Valve job causes increased blow-by?

[prometheus578]

Thoughts on a post in the xs/xj forum "Piston Ring Questions" from JAXX79. TopcatGr58 made a comment that he heard that ""Doing a valve job, without doing a ring job, could increase blow-by past worn rings.""
No disrespect to anyone, nor am I such a genius as to be able to question anyones mechanical abilities or knowledge, but I have problems with that statement. I too have heard it repeated over the years, but no one has ever been able to demonstrate to my why this is so.
I now question conventional wisdom once again.

Ok, so the time honored theory says.... That if you do a valve job, without doing a ring job, you will now get more blow-by past the worn rings. On the face of it, it might seem to make sense... but does it hold up to scrutiny? Time for a mental exercise.

I have a cylinder on my bike with bad compression. I screw an airhose into the spark plug hole and hear air escaping through the carb and through the crankcase. Ok... I have a bad intake valve, and worn rings (blow-by).
Now.. the theory says that if I fix one, it will increase the leakage of the other.
Now... to test this, I'll go to another bike that is in perfect condition and try to duplicate the problem. I'll use a bike that doesn't have shims for valve adjustment, but uses a set screw and lock nut to change the valve clearences. With this arrangement, it is possable, by overtightening the screw, not only to get zero clearance, but also to hold a valve open. By doing this, I will be simulating the leaking intake valve. Since rings are not adjustable, I will overtighten the exhaust valve... simulating the worn, leaking rings.

I screw in a leakdown tester into the plug hole and apply air. No leaks. I now overtighten the intake valve, popping it open a little until the tester shows a 3% leakage. This represents the leaking valves. I now overtighten the exhaust valve(simulated rings), until it opens and I adjust it so that the leakdown tester shows 6% leakage overall. (3% from the intake valve, and 3% from the "rings")

This is now the starting point for the experiment. I have a cylinder that is leaking from bad valves and "worn rings". The theory says that if I get a valve job... the rings are now going to leak worse. Let's see.

If you recall, I have 6% total leakage. 3% from the intake valve, and 3% from the rings(Exhaust valve). I now do a 'valve job', by loosinen the screw and closing the intake valve. I have now cut off the 3% leakage from the bad valve, leaving only the "bad rings" to leak. And what does the tester show? It now shows 3% leakage left in the cylinder. No "Extra" leakage past the "rings"!
Restated... I put in 3% leakage, added another 3% , and got a total of 6%(bad cylinder) I then took away 3%(valve job), and was left with only the remaining 3%, which didn't (nor couldn't) rise to 4 or 5%(The supposed increased blow-by.)

Now... maybe I'm wrong. If so, can someone point out the flaw in my logic to me?
As I've stated, I've heard this valve job story many times over the years, but no one has ever been able to explain it to me, nor show me anything in the books as to why it is so.
I welcome any thoughts or comments. (Skip the comments about me being a low-grade moron with too much time on my hands... I get enough of that at work)

[TopCatGr58]

Hey Prometheus,

I'll bite! First, the connected air hose and it's pressure is stagnant, constant if you will, and the size of the opening in the simulated rings=exhaust valve once you had the 3% value set is also stable, constant. The size of opening limits the amount of air/volume=% of leak at that same pressure to a constant value!

The compression cycle of the engine is very dynamic. So... as the piston rises, pressure values increase. The more pressure, the more volume of air/gas that can be squeezed past a set size hole, passage. So now with a pressure of let's say 140psi possible on well sealed engine, but worn rings that can only hold up to 100psi, and leaking valve which also will leak at 100psi, then the max pressure seems to be about 100psi. Now, fix the valves so that they don't leak and can handle pressures beyond the 140 psi.

Now with piston coming up building pressure values up to 100psi, before the leakage was shared with the valves, so not as much pressure and volume was being pushed by the rings. But now the valves don't leak, so as the pressure builds up, it now has only 1 place to escape, past the rings that can hold only 100psi, but due to increased head pressure capable, more of that total volume of gas that could be compressed will be blown by the rings since the compression process has the potential to develop more pressure...but is released past the rings as a total greater of volume of gas as it maintains the 100 psi capability!?

SO..this is "MY" basic common sense and simple highschool science class understanding of this process. Anybody else?

[Hobbit]

I'll give it a try

For the sake of argument, let's say that the cranking compression on a given engine is 100 psi.
However, during the compustion stroke the pressures are ~10 times greater, combustion pressure is ~ 1000 psi.
With 6% leakage, that leaves a combustion pressure of 940 psi.
So if you repair the valve leak, you have raised the pressures that the rings must contain to approx 970 psi.
So, I agree with you that as long as you don't disturb the rings or contaminate the cylinders during the head work, the rings should not leak at a higher percentage, BUT there will be more pressure leaking past the rings into the crankcase. Thsi blow-by does contaminate the oil as well as causes external leakage if the breather system cannot vent enough.
So I believe that the "time honored theory" is based on the premis that if you are this deep into the repairs, do it all right the first time.
In reality on a customer pay vehicle, I have done many valve jobs without touching the pistons with no problems.

