
I had measured the shim carefully before starting, & it was perfectly flat. What's more, it was a shim for a Yamaha XJ, & these are 29mm in diameter, (as opposed to 7mm on an SV), & theoretically easier to hold squarely, especially when using a magnet. My conclusion, was that it's not practical to grind shims in this fashion. (£3 wasted too).

Shim grinding is an excepted practise in the automotive re-conditioning industry. However a proper shim grinder is a precision machine tool, that holds the shim squarely on a magnetic base, & the grinding wheel, which is of a very fine grade, passes over the face of the shim in a horizontal plane.
What's more, car engines generally have shims that are in excess of 30mm in diameter, & over 3mm in thickness, & therefore are easier to hold squarely on the magnetic base of the grinder.


Cheers.

OMG, this is another language! But very interesting. I'm sure we all check these at some point, or just pay someone else to do it

Checking your valve clearances is the most intimidating thing in the service schedule, imo. It is, however, actually really easy.

Agree, quite scary, as well as checking throttle bodies and getting all the hoses changed

[5hort5]

Just a quick one, I posted these on another thread but didn't really understand the reading, looking at the manual I think they are ok but are any of them getting close to needing changing? They from a 99 Curvy.

Exhaust:
Left - 0.21mm Right 0.25mm

Inlet:
Left - 0.13mm Right 0.13mm

Rear cylinder

Inlet:
Left - 0.15mm Right 0.15mm

Exhaust:
Left - 0.23mm Right 0.23mm

Thanks

Si