Tappets

[Zombie Master]

So what are the current popular valve clearance these days on post 70 airheads?

My book says .004 and .006.

If you deviate from the manual....why?

[Kurt in S.A.]

The numbers actually changed a bit, even by BMW. I use 0.15mm (0.006) and 0.20mm (0.008). The exhaust is more important because it allows the valve face to remain on the valve seat for just a little bit longer thus extracting more heat from the valve. The intake 0.15mm is not that important as the air rushing in is cooler so things aren't as hot. But it couldn't hurt IMO.

Kurt in S.A.

[Zombie Master]

Wouldn't a tighter clearance actually leave the valve in contact with the seat longer?

[gspd]

Kurt said “The exhaust is more important because it allows the valve face to remain on the valve seat for just a little bit longer thus extracting more heat from the valve.”

The theory is correct, but exactly how much difference it makes as far as engine longevity is debatable and incalculable. Racers run little or no clearance for more lift (more power). Most vehicles now have hydraulic lifters and run at zero clearance their whole life without ill effects.

As far as adjusting clearances, I still don’t comprehend why some people still insist on feeler gauges when a there is a more accurate and easier method to adjust clearances on airheads.
OK, here it is (again) if you’re interested; It's almost too easy.
Keep in mind…the adjuster threads have a 1mm pitch. One turn = 1mm.

TDC, valves closed.

Roll the pushrod between your thumb and forefinger while slowly tightening the clearance until you can no longer roll the pushrod between your thumb and forefinger…stop! That’s zero clearance

Now back off 1/6 turn. one flat of the hex.
that’s exactly .16(666666)mm clearance (.15 to .20mm is recommended for exhausts)
A smidgeon tighter will get you to the .10 to .15mm recommended for intakes

K.I.S.S

[Zombie Master]

I forgot about this method. If my tappets need adjusting I'll try this out. But I'll have to check my work with feeler gauges until I'm sure I've got a feel for it. 1/6 of a turn is a fine adjustment!

[Rob Frankham]

I'm sorry but I have to take issue with some of your comments...

Kurt said “The exhaust is more important because it allows the valve face to remain on the valve seat for just a little bit longer thus extracting more heat from the valve.”

The theory is correct, but exactly how much difference it makes as far as engine longevity is debatable and incalculable.

Agreed.

Racers run little or no clearance for more lift (more power).

Very few racers run with NO clearance. That would be incredibly foolish because, when the engine is hot, the exhaust valve won't close properly. I would suggest that a good racing machanic would adjust for zero clearance when the engine is at operating temperature which is a very different thing, especially in a push rod engine, and which is actually the ideal situation. The clearances, set when cold, are, in fact, a way of achieving a minimal safe clearance at engine operating temperature without the complication and expense of adjusting when the engine is hot.

It's also worth considering that most racing engines are rebuilt regularly so the whole comparison is somewhat academic.

Most vehicles now have hydraulic lifters and run at zero clearance their whole life without ill effects.

Totally different system which eliminates the need for valve clearance... which is precisely why it's used... in no way relevant to airhead engines.

As far as adjusting clearances, I still don’t comprehend why some people still insist on feeler gauges when a there is a more accurate and easier method to adjust clearances on airheads.
OK, here it is (again) if you’re interested; It's almost too easy.
Keep in mind…the adjuster threads have a 1mm pitch. One turn = 1mm.

TDC, valves closed.

You're right... it is too easy (although I don't see it as being any easier than a feeler gauge). It completely ignores the first, diagnostic purpose of valve clearance checks and the third equally important part of adjusting the clearances.

The first part is to see if the clearances have changed in the service interval... If they have, you need to be looking to find out why. It could highlight a number of problems including but by no means only the issues caused by VSR. You're 'system' cannot do this. Incidentaly, if they haven't changed at all, there is no point in carrying out part two which is to actually adjust the clearances.

The third part is to carry out a QC check. Turn the engine over at least once, preferably two or three times, and then check the clearances. This ensures that no random element has affected the checks you've just made. It also may highlight uneven wear to tappets. Again your system can't do this.

Your system may well result in acceptable valve clearances in a 'perfect' engine but it ignores the main reason for servicing which is to examine parts and to diagnose incipient issues before they cause problems.

Roll the pushrod between your thumb and forefinger while slowly tightening the clearance until you can no longer roll the pushrod between your thumb and forefinger…stop! That’s zero clearance

Now back off 1/6 turn. one flat of the hex.
that’s exactly .16(666666)mm clearance (.15 to .20mm is recommended for exhausts)
A smidgeon tighter will get you to the .10 to .15mm recommended for intakes

K.I.S.S

Sorry, can't find a ruler with markings for a 'smidgeon'.

Rob

[Rob Frankham]

Wouldn't a tighter clearance actually leave the valve in contact with the seat longer?

