How Much Lapping?

[eschrom]

Question for Fred or anyone else who uses air pressure to check valve lapping... I hear only a whisper but can make high pitched fart noises with the heel of my hand over the exhaust port. Is that lapped enough? More doesn't seem to improve it.

Thx,
Ed

 

[soneraifred]

I use a differential compression tester when I lap the valves. If I can get 78/80, I'm happy .

 

[Des]

Hi Eschrom,

one of the methods I use is to keep the valve pressed against the seat, with your thumb or any finger, which is the most convenient, using "moderate pressure". Hold the head, so the seat area is somewhat level, and then pour in from the intake- or exhaust channel some petrol, making sure there is full coverage of the seat area. (A few cc's will do)
Then, manipulate the head as such so you can observe the seat area, while keeping pressure on the valve.
If after a few seconds, the seat area remains dry, one can be sure the lapping job is good.
I use petrol, since this is a very cappilar liquid, but maybe many other liquids may do as well.

Best regards,
Des.

 

[eschrom]

Thanks, guys.

Ed

 

[soneraifred]

One thing that I do when the lapping is complete, the valve is reassembled in the head, and the air is applied, is to rap the end of the valve stem with a plastic hammer to pop the valve open. With 80 psi in the combustion chamber, that blows any residual lapping compound off the seat and allows the valve to seat better.

 

[eschrom]

I tried the liquid method with gasoline and saw no leakage so I suppose they're good.

Every time I lap valves, which is way too often, I wonder how good is good enough and then I wonder does it really matter? Within a flight or two one of the cylinders will be noticeably softer so why stress over a perfect seal when it vanishes so quickly?

No need to answer. I'm just grumbling.

Thanks again, Fred and Des.

Ed

 

[RobinHou]

I always wonder why VW aero engine owners lap valves much more often than VW car owners. Is it because we aero engine owners maintain their engines to a higher standard?

Or, do we run aero engines much harder than average car engines?

What is your opinion?

 

[soneraifred]

Here's my opinion (for what it's worth): First of all, we do run our engines harder than the average auto engine. A car's average speed is probably around 35 mph, so it's not turning between 3200 and 3300 rpm continuously like our airplane engines do. Second, most of us use 100LL avgas and run with short exhaust stacks. When the engine is stopped, at least one of the exhaust valves is open, allowing the air in the short pipe to contact the valve seat. Because lead is fairly corrosive, when it reacts with moisture in the air it causes the exhaust valve seats to rust. Then, the the valve starts to leak. I always plug my exhaust pipes after each flight to slow down this process, but over time, the heads have to come off to clean up the seats. I've often wondered how the Sonex drivers fair with their two two-into-one exhausts which should do a better job of keeping atmospheric moisture out of the engine when it is parked. Any of you guys with long exhaust systems have any input? Or anyone running auto gas?