So WHY are valve clearances so blasted important?

I was thinking about this on the way home; so many people are reluctant to touch their valve clearances because the process seems daunting, and they see no "immediate need."

In my previous rant on this subject http://xjbikes.com/Forums/viewtopic/t=32118.html I explained the lack of maintenance vs. the recommended service interval situation, so I won't repeat it here.

No, what we're going to do here is break down the science a little bit and explain exactly why it matters.

Experienced engine guys, please feel free to jump in and add on!

HERE WE GO:

-The internal combustion engine is at its heart, an air pump.

-The valves we are discussing open and close to allow fuel mixture in, compression to occur, power to be applied, and exhaust gasses out. They are the "gatekeepers" of what flows into and out of your cylinders.

-The timing and duration of those openings and closings are vital to proper flow in and out, and proper compression and combustion; therefore, power.

-Let's use 6000rpm as our "data point." At 6000rpm, the crankshaft rotates 6000 times in a minute; or 100 times a second. (see why I chose 6K, the math is easiest.) Therefore (it's a four-stroke motor, remember) at 6000 rpm, the valves have to open and close 50 times per second. FIFTY. Let's do it: One-Mississippi- FIFTY openings and closings!

What happens at 10K rpms? One whole hellova lot more than 50 per second!

-Now then. The valve clearances are what control exactly when a valve opens, and when it closes. Too tight (and that's usually the case) and the valve gets opened too soon; and stays open longer than it should. In the case of an exhaust valve, this means that the valve is spending longer than expected exposed to the stream of hot gasses.

-If a valve is tight enough, and it doesn't need to be dead tight, if it's opening too soon and staying open too long, there's a point where at oh, say 50 OPENINGS AND CLOSINGS A SECOND (6K rpms remember) it might not have time to get fully closed before being opened again, meaning it's effectively always open. (That's known as "valve float" to the old guys.)

What if it's an exhaust valve? It's spending all of its time in the stream of hot exhaust gasses. Now you know why we caution about the possibilty of "burning a valve" if you neglect your clearances.

KEEP YOUR VALVES IN SPEC. Above is the main reason why. Make sense? I sure hope so. I get tired of typing the same thing over and over; especially when restrained by XJ LAW #2: No Saying I Told You So.

Ride safe--- Fitz

 

If the crankshaft is turning @ 6K what is the RPM of the cam shafts ?

 

3000 rpm.

It's a four stroke motor. Therefore, TWO revolutions per power stroke.

Camshafts on four-strokes turn at half crankshaft speed. C'mon Wiz...

 

And it's not just that the valves (particularly the exhaust valves) are spending too much time "in the stream of hot exhaust gasses"; the lack of enough "contact time" (meaning: fully closed valve, in contact with the valve seat in the cylinder head) does not allow the valve to transfer off its accumulated heat to the cylinder head, and thus the valve face and seating area gets "burned" (warped, etc.), thus preventing it from ever fully seating (therefore it will no longer seal off the combustion pressures).

 

Hey Fitz, well put !

Compression, Ignition, Fuel, IN THAT ORDER !!

 

So TO CLARIFY:

If the crank is turning at 6000 rpm and the cams at 3000 rpm, the crank is rotating at 100 times a second, so the valves have to open close HALF that often, or 50 times a second... at 6K.

(Which is why I chose 6K, dammit; I've had one too many Labatt's to do more complicated math right now.)

Do your own math for 10K rpm.

 

VERY GOOD POINT and equally important to the "science of not burning a valve." And another reason why the amount of time spent closed is as important as the amount of time spent open.

They generally don't actually "burn" until they've warped/quit sealing.

Thanks.

 

I just measured my clearances and got the following measurements:
Cylinder: 1 2 3 4
Exhaust: .006 .009 .007 .008
Intake: .006 .004 .006 .004

However, when I put my caliper on the feeler gauges they measure slightly bigger than they claim to be. The .004 measures at .0045, the .006 measures at .0065. This is important because depending on which measurement I go with determines which of the two intakes I need to swap out as well as determining whether I need to swap out #1 exhaust (.004' = .1016mm, not in spec, whereas .0045 = .1143, in spec; the problem is the opposite for the other two intakes which are in spec at .006 but not .0065).

