View Full Version : Valve Train Clearance

Matthew E. Herd
03-17-2004, 10:46 AM
After finally getting my clutch situation sorted, it turns out that my mechanic discovered excessive valve train clearance on my newly built (760 mile) engine. What could be some possible causes of this clearance, as the valve clearance was set when the engine was first run in? Seems a little fishy, but I wouldn't think major wear would develop that early ... apparently the clearances were about 3/16" (huge, esp. when the clearance was supposed to be .024"). Do I have a problem?

03-17-2004, 10:54 AM
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Matthew E. Herd:
.... apparently the clearances were about 3/16" (huge, esp. when the clearance was supposed to be .024"). Do I have a problem?<hr></blockquote>

Your clearance is supposed to be 0.010", measured cold (not 0.024")

Retorque your head and rocker arm shaft to the correct settings, set your valves correctly and check again in several hundred miles. I guess I'd hope that it was set wrong initally.
You may want to change your oil and filter and inspect for metal debris (and I hope you don't find any).

03-17-2004, 11:48 AM
Matt, it sounds to me as if the lifters were on the cam ramp, rather than the heel of the lobe when they were initially adjusted. Do as Nial suggests, though, and drain the oil and check for "shiny bits".
Were all of the valves that far out? If they were, I would tend to believe that the cam was OK.
It still probably wouldn't hurt to set up a dial indicator, and check the lift at the valve, just for peace of mind, if nothing else.
Also, I note that you got the clutch issue resolved. What did you find for a root cause?

[ 03-17-2004: Message edited by: Bugeye58 ]</p>

Matthew E. Herd
03-17-2004, 12:41 PM
Concerning the metal, filter change was done first week of Jan, and hadn't driven the car since then hardly at all (due to shifting problem). No metal debris at that time ...

Valves were initially set during run in of the cam by my engine guy, at .022 intake and .024 exhaust as the kent cams kit information specified for the spit/midget 1500 engine. On the shifting issue, the problem turned out that the cylinders would not bleed properly. They were removed and bled individually, then installed and bled again. Also, the clevis pin was replaced. As a result of fluid probably seeping into the clutch disk, the car apparently drives better and better as the fluid is most likely being burnt off. Unfortunately, since I'm at school, I have to address these issues remotely.

Another question: I discovered (second hand) that the cylinder pressures are not equal. The C.R. is 10.49:1, yet the first two cylinders read 185-195 while I understand cylinders 3-4 read at around 145. Now, am I correct in calculating static cylinder pressure to be C.R.*Atmospheric Pressure? If so, that would indicate that the pressure should be about 154 psi, so why (especially w/ a reasonably long duration cam--284 degrees) am I seeing such a high number? And what could this discrepancy be? The entire engine was just done, top to bottom. New pistons, rings, valves/valve seats, etc. Is it possible that the cylinders are seeing greater manifold vacuum as a result of starvation from the air box? I am running a dual sidedraft 40mm weber setup, so each cylinder has a separate runner, merged at the airbox.

03-17-2004, 01:03 PM
Matt, static numbers look good on paper, but in reality, the cylinders 'overfill' themselves, during the intake stroke, hence the higher observed pressures.
But, a 40 psi delta between the front and rear two cylinders shouldn't happen.
Have you done a leakdown test to determine where the loss is? I know it's hard to do at a distance, but that should be the primary tool used in determining such a variance. It sounds suspiciously like a bad head gasket between 3 and 4. I'm assuming that the valves were re-adjusted correctly prior to the compression test.

Matthew E. Herd
03-17-2004, 01:14 PM
Correction, I just talked w/ my mechanic and I found out the initial info was incorrect. Actually, cylinder 3 is reading 125-135 and the other three are fine. He suspects that it is a valve seating issue. Perhaps the extra violence due to excessive clearances unseated a valve, or it was never seated properly to begin with. That is his theory. He suggests driving it a bit before pulling the head, as it is in otherwise fantastic working order (best its ever been since I've had it).

03-17-2004, 01:21 PM
Well, Matt, what I said about a leakdown test still applies. Driving and hoping a problem will cure itself is a crapshoot at best, and could lead to loud, expensive noises. A leakdown will pinpoint it exactly. Were the valve seats replaced during the rebuild?
Running it with such excessive clearances could have conceivably even bent a valve, in which case, nothing short of pulling the head is going to fix the problem, and driving will only make things worse.
I don't mean to sound like a doomsayer, but better safe than sorry!

Matthew E. Herd
03-17-2004, 02:16 PM
Well, When you say leakdown, you mean wet vs. dry compression test, correct? A wet test was done and read the same values for compression. This would seem to indicate bent or unseated valve. Yes, the seats were done.

03-17-2004, 03:05 PM
Matt, a leakdown test is done with a leakdown tester, basically two gauges in series.
A set pressure of air is introduced into the cylinder, with the piston at TDC. (I use 100 psig as it makes the math easier.) The first gauge measures the pressure of the air applied, and the second measures the pressure maintained in the cylinder. For example, using 100 psig as the pressure applied, if the cylinder pressure gauge reads 90 psig, the leakdown would be 10%.
Once the cylinder is pressurized, you listen for the escaping air at various points. If you hear it at the exhaust port, that would indicate a leaking exhaust valve If it can be heard primarily at the dipstick tube, that would point to rings not seating, etc.
An inexpensive tool, and invaluable in diagnosing internal problems.
A wet/dry test differentiates between the valves and the rings, and is generally used to isolate bad ring seating.
I hope I haven't muddied up the water too badly! images/icons/smile.gif


