Maximum valve clearances. How big is too big?

Question

I have a 1700 Leyland "O series" engine which has what I believe to be a direct overhead cam (OHC). The lobes on the overhead cam push directly down on buckets with shims.

I have just transferred all the valve assemblies over into a used (probably), aluminium cylinder head and checked the valve clearances. They are all a fair bit larger than recommended ideals.

The manual says "...note that adjustment is only required if the clearance is less than the specified minimum" and this thinking is confirmed in the Specifications - as follows:-

Tappet clearance (cold - inlet and exhaust) 0.012 +/- 0.001 in. (0.3 +/-0.03 mm) Adjust only if less than: Inlet....................0.008in (0.20mm) Exhaust..................0.011in (0.28mm)

I've come to understand that there are many reasons why you wouldn't want the valve clearance to be much higher than the ideal. These reasons are clearly laid out in a clever demonstration on https://www.youtube.com/watch?v=Ou_3Qo8wTec video starting at 5:50 for loose valves.

So, can anybody explain why the Haynes manual is so indifferent to loose valves and what would a realistic maximum clearance be for the inlet/exhaust valves. I know I can get valve shims reduced in size at an engineer works but getting thicker shims (15.5mm diameter discs) seems quite difficult. I am quite prepared to go out my way to get a better performing and longer lasting engine but I don't want to waste my time if it doesn't really count for anything. I will at least swap around the shims amongst themselves to balance out the clearances.

Answer 1

First of all, thank you for this awesome question!

Haynes manuals are written for consumer/mechanics that work on their own vehicles. This means that some of their advice is centered on what you are likely to be able to do while working at home. I also believe that what they mean is that having too much clearance is a very rare problem, and it won't harm the engine, much, if it IS too much. Too little will destroy your cam shaft within moments of starting the vehicle.

Now to answer your actual question! The answer is in your post:

Tappet clearance (cold - inlet and exhaust) 0.012 +/- 0.001 in. (0.3 +/-0.03 mm) Adjust only if less than: Inlet....................0.008in (0.20mm) Exhaust..................0.011in (0.28mm)

It lists the tappet clearance with a +/- number, that's your clearance range. Ideally, you want the clearance to be between .011in and .013in. However, you have a little more leeway on your intake side, thus the note after that you should leave it alone if it is above .008. Anything less than .008 WILL destroy your cam shaft. So, using both parts of the notes, your actual clearances are:

1. Inlet .008in - .013in
2. Outlet .011iin - .013in

Sadly, the specifications are written by engineers for engineers, and sometimes that means reading all the little foot notes or doing unnecessary math in order to get any kind of usable number. And frankly, the only way that you could have known that is to ask someone that's had to translate these before, even in the Haynes manuals.

In your post, you mention wanting to understand what happens if your clearances are too high or too low. On the intake side, if the clearances are too high(More than .013"), then the valve won't open all the way. This causes the engine to run rich, which will damage your catalytic converters over time. I'd also be concerned about the amount of pressure being generated against the rings as the pistons struggle to pull air into the cylinder through partially closed valves.

When the clearance is too high(More than .013") on exhaust valves, some of the exhaust gases may not be able to escape the chamber, putting pressure, again, on the rings as the piston achieves top dead center(TDC). Eventually, some of the exhaust gases will begin to escape around the rings, creating excessive pressure within the crankcase.

Now, with the clearances too low(.008 on intake and .011 on exhaust), then the valves may open too far, allowing the piston to hit them. This would damage the piston, valves, and camshafts. If low enough, the lobes will press the valve springs up to their travel limit and still have some way to go. That pressure on the lobe must go somewhere! So the camshaft begins to warp and grind against the journals. Ultimately, low clearances will absolutely destroy camshafts, if you're lucky. If not, it'll take your pistons with it.

The long story short is this: do not exceed your clearance ranges.

Please let us know if this answers your question or if you need any further information.