So my two questions are as follows:
Which of these measures should I trust?
Are any of these so out of spec that they run the risk of damaging the engine if I continue to drive it for the next couple of weeks while I await my valve holder tool and then a couple more while I await the appropriate shims?

I had posted this in a different thread in 'FAQ suggestions' but the traffic there seems quite low. Sorry for the double post.

 

I learned it a little differently.
How Long the Valve is Opened and Closes depends on the Lift and Duration of the Camshafts Lobe.

Valve CLEARANCES must be maintained to assure that the Valves remain within their Critical Operating Tolerances to prevent Engine Damage.

Too Tight:

Valves which are too TIGHT place the Valve in a condition which is harmful to the Engine.
The VALVE needs to CLOSE fully tight for there to be HEAT TRANSFER from the VALVE to the SEAT.
A TIGHT Valve will retain Heat and cause Temperatures to rise in the Valve.
Leading to overheating.

A TIGHT Valve which fails to remain GAS Tight, ... allows a stream of Super-heated Gasses to Escape the Combustion Chamber.
Igniting Gasses not meant to be exposed to Ignition yet.
Causing Performance deteriorations.
Back-fires.
Eventually, scorching the Valve Face and Seat >> "Burning-a-Valve" & "Burnt Valves", ... requiring the Motor to need a Valve Job.

Loose Valves:
(Not generally a Condition that is cause for alarm. EXCEPT!!! Our Engine Design has VALVE Clearances maintained by SHIMS at the Top of the Valve Spring Bucket)

On our Bikes, ... LOOSE Valves present a Danger.
If the Valve gets too loose, ... ( a rarity ) ... there is a Danger of "Throwing a Shim" -- "Spit the Shim" "Flipped a Shim"

A Situation that can have Catastrophic results.

So "HEAR" we are.
Learning about Valves.
Take this away with you.

If you can HEAR your Valves, ... that good. They're closing.
They should "Tick" a little bit.
If the Ticking gets too loud,... Investigate.

Don't run them loose because of Burned Valve paranoia.
But, ... DON'T Ignore them because the Bike is running good.
If you have a rainy weekend.

Check your Valves, ... if you dont know how they are right now.

 

Obiter_Dictum Save yourself a lot of consternation and just get a metric feeler gauge: http://www.thetoolwarehouse.net/p-2399- ... -2274.aspx widely available, even in the frozen north.

I would re-shim the tight valves, being very concerned about the tight exhaust valve.

That being said, as long as you take it easy neither revving nor lugging the motor it should be OK as long as you're not a 120-mile a day guy.

 

Rick's got it just right. They need to be loose enough to transfer heat to the head.

Theoretically too loose will lose some performance, but I think the noise would drive you insane before you noticed the performance fall off. Of course there is the issue of throwing a shim if way loose.

Now another piece of interest: Why are the clearances specified larger on the exhaust valves than the intake for most engines (including the XJ)?

The stem of the exhaust valve is exposed to those hot combustion gasses escaping from the cylinder. This makes the valve stem heat up and get longer, reducing the clearance. The intake valves on the other hand are exposed to cool intake air and don't lengthen as much. So there doesn't need to be as much slack for expansion on the intake side.

I routinely see Chinese scooters with less than 1,000 miles on them that start and run just fine. When the rider comes to the first stop the engine shuts off and won't restart for 5 minutes, at which time the cycle repeats. This is because the exhaust valve clearance is too tight and the heat of high speed riding lengthens the stem to where the valve doesn't seal. At low rpm the fuel-air mixture leaks out fast enough there is insufficient compression for ignition. After the valve cools it re-seals and the engine will start again. The only reason the valves aren't destroyed is the engines are so low power. In a real engine (XJ) they'd be toast in a hurry.