[ 03-17-2004: Message edited by: Bugeye58 ]</p>

Matthew E. Herd
03-17-2004, 05:13 PM
No, I understand exactly what you're talking about now, I just wasn't familiar with the term. Indeed, that would pinpoint the problem, and will probably be my next project once I get home to work on the car (figure 2 months from now ... ) images/icons/tongue.gif

Matthew E. Herd
05-11-2004, 11:01 PM
Ta-Da! And the cause to this issue is discovered at last! To the sound of a tremendous BANG! I found that all of the sudden my engine ran 'very badly.' I shut it off, and had it towed back to my mechanic (where I'd just come from and was about half way home). We popped off the valve cover and discovered that the number 4 exhaust pushrod was headless. The custom made pushrods (slightly shorter than stock to accomodate for the shaved head) were poorly joined where the rod and cup portion meet, and the cups were actually spreading out as the engine operated, making the clearances increase as the rods deformed. Very thankfully, the top of the failed rod landed on the block, sitting between two pushrods. Came very close to a very expensive repair. Since the difference between cast and the custom pushrods is only slight and didn't alter rocker geometry measureably, I simply swapped the cast ones back in, which are of significantly superior construction. It never really occurred to me to check this detail. Also, I decided that the installation of a rocker feed tube on the 1500 is a must as mine wasn't pumping a sufficient volume of oil (perhaps a head gasket issue) but the pressure was (without rockers installed and while cranking the engine) 35 psi, too much with what should be no restriction.

At any rate, hope this helps for future diagnsis.

Dave Russell
05-12-2004, 02:29 AM
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Matthew E. Herd:

Now, am I correct in calculating static cylinder pressure to be C.R.*Atmospheric Pressure? If so, that would indicate that the pressure should be about 154 psi, so why (especially w/ a reasonably long duration cam--284 degrees) am I seeing such a high number? <hr></blockquote>
You are thinking of isothermal compression. Look into adiabatic compression with an exponent of around 1.1 to 1.3. Not all heat is lost during compression.

You are right about the cam timing effects. The EFFECTIVE compression ratio is considerably less at cranking speeds due to the intake valve closing after bdc. No compression can begin until the intake valve gets closed. Note - I said at cranking speeds. Gas column inertia effects at higher rpm tend to overcome this late valve closing & EFFECTIVE compression ratio will approach the designed 10.5/1.

Your engine is probably around 7/1 effective ratio with an exponent of 1.13 which would give around 180 psi. At atmospheric pressure of around 14.0 psi. (Just guessing at these numbers) Too many unknown variables to be exact.

Jeff is certainly correct on his diagnosis recommendations & I would still do the test after repairs.

Matthew E. Herd
05-15-2004, 12:06 PM
Actually, I did the tests before doing any driving and detected a consistent 210-220-230 compression at cylinders 2, 3, and 4 (3 being the suspect one). I didn't bother to remove the alternator to get at number 1 cylinder, however, my own testing seems to show that compression is quite consistent, with the variation less than 10% and seemingly not the worst in 3. If the valve was unseated, it must have re-seated itself, or, it is also possible that the test was incorrectly recorded. At any rate, I tested it myself and am satisfied with the results ... hard to say. However, I did have some trouble with the Moss TR-6 (and therefore also Midget 1500) rocker feed kit, as a critical bolt broke. However, when tested (it leaked but supplied oil at an excellent rate) the kit solved the problem of insufficient lubrication. However, I do not believe that the lubrication was the only reason the pushrods failed, as they show that they aren't worn out, but rather fatigued to failure. As soon as I get the replacement bolt, it will be a running machine.

Conclusions: In the Midget/Spit 1500, oil feed is insufficient, due to whatever reason, without the auxilliary feed. Also, stock pushrods are of solid construction and are most likely suited to high performance applications, as they seem to have been barely worn after 77,000 miles and are suitable for re-use in a rebuilt engine.

05-17-2004, 12:01 PM

Thanks for the pushrod update. I'm running stock pushrods at the moment so I don't really have any opinion on the race-units. If I consider switching, I'll keep this info in mind.
I don't have any bad experiences with rocker arm / valve train oil-feed problems on my 1500....the more familiar problems to me are thrust washers, rod bearing, weak rods and flexible cranks.
Anyway, I'm really writing because of your reference to the 0.024" valve lash (again). As you may recall, I mentioned the stock lash in a previous note (I assumed that you had a stock cam). I was just checking valve lash on my car after my first event and I was reminded that APT suggests 0.016" (intake) and 0.018" (exhaust) for their TH5 Kent camshaft. I think this is the cam you have too. These setting are adjusted hot (kind of a pain in the neck). Mine were only about 0.002" more when cold. You may want to double-check your's.

Matthew E. Herd
05-17-2004, 09:48 PM
Its entirely possible that I have a plugged passage or a gasket which blocks flow, as it comes slowly and only after extensive cranking. Also, the oil pressure reads 32 when it should be reading approximately zero (because the rocker shaft is removed). At any rate, my problem (as I should have noted) is not necessarily universal, but can be easily fixed with the addition.

Also, thanks for the note on the valves. My kit was sent directly from Kent Cams and I believe it was .022-.024 intake and exhaust. However, I'll be sure to check my literature (which needs digging up) in order to be sure! The problem with my custom rods was that the shaft was hollow and the ball and cup were attached via an unknown method, but it was apparently not sufficient as, upon inspection, there was hardly anything holding the cups to the shaft portion of the pushrod (if that makes sense). The reason was not the actual shaft strength, but rather the stress increase at the attachment point. But by looking at the standard rods, assuming you're willing to live with the marginally increased weight (it was noticeable when comparing rods by simply handling them), the basic shape (as I'm sure you know) is very good at minimizing stress as compared with the ones I happened to get, even though they were supposedly race quality.