Valve clearances - advice

[deequeue]

Hi All,

So in 13 years of bike maintenance, I've never had to change a valve shim - that's changed this evening, as 2 of the clearances are out of spec on my NC30.

I just wanted to confirm my thoughts really:

Am I correct in thinking that the clearances will only close up? As I understand it, the spec values are to account for the valve stems stretching - Eg: an exhaust clearance of 0.27 will close up to 0.26, keeping within spec, whereas a clearance of 0.21 would close to 0.20, taking it out of spec.

Is this correct, or am I simplifying things too much? I ask as I have 2 or 3 inlets at 0.13, and a few exhaust at 0.26 and one at 0.27.


Also - the 2 that are out of spec, one inlet at 0.20 and one exhaust at 0.30. I've removed the shims, and have a 248 and 220 respectively - am I correct in thinking that if I switch the 248 to the exhaust valve, it will drop the clearance from .3 to .27?

Apologies for the questions, but I want to get my shims sorted over the weekend and would rather ask you guys than guess.

Thanks
Dale

[speedy231278]

Yes, they close up. The valve stem might stretch, but you also get wear on the seat and face. Whatever is causing the difference, I can't think how the clearance might become larger.

What exactly do you mean by 248 and 220 shims? Is that the number on them? I'm not sure how that translates to the actual thickness as I would interpret that at 2.48 and 2.2mm, which doesn't tally with your suggestion of it tightening up by 0.03mm. Or is it 2.248 and 2.220? However, that doesn't tally with standard Honda sizes, which go from 1.200 to 2.900mm in 0.025mm increments.

You have the right idea though. You add or subtract the difference in shim thickness from the clearance you have measured to work out what the new clearance will be. For example, if one of your 0.13mm inlets has for example a 2.275mm shim in it and you want to take it back to the full 0.18mm, you need to remove 0.05mm from the thickness of the shim. In this case, you'd want a 2.225mm shim, as 2.275-2.225=0.05.

Hope that makes sense and/or I'm not teaching you to suck eggs! I have the pleasure of doing this job very soon...

[deequeue]

Thanks for the quick reply Speedy,

Cool, i thought that was the case - The bit that I don't get is how the two clearances have increased to beyond their original specs - the only thing I can think is that they were set incorrectly when the clearances were done at 16k. Its only done 19k now so, I cant see how that would have happened in 3000 miles.

I was quite surprised actually - its only done 400 miles since 2007, but everything else has been spot on, underneath the fairings is really clean, the carbs were very clean, looks like it has new oil in and its been loved by one of the previous owners

As you thought, the shims themselves have 248 and 220 stamped on their tops. I think I may need to get my vernier caliper out to confirm their sizes - although I might need to buy myself a digital one, as mine is... shall we say.... manual - I assumed the numbers would make sense to you guys :)

Good to see im on the right path though - why do I get the feeling I'm going to end up pulling all the shims out... worst thing is I have 3 bikes to do this on - at least with the NC21 it was an adjuster and locknut!

[speedy231278]

Numbers might make sense to someone who's done shims before. I've not actually done it yet, but I've read everything written on here and gone through the Haynes a few times (fortunately it is pointed out here that their pic of the feeler gauge shows it in the wrong place!). Maybe there's a code on the shim rather than an exact size, or maybe your shims are simply different increments. I don't know, but I bet other 10 people here do....

[bikemonkey]

If you need a bigger gap, get a smaller shim.

If you need a smaller gap, get a bigger shim.

Simples

[pip]

The shim sizes are 2.20mm & 2.48mm thick

You need a 2.25mm shim to bring your exh clearance down to .25mm
Ideally you need a 2.52/3mm shim in the inlet or a 2:50mm at a push (please note, I don't have the clearance specs to hand & am assuming exh is +/-:25 & inlets are +/-.15mm, crap memory l!)
It's always wise to use a micrometer when swapping shims)

When changing shims bear in mind that replacement new shims come in .05mm increments which can often mean that you either choose to be at/near the upper OR lower clearance.

I have just had the 4 cams out of my vfr750 (probably the 1st time they have been done?) - some clearances were larger, some smaller than spec (some were in spec too), so I believe clearances can increase despite logic suggesting otherwise.

Another point - don't get hung up on getting them all perfect, you'll be there for ever. As long as they are in spec they'll be fine, in fairness the engine will run fine even if clearances drift a little out of spec.

My 1st NC 30, 13 of the clearances were 'quite' tight when I checked the valves, it started & ran perfectly but obviously it's not ideal.
Another bike I serviced for a mate (Honda 600 v twin) had pretty much zero clearance on any of the valves but still ran fine before & after valve adjustment.

But yeah, shims are a PITA - bring back screw & locknut

Phil

[speedy231278]

2.48mm isn't a size on the Honda parts list, 2.475 is the closest, so I'm interested as to where it might have come from. IIRC, the NCs use 7.48mm diameter shims. You can get 2.48mm thickness shims, but these appear to be only available in 8.9mm diameter. Is it possible that someone has put an oversized shim in by mistake?

[bikemonkey]

The way I'd see it is the shims are being bashed about constantly while the engine is running, especially seeing as its a high revving engine. Add that to the multiple heat cycles the shims go through, I wouldn't be surprised if they got flattened or expanded like the valves themselves.

[deequeue]

Ta for the replies all - Its nice to see my initial thoughts were right - even if my maths were out on shim sizing

Speedy, if you've not done the NC before, you're in for a treat after the nice simple ZXR, I have a ZX6 myself and the front Vee is quite simply the most infuriating thing I've done to a bike, mainly getting the damned cover off in the first place :) At least you don't have to pull the cams to get the shims out on the NC

Pip - many thanks for confirming the exact sizes of the shims - the specs are .21 - .27 ex and .12 to .18 inlet. I'll be measuring their diameter to check they are the right sizes, and if it comes down to it, I can get them measured at the mechanics for peace of mind.

I was going to try and get the .13's up to middle spec, but if they only come in .05 increments, as you say, it'll put them at the very end top end - Maybe I'm just being a bit anal with them, but she's going to be treated properly from now on :)

Fortunately the ones at the bottom end of the spec are in the back Vee, so are easier to keep an eye on.

I'll let you know how I get on.

[speedy231278]

The way I'd see it is the shims are being bashed about constantly while the engine is running, especially seeing as its a high revving engine. Add that to the multiple heat cycles the shims go through, I wouldn't be surprised if they got flattened or expanded like the valves themselves.

Seems a fairly sensible conclusion, although I'd also suggest that shims made from a material that would get squashed by the rockers over time wouldn't be much cop as their job is in an area of the bike where extremely fine tolerences are required. I guess that's why they are checked fairly often. Not as often as my ZXR750 according to the service manual, and that revs 2K less.... I guess it's whatever the manufacturer feels comfortable with.