No, with a tight valve clearance, the valve opens earlier and closes later so 'closed time' is less.

If the clearance is too tight, the valve will never close which will have a very undesirable effect on the performance and longevity of the engine.

It's useful to understand why valves are set with a clearance in the first place... it's not to ensure that then engine makes a pleasant tinkling noise when it's running, it's to allow for the expansion of the components in the valve train as the engine gets up to temperature. In a system where this expansion is allowed for or controlled (such as a system with hydraulic lifters), there is no need for valve clearances and, indeed, there aren't any. In a pushrod engine like the airhead, the ideal would be to adjust the clearance to a minimum with the engine at maximum operating temperature, however, that's hardly practical so almost all pushrod engines are adjusted when cold to a figure that allows for calculated expansion (and a bit more to allow for the cag handedness off the average fitter). In an air cooled engine, it is desirable to have a little more clearance than in a liquid cooled engine because the head temperatures aren't so closely controlled.

Rob

[barryh]

I run a tightly set 6 thou and 10 thou because I prefer the slightly milder cam timing it provides. It's a very small effect but noticeable. It also in theory cools the exhaust valve a little but I once worked out in approx. terms how much longer the valve remained on the seat and it wasn't much bearing in mind the exhaust valve only opens once every 720 degrees of crank rotation so if it was to stay closed an extra 10 degrees that's not a lot in percentage terms. I think after allowing for how long the valve is open it came out as something like 2% more cooling.

I've found the valve train has been carefully designed so that there is almost no difference in clearances between hot and cold. I will usually change the oil hot then check the clearances while the engine is still warm if not hot. I've done the experiment to check them again when stone cold and there was no measurable difference. In any case it matters more that both cylinders are set the same than they are accurate to a half a thou. Everyone should try that experiment at least once. I expect very few if any of us would have the bike serviced by BMW but do you think they ever wait until the engine is stone cold ?

On the subject of measuring them hot the ultimate would be to check them with the engine running. I once had a Vauxhall car where that was the prescribed method but it was messy and tended to chew the feeler gauges.

[Airbear]

Reading GSPD's instructions again, and without pedantically going through the maths (math), I'd say that a smidgeon is around about 1/24th of a rotation, y'know, give or take. Seems simple enough to me.

I like to use 0.10 and 0.15 (4 and 6 thou), so far using feeler gauges and checked every 5000kms at oil/filter change interval. I suspect that for BMW it was better to have called for slightly wider gaps to add a safety margin since they could never know how frequently the gaps might be checked and, well, BMWs lasted forever, didn't they?

I recall the advice from How to Keep Your Volkswagen Alive: A Guide for the Compleat Idiot, that it's perfectly ok for your tappets to "click a bit".

I like the logical simplicity of GSPD's method and will give it a shot; thanks for mentioning it again, GS. I will measure clearances with the feeler gauges as normal - 'go, no-go' fashion. But I'll admit to finding it difficult sometimes to be happy with the precision I can get with flimsy feeler gauges. Rotational angles are easy - it's simple jommetry (geometry), in'it?

The old girl's gaps have been slowly closing in the last five years, valve edges are razor sharp and seats and guides are well worn, so time for a head job after a couple more rides maybe(?) It's almost the last thing to deal with as we've worked through her 'motorcycle menopause'.

[gspd]

Rob said:”The clearances, set when cold, are, in fact, a way of achieving a minimal safe clearance at engine operating temperature without the complication and expense of adjusting when the engine is hot.”
The valve clearances do not change measurably with temperature. The clearances are to allow for long intervals between adjustments. You would be amazed at how powerful, smooth and quiet your engine will run with zero clearance. But then you’d have to check clearances every few hundred miles to be sure there is some, as (all?) racers do. Also, the recommendation to adjust valves when cold is primarily so the mechanic doesn’t burn his hands.

Rob also said:”The third part is to carry out a QC check. Turn the engine over at least once, preferably two or three times, and then check the clearances.”
You can recheck all you want, but turning an engine once, twice or a hundred times, does not affect valve clearances.

The method I described provides guaranteed in spec clearances with zero trial and error.
Feeler gauges, especially worn and bent ones, do not compensate for unevenly worn surfaces or operator inconsistencies.
Visualize a feeler gauge between a set of points that has a valley in one side and a peak on the other; your rocker arm to valve stem surfaces may be more like that than you think. They are rarely perfectly flat on a worn engine. Also, you can ask ten different people to set a gap with feeler gauges and get ten different results, depending on uneven wear and how tight or loose they decide the blade should fit between the parts. 1/6 of a turn, one flat of a hex, is a consistent no-brainer.


On my bike, I pop the covers off every oil change (5000kms) and check clearances by feel. I only have 520,000kms on my bike, so I'm not sure how my method will pan out in the long run but I'll keep you posted..

Oh and btw, for those of you who can't extrapolate, smidgeon means c%$@hair,