If the Chinese ever learn to properly machine a head I'll lose a nice revenue stream. :wink:

 

Thanks BigFitz, I actually thought I had bought a set of metric since that is what they were labelled as on the package.
Today's lesson for this noob: BUY QUALITY TOOLS because buying cheap may prove to be just too costly.

 

I've had that problem before...you can't just get a "Metric" feeler gauge and expect it to work easy. This is a LOT harder with a standard gauge set. .004" is .1016mm, so .05mm smaller shim brings you to .1512mm ...so you go from out of spec tight to out of spec big? AAARgh!!!

You need to look at the "options".

Standard is.001, .0015, .002, .0025, .0028, .003, .0035, .004, .005, .006, .007, .008, .009, .010, .011, .012, .013, .014, .015, .016, .018, .020, .022, .024, .025, .026, .028, .030, .032, .034, .035, .040", the metric equivalents are all over the place!

Metric is 0.0400 , 0.0500 , 0.0600 , 0.0700 , 0.0800 , 0.0900 , 0.1000 , 0.1500 , 0.2000 , 0.2500 , 0.3000 , 0.3500 , 0.4000 , 0.4500 , 0.5000 , 0.5500 , 0.6000 , 0.6500 , 0.7000 , 0.7500 , 0.8000 , 0.8500 , 0.9000 , 0.9500 , 1 mm, nice, even, orderly....of course it only matters because the shims come in mm's!

If you could get more precise between .10 and .20 mm, then you could convert a bit easier, but the resolution of the gauge is too big...basically, there's only one "correct" gauge....if a .10 mm fits, it's too small. if a .25 mm fits, it's WAAAY too big. It says .11-.15, but since you can only get .10 or .15, it might as well say .15 only, since that's all you can measure....but a nice, tight .15. If a .10 goes in..you got some swappin to do!

Of course, the shims go in .05 mm intervals too, so if you measure .09 mm, you know you can get one size smaller, go to .14, and be ok.

I don't think chacal has a true metric gauge, but he does have one with a lot of blades.

And the way to properly check requires that there is some "drag" in the gauge...so that's probably why they mic at .0005 over. If you put a .15 blade in a .15 hole, you wouldn't get any drag, after all.

Next time I do it it's gonna be with plastigage! If I can figure out how! (Not the reading...the crushing)

 

http://www.harborfreight.com/32-piece-s ... 32214.html

So what about this one?

 

http://www.autozone.com/autozone/access ... 66516_0_0_

Or that?

 

Nooooo, Compression, Fuel, Ignition.

 

Standard SAE set, incremented in 0.001", may also have metric equivalents. I use one like it. Problem is metric equivalents don't hit those .11, .15, .16 and .20mm values you'd like.

It's metric but doesn't list the sizes. It takes more than 25 to get from .04 to 1.00 mm in .01 increments. Somewhere it's got bigger gaps than we want.

Personally I want a set that goes from .02mm to .25 mm in .01 increments. I have yet to see such a beast.

 

Your right thats the way i learned it. Something to add when yoh have an excessively tight valve its keeping spring pressure off the seat which is keeping pressure on the cam lobe causing heat and acceleration wear on all valve train parts .
To loose and the and the valve train is actually smacking the cam lobe rather then following the contour of the cam lobe and the duration of time that the valve is actually open is less because there is a greater gap between the lobe and valve train. And what a lot of people don't know thru all the opening and closings of valves after time they start sinking in the head and thats what tightens up clearances

 

The harbor freight one is in inches (and metric, but it's integer denominations are in inches).

I can't quite tell about the autozone one, but I do remember that I thought they had a true metric feeler gauge when I was in their store one time....it wasn't the one I bought though... I wish it were.

I guess my point is that just because it's labelled in metric doesn't mean is numbered in nice, even metric sizes. It would be like saying a 1/2" wrench is metric because it's exactly 12.7 mm.....still won't work on a 13mm bolt though.

