Issues when replacing only bent valves and not heads?

Dwayne Owner/Technician Pennsylvania Posted   Latest   Edited  
Resolved
Propulsion
2004 Audi A4 3.0L (T AVK) (01J)
Bent Valves

Good evening techs, this is my son's car, purchased with torn timing belt and bent valves. He purchased some valves to replace the bent valves. Most of the time we have either purchased other heads or got a machine shop to replace valves. My question- are we liable to run into compression issues if we just replace the bent valves and reinstall the heads? I would never do that for a customer car, but in this case??? 

He did install the valves in the heads- now on a couple of them when you fill the ports with water they will slowly drip water out past the seats. Like a drop every 15 seconds. Would we be crazy to reinstall the heads that way?

0
Sam Mechanic
Missouri
Sam
 

Can't say I've seen it on an audi but some engines damage the pistons when the valves hit, may not be visible on the face, but the oil and compression rings and piston skirt could be compromised, there is that risk. Otherwise I tend to lap valves until they seat properly without leakage when you dont have a valve grinding machine, just making sure you dont go too far(mind the machine limits)

0
Ð Awarded
Frank Manager
Tennessee
Frank
 

It would be wise to use carb spray/ throttle body cleaner and determine the leakage on the other valves. I dont use water. A bit of fine clover lapping compound with a suction cup may cure the leakage.

+2
Ð Awarded
Agree
Helpful
Glenn Owner/Technician
Texas
Glenn
 

Hi Dwayne, Bent valve usually mean the guides will be flared at the ends (the whole guide is not damaged) on the combustion chamber side. They should run okay for a while, but I recommend lapping in valves anytime they are replaced regardless.

+3
Ð Awarded
Agree
Helpful
Obie Technician
Washington
Obie
 

Spend some time looking at the TSB's for engine issues, then decide if you want to do a partial repair to an unknown engine. That engine is a very tough customer, especially with lousy service history. I can tell you it is a heartbreaker!!!

+4
Ð Awarded
Agree
Interesting
Gary Owner/Technician
Colorado
Gary
 

Hello, Dwayne. I'm not an Audi mechanic, but I've taken brand-new OE valves out of the box and found some that were bent a few thousandths. I have a really accurate valve grinder and I seldom find new valves that run exactly true. As others have said, I'd check seating by lapping with a little fine oil compound just to make sure. If the valves are badly bent, you might have guide and piston…

+3
Ð Awarded
Agree
David Business Development Manager
Ohio
David
 

I would make sure to check the valve guides. Spent my younger years in a machine shop and it wasn't a rare occurrence to find a damaged guide when dealing with a bent valve. Also, since you're going through all of that work to disassemble the head, why not have it checked for flatness and machined (if needed) so your son will have years of trouble-free running? – Dave

+2
Ð Awarded
Agree
Albin Diagnostician
Washington
Albin
 

Why would you try to take a short cut like this on one of your personal vehicles, and not on a customer vehicle? What are you going to do about the possibility of having cracked valve guides? There are a lot of gremlins out that are lurking when you just put in some valves and hope for the best. Your water test found one. Now, if you really want to find the leaking valves, use diesel fuel. The…

+5
Ð Awarded
Agree
Helpful
Interesting
Louie Technician
New York
Louie
 

Hi Dwayne I have extensive vw/Audi dealership experience and I hate to say it but every time I or any of my coworkers attempted to install new valves for bent ones never ended good​.​Even if the engine runs it will be plagued with intermittent misfires and minimal compression in extreme cold​.​You are better off installing a known good used cylinder head

0
Ð Awarded
Sherman Owner
Texas
Sherman
 

I've bought valve lapping compound and chucked the new valve in the head with a drill….. and done a quickie seat in job many times over the years. So far, I've had 100% success…. but you never know! It's a risk that you'll have the final say about.

+1
Ð Awarded
Helpful
LaMont Owner/Technician
Ohio
LaMont
 

I would lap them in by hand and check to see where the valve seat is contacting the valve face. I don’t see why you should have a problem unless you have a damaged seat, which is highly unlikely.

0
Ð Awarded
Bill Owner/Technician
Michigan
Bill
   

We have done many VW Audi cylinder head jobs over the years. Bent valves due to belt/chain issues are common. Long as you get them lapped in good (valve grinding compound and suction cup tool by hand has worked for us) check them with mineral spirits after. I see no reason for a new cylinder head unless there is a cracked or damaged guide or seat but that very rare!

+1
Ð Awarded
Helpful
Bill Owner
Maryland
Bill
 

Try smacking the top of the valves a couple of times with a small hammer and brass punch. There might just be some carbon on the seats. If water is leaking through then there is more than likely a problem. That being said, a lot of times when they bend the valves they will also hit the seat pretty bad and will require regrinding the seats. It could also have cracked guides.

0
Ð Awarded
David Owner/Technician
Alberta
David
 

Surely this car has had lots of use and there will be wear evident with the engine. The seats may well have been damaged and they were already getting wider. If you lap them you will make them even wider. How well will they stay on centre if the guides are damaged or worn? Lapping compound does not easily wipe off after as it needs to be washed off with hot water for proper cleaning. You asked…

0
Ð Awarded
Agree
Disagree
Sherman Owner
Texas
Sherman
 

Cars are fungible….. disposable. We professionals tend toward a “permanent fix” much too often, IMHO. So many variables not alluded to here would make for their own basis for debate on which way to go on this repair. I've seen many folks hire done, or do their own patch jobs to get by until they could afford either an entire proper repair or another vehicle. It's all fun!