[Jaxx79]

Thanks for the great posts.

This is a customer bike, and three of the cylinders were close to 120psi with the fourth at 90+ psi (damaged intake seat).

All valves had heavy deposits and buildup inside the ports (especially the exhaust side). Maybe it was just a valve issue.

I may just volunteer to be the guinnea pig on this and post my results.

Ride safe,
Jack

PS - previous experience with this exact issue.
1988 Honda Civic 1.3 L with 150,000miles
Had to replace the rocker arm assembly and figured I would clean and reseat the valves while it was apart. Long story short, I had a pretty cool mosquito fogger that sucked about 1/2 gallon of oil per tank of gas.

[Ken Talbot]

Interesting theory there, Prometheus. If all that was changed was the valves and seats, it would seem reasonable to expect the leakage past the rings would not change. However, when you remove the head on an XS, you remove just about all of the bolts that hold the cylinders to the crankcase. That might be enough to disturb the seating of the rings in the cylinders, whether you completely removed the cylinders or not. Maybe that disturbance, and the need to reseat the rings, would be enough to increase blow-by.

[rneal55555]

Or if the rings were holding reasonably well at 90PSI all that you could get with the leaking valves. With the valve leakage stopped it is now possible to get 120 psi which the rings may not be seating well enough to hold 120psi which would incrase the leak rate past the rings. How it works out in the real world would really depend on what caused the bad valve was it just high milage or did something cause the valve to go bad bad lifter not adjusted properly, ran lean, or cams got out of time. Or so it seems to me. If the milage was way up there I'd probably go whole hog but if the miles weren't that high and I knew why the valves were bad I'd probably just do valves.

[prometheus578]

Excellent! This is just the type of thought and discussion that I was hoping to provoke.

TopcatGR58... Thanks for pointing out the flaw in my logic, as was also alluded to by Hobbit and rneal55555. The volume of the remaining 3% pressure leakage would surely rise as the overall pressure rises. Simply... that there is a higher volume of air leaking with 3% leakage at 140 psi than a 3% leakage at 100 psi. Hence, increased blow-by. The part about the rings that can hold 100 lbs of pressure, but maybe not 140 lbs, seems to be key.
Ken, I'm sure, agreed with the other posts, but threw in a supplimental thought regarding disturbing the rings. That may hold true for XS's, but when pulling a head on a Chevy to do valves, the cylinder stays put and cannot affect the rings. I first heard about this valve/ring blow-by thing from my H.S. autoshop teachers, Mr. Ben DeBumper and Mr. Denton Fender.
(When I pulled the head on my XS due to a bent valve, I wrapped safety wire around the fins on the cylinder, and then wired the cylinder to the case. I was concerned about shifting the cylinder, screwing up the base gasket, and having my fine machine leak like a Harley)
Though rings are designed to spin on the piston on their own, they eventually find a position that suites themselves and stay with it. Jostling the cylinder a little bit removing a head can no way compare to the violence the rings are subject to moving up and down with the piston.
Now, obviously, if one removes the cylinder, you'll never get the rings back aligned in their previous tracks, so that would increase the chance of more blow-by.
Yeah, Jaxx79... when it comes to customer's bikes, it's hard to decide what to do sometimes. Sure, it's their money, but I don't like blowing more of it than I have to. Once the head is off, if I can still see some of the original cross-hatch pattern on the cylinder, then I leave the rings alone. Otherwise, I look at things like mileage, condition of the plugs, or any other clue I can find short of pulling the cylinder and rings and checking the end gap. Of course, before doing any work on a customer's older bike, I always inform them that once I start tearing into it, I may keep finding more things wrong as I go along.
So... I stand corrected. Dependant upon the condition of the rings, doing a valve job can cause increased blow-by.
Next month's Mechanical Question will be concerning the need to replace the winter air in your tires with summer air, and can this be best accomplished by letting the old air out through the valve stem, or by removing the tire from the rim, as recommended?

[Hobbit]

Winter air

That winter air/summer air must be a regional thing. Where I'm at there isn't much of a difference!

[Jaxx79]

Great stuff guys

Well, I think I will go the route of new rings.

With the ring gap slightly wider than spec, but still within usable range (its about 0.7mm), it's close enough I will put in new. Too much work to replace vs. the cost of new. Pistons still in good shape and within spec, as well as the cylinder walls.

Noticed an interesting spot on piston skirt 2 &4. A funny dicoloration (looks like an off grey wear mark) on the RH side of the leading (exhaust side) skirt. The size is smaller than a dime and there is no correspoding wear (to the naked eye) on the cylinder walls. Odd that it's the exact same wear on the exact same spot of both pistons but one is an inside and one outside cylinder. Hmmmm.....

Thanks,
Jack