 

NOOOO fuel compression ignition exhaust or suck squeeze bang blow

 

A couple of cool videos
http://www.youtube.com/watch?v=5JtcqAtj ... er&list=UL
http://www.youtube.com/watch?v=_REQ1PUM0rY

 

Another great write up!
Thx again for taking the time to explain in detail "why"
we need to do it the right way every time.

Since joining our little group I've had the opportunity to
help many friends get thier bike's running the way they
should, and always with the statement "here's what I've learned
this week on the xj site"

ron

 

You're more than welcome. What I was hoping for was exactly what I got; some good "add on" info forming the basis of a solid technical discussion as to WHY what happens inside our motors happens, and the reasons behind the need for proper maintenance.

For a lot of folks, especially those new to old motorcycles, simply ignore that which they can't see, hear, or understand. I'm just trying to help spread some understanding.

 

Thread removed.

 

It's all related. What I said and what Carl and Rick (and you just now) added is ALL part of the same process. My point to begin with was to start just such a discussion to raise awareness and expand understanding of the function of the valvetrain and the importance of maintaining proper clearances. Our points are not in opposition; Rick simply added even MORE facts to what I had already put forth, as did Carl.

I want the new members, the ones who don't understand WHY we keep insisting on keeping the valve clearances in spec, to read this. We already know and understand; what I'm trying to do (with your help and everyone else's) is to spread that knowledge and understanding through a technical discussion; not have a debate.

 

Post removed, I have nothing to offer.

 

No, no. XJ LAW #2: No Saying "I Told You So."

And you're right; it's not a debate if we're all in agreement, which we are.

 

It has certainly enhanced my understanding. I always thought it had to do with the contact FORCE with which the valves hit their seats as opposed to contact TIME (which left me very confused as to why we want them loose instead of tight).

 

If you put the fuel in there before you are on the compression stroke, it won't stay there.

 

Wheres it gonna go?

 

Ya just so everyone knows. Autozone does not carry a true metric feeler gauge

 

I didn't even think to post where I got that other one that I copied the numbers from!!!

http://www.google.com/products/catalog? ... 363030292#

 

Fitz, Wiz, Rick and Carl all gave some really good info on this thread!

There is one important thing that wasn't mentioned... if the valves are out of adjustment then the vacuum will be wrong!

If you are going to sync your carbs then the vacuum becomes a real factor to consider.
That is the other reason for checking valves before syncing and colortuning your carbs.
That is also the reason for doing that job 1st.
Valve adjustment is important to a proper tune up.

 

Precisely.

Per the manual(s) "valve clearances must be within specification before synchronizing the carburetors."

Thought I'd add a little bit more about INTERVALS:

Book says initial check at 3000, then every 5000 thereafter. The initial check was the most important, and often never done.

When I got my '83, it had never been touched, and all 8 were tight. So I put them all in spec.

Now I'm 5704 miles down the road from that adjustment, time to check them again.

I now have one out of spec, and one that I need to keep an eye on. The others have all tightened up by about .01mm as expected but are still in spec.

GOTTA check 'em every 5000 miles; the book is right. And check them right away unless you have solid proof that it's been done at some point less than 5000 miles ago.

 

Every 5K wow, that is like 90 days for me. The actual mileage was 95 days I put 5K on my bike.

So here is my noob question: I need to check the clearances first, then order the shims needed to bring it back in to spec. So, the question is will this change in the few days it takes to get them from Len? Or should I do my check, order and wait for them to come before riding the bike. Though right now it is my primary transportation.

 

Check the clearances, then button it back up and ride it until the shims come; unless you have a valve or two that are critically tight.

The advantage to doing it every 5000 miles as recommended is the fact that you can "catch" them when they first go out by only .01~.03mm or so, so riding it a little longer while you wait for a shim won't hurt anything.

For instance, I now have an exhaust valve that's at .15mm. Out of spec, yes, but only by .01mm so no danger of hurting anything riding another couple hundred miles on it until I get it re-shimmed.