0
Ð Awarded
David Owner/Technician
Alberta
David
 

Sherman, I do not disagree with how great of a job we may perform with repair quality. Purchasing a vehicle of this type, in this condition, is not likely where some one is trying to get by as a matter of costs. It is an old car but likely still could be worth 4 or 5 thousand? The big work is tearing it apart. This is at least a 10-12 hour job without any big complications. Spending another 4…

0
Ð Awarded
Tommy Owner/Technician
North Carolina
Tommy
 

Dwayne, Also check to see if this engine has pressed on camshaft lobes. If it does, they can be shifted on the cam when the valves hit the pistons.

+1
Ð Awarded
Bill Owner/Technician
Idaho
Bill
 

I would lap the valves in by hand to make sure the valves seal in the seat.

+1
Ð Awarded
Helpful
Travis Diagnostician
Texas
Travis
 

Have the head rebuilt at a machine shop it’s very inexpensive and they can make sure everything is how it’s supposed to be. I’ve done many Audi jobs for bent valves this way and have never had any issues.

0
Ð Awarded
Dwayne Owner/Technician
Pennsylvania
Dwayne Resolution
 

I realized I never closed this post, so here are a few thoughts. My son did lap the valves, and that definitely helped. The car has been running now for quite a while, its his daily driver back and forth to the shop. As far as I know, it does not set misfire codes although it may have the light on for various other reasons, like bad cats… It was interesting to read the range of opinions on…

+1
Resolution Ð Awarded
Thanks
LaMont Owner/Technician
Ohio
LaMont
 

I’ve been doing this for 35 years. Every time I’ve had bent valves. I’ve replaced the bent valves, vacuum tested all of the valves. resurfaced the head if necessary and put it back together. I don’t consider it to be much more than a normal head gasket job. I’ve never had any complications. I guess you could overhaul the engine if you want to, but if the valves wouldn’t have bent, the customer…

0
Ð Awarded
Obie Technician
Washington
Obie
 

“ I’ve been doing this for 35 years. Every time I’ve had bent valves. I’ve replaced the bent valves, vacuum tested all of the valves. resurfaced the head if necessary and put it back together” That must be because of the engines you have worked on. Some engines can break a timing belt and not bend a valve. The exact same engine can bend valves. Many engines bend valves every time. Others bend…

0
Ð Awarded
David Owner/Technician
Alberta
David
 

Obie, I feel you are pretty accurate with your “drop” test. An other matter is how many seats get “dented” when the piston hits the valve. That is common. Lapping one like that is only going to make the seat too wide and it invariably leaves some grit stuck in the seat. Note how the seat is never shiny and smooth after lapping. The seal is most likely doomed to an early failure. Lapping is too…

0
Ð Awarded
Gary Owner/Technician
Colorado
Gary
   

Hello, David. Just my opinion because I've built a number of racing engines: Sometimes I use oil lapping compound to check the valve contact area and remove any “fuzz” left from machining or grinding the valves. I don't do many heads, but I bought the small can oil compound that I use ~60 years ago and it still works. That said, using water compound to correct valve face/seat irregularities…

0
Ð Awarded
Glenn Owner/Technician
Texas
Glenn
 

Hello David, So, you are saying not to lap in new valves? That's a new one on me. Original new factory engines are built with tighter specification, so the valves are not lapped in when everything is new. Lapping is very important when installing a new valve in a head that has been run for any period of time. The lapping process trues the new valve to the worn in valve seat. If the lapping…

0
Ð Awarded
Bill Owner
Maryland
Bill
 

Glen, If the seats need lapping to allow the valve to seat then they really need to be machined. Like Gary said above I use a real fine (320 grit) lapping compound just to verify contact area but that's it. Lapping with the water based compound that most guys see is like running sand through the valves. I think in this thread he is talking about buying new valves and lapping them in to the…

0
Ð Awarded
Gary Owner/Technician
Colorado
Gary
 

Hello, Bill. I've always found that any wear in the valve guide causes wear in the valve seat, especially on pushrod engines where poor rocker arm geometry can wear both guides and seats…

0
Ð Awarded
David Owner/Technician
Alberta
David
 

Hi Glenn, if we think of the whole purpose of interference angle, the idea was to keep the seat/seal surface as thin and smooth as possible. I learned it was to keep the loading of the valve face surface to the head at a high enough force per unit area. Lapping is definitely going to be counter productive to that idea or surely not as great an idea as we once thought. As I mentioned before, a…

0
Ð Awarded
Gary Owner/Technician
Colorado
Gary
   

if we think of the whole purpose of interference angle, the idea was to keep the seat/seal surface as thin and smooth as possible. I learned it was to keep the loading of the valve face surface to the head at a high enough force per unit area. Lapping is definitely going to be counter productive to that idea or surely not as great an idea as we once thought. In my book, intake seats should be…

0
Ð Awarded
LaMont Owner/Technician
Ohio
LaMont
 

Not sure how you could remove enough material to widen a valve seat by lapping. Maybe by grinding, but not by lapping.

0
Ð Awarded
Bill Owner
Maryland
Bill
 

You would not believe what some people can do :(

0
Ð Awarded
David Owner/Technician
Alberta
David
 

You made my day Bill. Valves, springs and heights, shims & rotators, guide installed heights, stem seals, and such have been an interesting subject for me. One of the “classics” of what people can do when they really get after something without thought is a story for me to share: Customer purchased a Buick 3.8 engine kit for his carburetted ‘77 Regal. It was a whole long block kit to…

0
Ð Awarded