exhaust valve guidance on 8-71

[Larry B]

  New compression tester arrived a week ago. Did a test and both sides showed only 380 psi-tested one hole each side. Moved engine from running location at rear of bus back to hoist area. Removed head that had noisy valves first.  The distance the valve was below head face was at book maximum. If you look at picture of valve, the narrow shinny ring on the seat surface towards the stem is the only thing making contact with seat in head. If you look at picture of head the angle of seat looks too flat to match valve face. The seats look to have had several grinds. The contact area in the seat is the inside i.d.  Surely this can't be acceptable for a rebuilt head? ($1750.00per head -30 hours of run time). If you measured the o.d. of the step in the seat insert and the od of valve there is .003 clearance for valve to go in " hole" and make a seal when it closes. Need some guidance on what is acceptable.

[luvrbus]

Larry where people go wrong is the seat is a grounded at a 30 degree angle and the valve it's self is grounded a 31 degree angle check the letter on the valve and be sure the lettering is the same on all so many different valves and thickness for a DD

good luck

[Larry B]

I am sure the valves are all new. They all have have letter U on valve face center indent. There are numers on stem end, will compare today. Why are the two angles different? With different angles the seat and valve will only touch in a very narrow ring when new. Is the thought to form together as engine wears in? Why are they not ground the same to start with?? On a new grind job, if you ever let engine idle too long or run cold, carbon would build up in this 1 degree difference. As seat and valve wear together would carbon not be forced into valve or insert seating area. The head will probablly go to shop Monday. The seats need to be changed. Is welding on the seat insert an acceptable thing to do to cause them to fall out??   

[luvrbus]

They done it that way for years it works only problem I run into is when someone get it reversed and grinds the seat at 30 and the valve at 31 degrees and don't center the valves find you a good machine shop that knows what they are doing,me I would not weld on the seat to remove it JMW I use the tool 

good luck  

[bevans6]

That valve job looks bad to me.  I went into the manual to see what it should be like and I agree with you that the seats need to be replaced.  As said the seat should be at 31 degrees, but the top cut should be 15 degrees and the bottom cut 60 degrees to set the seat width and location to where it should be.  Your seats don't have the 15 degree top cut at all, so that you have that recess straight down to the 31 degree cut.  Also, the seat contact should be in the middle of the valve face, not at the edge.

Are you sending them back to the same machine shop?  I might, but only after a long conversation with the manual in hand to explain the correct process, including pictures...  

Do you have the manual?

Brian

[junkman42]

The angle difference between valve and seat is common on many engines.  I was told many years ago that the difference is due to angular change when hot.  I am surprised at the shallow angle on a exhaust valve.  Most exhaust valves are at 45 degrees.  However You can bet that DD had a good reason for the parameters for the valves.  Regards John L

[Larry B]

I took other head off to have a look as origin was same place. This head looks much better, the way the book says it should. No I will not be going back to that shop again. I wish I could tell you who you might want to consider avoiding. I did the mechanical first on my conversion and I have timed out calendar wise for any warranty or money refund. Any ways that past, have to deal with what I got. After seeing the difference in heads, I went to a mchine shop to ask questions. this shop was recomended by a couple of friends that I trust. He suggested I try some fine lapping compound not with the intend of lapping the seats in but do only enough lapping to see where the seat and valve touch, how wide the contact ring is, check for concentric seat grind and if a valve is bent enough this wiil also show. He claims a bad insert grind can even be egg shaped. These sound like good ideas to me. Going to try this on the better looking head first. Got a new mission and a reason to play with my  toys for a couple of days.He also claims a good valve job should be varsol tight. He was very specific on minimal amount of lapping compound.     

[Brassman]

Have you tired Prussian blue. It will show contact area.

[Larry B]

 I have been trying to get an update for 3 to 4 days. I tryed the lapping compound, It  showed the contact spots really quick. I took both heads to shop (with valves removed).The one I did lapping on he said it was good, put it back together and install. The other one being so close to maximum distance of valve countersunk in head, he suggest changing seat inserts before grinding. You were right Brian the top should have been cut back. He claims the sharp corner at the bottom of "hole" would soon fill with carbon deposits just from idling and engine starts causing valve and seat to loose sealing capibility. Some might remember I had talked about valves making a rattling noise a short time ago. The likely cause was the valve trying to find its way into the "hole" with seat at bottom. (only .003 to .004 difference in valve o.d. and hole i.d.)  Head will be back in a week or so. Thanks again for all help and quideance    Larry     

[bevans6]

Kudo's to you for being so ornery as to take the thing apart and find the problem!  Good job, I say!

Brian

[luvrbus]

Geoff hit that on head when way back said you had a valve seat problem,fwiw shops can screw the heads up be care where you have machine work preformed on 2 strokes a regular automotive machine shop not set up for the V 2 strokes can screw a crank up in a hurry also.
I have one place that does mine here in AZ they do all of WW Williams work in Az and Nev on all engines models and brands they sell the place is as clean computer chip mfg alsmost

good luck