My round-trip commute is 120+ miles; running one slightly tight valve for a few days won't keep me off the road. They don't change that quickly.

 

That's hilarious guys ... I was referring to the order in which you tackle the tuning issues on the engine, not the order of the Otto cycle !

Any 4 cycle engine needs these three elements to operate but tackling them in the Compression, Ignition, Fuel sequence will help prevent red herrings in the tune-up process.

 

Solid advice.

But people still seem to think that simply denying that their valves need periodic checking and adjusting ("the bike runs fine") will make the requirement go away.

So I'm bumping this thread to save a whole bunch more typing.

If you don't understand why your valves need to be periodically adjusted, then go back and read this whole thread.

 

Honestly, anyone here should be able to check and replace their valve shims. There is a great how to with detailed pictureas well. I am far from being machanically inclined. But I managed to do this. Ultimately I think people are afraid to do something interal on their bikes. Some I fear think it is just something to make money for the dealers a 'suggested' maintenance item. Personally, I am estatic this site is on the net and all this info is here because without it I would just ride my bike until it just died. These bikes do require a certain level of maintenance perhaps higher than most cars or trucks and modern bikes. Like it has been said here many times these bikes because of age and special knowlege needed are pretty much DIY. But at the end of the day you'll take more pride in your bike because you know you did the work yourself. Bottom line, I love my XJ I am going to do everything I can to keep her on the road for as long as I can. If that means checking valve clearances two or three times a calendar year so be it.

 

Me too. It simply means sticking to the maintenance schedule or it will bite you. Most people are used to cars that they can simply ignore everything in the chart except maybe oil changes and get away with it.

You simply can't do that with an XJ; and they do have certain details about them that are important. They're not completely unique; a lot of Kawasakis have shim-over-bucket valve trains and also require a similar maintenance regimen to keep them running right. Unfortunately, a lot of the correct knowledge is too "old school" for today's shops.

And trust me gentlemen, if you think the XJ is difficult don't even think about a Ducati.

 

My buddy with his Monster is nearing the initial valve maintenance mark. When I explained why that's a big deal, he started to freak out.

I don't blame him.

Instead of setting aside a weekend to check your valves, you're better off setting aside a weekend to find a second job.

 

If I could afford a Ducati, I think I could afford to take it to the dealer so they can work on it.

 

Yes, but there are a lot of people who buy a second hand Ducati thinking they can treat it like any other bike lol

 

No valve springs. Go ahead, neglect THAT motor and see how long it lives.

(Do you know how the desmodromic system works, and how the clearances are adjusted? It's a degree of mechanical insanity that could only have been devised by Italians.)

 

Ducati are nowhere near as bad as you're making out, on my Darmah they only needed doing every 10,000km so not much different then the XJ

Although it's easier to do the heads on the bench as the head only has 4 bolts and there's no head gasket to replace, the motor does have to be dropped to access the rear head easily, but you can have it out in 10-15 minutes

Do the same stuff, regular oils changes, carb sync, valves and they'll last a fair while you just might have to do some of those things more often than on a Jappa

 

I was speaking directly to the method of adjusting the valve clearances on the desmodromic motors. A tad more complicated than swapping shims...

 

Bump for the sake of the new members.
Give every thread by Bigfitz52, RicCoMatic, and Hogfiddles a good reading. Most of the XJ specific knowledge I have came from them.

 

Good stuff. Glad I did mine!
I'm going to say, that's not what "valve float" is (from the original post in this thread). It's when the cam is too aggressive and the spring isn't able to keep the valve stem (bucket, shim, whatever) in contact with the cam face. The float part is when the cam lobe comes past the valve, which is when the valve should close, but the inertia of the valve can't be overcome by the spring and the cam lobe comes away from the valve shim till the valve spring gets that valve shut again.

 

Correct on the valve float.

But too tight of a valve clearance can keep the valves open, and the result is effectively the same.