What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Workshop screw-up

Tom @ N269CP

Well Known Member
I had a local service shop replace my left magneto early this week (Slick 4371 IC mag). On my first post-work flight I noticed the cyl # 1 EGT climb to possibly over 1450F with Cyl #1 CHT of 425F...other three cylinders were lower and comparable to each other...only the Cyl #1 temperatures spiked. During the left mag drop test the rpm dropped by about 120 rpm. The planes climb rate was definitely sluggish compared to normal climbout. I normally take off full rich as the plane has bags of power and it helps keep my CHT's below 400F.

Today the workshop checked the magneto timing which he found to be 30 deg BTC versus the Lycoming spec of 25 deg BTC. The discrepancy appears to have been caused by an internal mag timing error which the shop set while installing the mag, compounded by their not electronically confirming the advance....timing appears to have been set solely off the flywheel timing mark..which is wrong if the mag internal timing was wrong to begin with.

Static manifold pressure at my home airport is about 24.5"Hg. Takeoff RPM is 2650 rpm....this puts the takeoff power at somewhat over 75% (24/24).

My question for VAF is this: Could this 5 deg advance timing error have resulted in detonation during takeoff which may have caused the higher than expected EGT/CHT?

I'm hesitant to fly the plane until I better understand the potential for detonation damage...may have another shop borescope the cylinders Friday morning before flying it anywhere.

I would be grateful for any suggestions as to my next course of action. I'd rather be safe than sorry.
 
Last edited:
if you are using AVgas vs Mogas I think you are fine. I had a shift like that from premature mag wear. it was even across the cylinders but I needed to reduce power to avoid exceeding 390F. the mag timing had advanced to 31 deg. I borescoped the cylinders and all was fine. no further issues in 300 hrs. borescope it to ease you mind.
 
if you are using AVgas vs Mogas I think you are fine. I had a shift like that from premature mag wear. it was even across the cylinders but I needed to reduce power to avoid exceeding 390F. the mag timing had advanced to 31 deg. I borescoped the cylinders and all was fine. no further issues in 300 hrs. borescope it to ease you mind.

Steve,

I use 100LL...I didn't want to reduce power because of the shallow climb rate and being lower than usual to the ground. I normally reduce rpm to 2400 rpm for climbout at around 400 ft AGL and leave the throttle WOT due to the higher altitudes here. All temperatures returned to historical norms at cruise ~20"Hg/2280rpm/7.2 gph/182 mph TAS.

I will borescope the cylinders.

Thank you for sharing your experience.
 
Last edited:
Internal timing (e-gap adjustment) will have no effect on the actual timing of the mag to the engine. Who ever installed the mag just didn't follow normal procedures or didn't do them correctly.
 
Internal timing (e-gap adjustment) will have no effect on the actual timing of the mag to the engine. Who ever installed the mag just didn't follow normal procedures or didn't do them correctly.


I'm going to have another shop review this in the morning. I'm getting conflicting advice.
 
not conflicting, just confusing

FIRST, you set the e-gap [which is what Slick calls 'internal timing']. A lot easier to define if I could show you one...
SECOND, after inserting a lock pin to keep the armature from rotating [maintaining that 'e-gap'], the magneto is installed on the engine , and timed to give you the spark at specified BTDC. [Aaand remove the lock pin, please]
So: if the timing checks to 25 BTDC after installed, and your engine requires 25 degrees, you are set to go --- both "settings" will be correct.

To your original query: by all means, scope, particularly the 'hot' cylinder. I infer you'll be having an A&P do that; tell her you're looking for det/ pre-ignition damage, they'll know what that would look like.

A suggestion: The more you learn about how to fix them [correctly], the more confidence you'll have in the work you pay for. And yes, a lot of shops are understaffed - and under-experienced; it is not going to get better for the foreseeable.
 
get it checked out, but don't get to torqued up right now. if you are running 100LL the odds of detonation damage are low unless you have built it with higher compressions than normal. the detonation margins that lycoming designed in are pretty wide. ask some of the guys at RENO what they are running there engines at, its actually scary.

bob burns
RV-4 N82RB
 
I'm going to have another shop review this in the morning. I'm getting conflicting advice.

The E-gap adjustment (internal timing), is setting the contact points opening point so that it occurs at the moment that the highest voltage is being induced by the magnet into the coil.
The timing of the mag to the engine is a totally different process and is done with an indexing pin locking the position of the distributor rotor so that the mag can be properly timed to the rotation of the engine.
So, even if the internal timing of the mag. was incorrect, if the proper timing adjustment process were used when the mag was installed on the engine, the spark would still occur at the proper time. The only issue would be that the spark output of the mag would be less than it is capable of (this will often cause poor starting performance).

I agree with others that it is unlikely that timing at 30 degrees has caused any type of damage to the engine.
 
And

?..:And yes, a lot of shops are understaffed - and under-experienced;...?

That is no excuse. Years ago, we had a C421 in for an annual. Long story short, the shop either did not replace or used a wrong sized cotter pin in the elevator trim tab linkage. This led to the linkage disconnecting while climbing through FL190. The tab then was free to move on its own causing the right elevator to enter a divergent flutter mode. This snapped the three bolts that joined the left and right elevators (thankfully). My dad landed the aircraft safely and the investigation began...

My point is, if you go to a shop and the aircraft is signed off, you should have a reasonable expectation that the aircraft maintenance was done correctly and the aircraft is safe to fly. Understaffing or lack of experience is no excuse for not doing something correctly...

The funny part is the shop that did the work on the 421 still expected the $10,000 annual to be paid...
 
Couple of comments:

1. Scott is spot on in his description. E-gap can be off, but final timing will be correct. Less spark energy if its off. I normally set e-gap by feel.
2. 30 degrees of advance wont hurt anything short-term.
3. Sounds like the mechanic didn't have a buzz box.
4. Pin on a slick is only used when inserting the mag into the accessory case. You immediately pull the pin after the nuts are snugged. DON'T rotate the engine with the pin in place!
5. A trait of an excellent mechanic is the ability to disassemble, reassemble, and time a magneto without any difficulty. A really excellent mechanic will be set up to re-magnetize rotors. Ones that can't or have to send mags off to have someone else do it...meh. Just my educated observation.
 
So, late yesterday afternoon after "Shop A" allegedly found the mag timing at 30 BTC, they reset the timing to 25 BTC despite the final alignment between timing mark on the flywheel and the casing splitline being off by about 5 teeth.

This morning "Shop B" checked the above reset timing and found it had been wrongly reset to 19 BTC. "Shop B" reset it to 25 BTC and now there is also good alignment with the flywheel timing mark. They also borescoped Cyl #1 and did not find any sign of damage.

Both shops used a buzz box.

Will soon test fly the plane to see if this resolved the high EGT/CHT issue on Cyl #1. Odds seem pretty decent that they were caused by faulty mag timing by "Shop A"...though we will never know what they had originally set it to.

I met with the manager of "Shop A" and advised him of the findings by "Shop B".
 
Last edited:
Couple of comments:

5. A trait of an excellent mechanic is the ability to disassemble, reassemble, and time a magneto without any difficulty. A really excellent mechanic will be set up to re-magnetize rotors. Ones that can't or have to send mags off to have someone else do it...meh. Just my educated observation.

That's a pretty arrogant statement.

My own observation is that some folks are just to cheap to pay for component overhaul by a qualified shop that specializes in these items. When I see a log entry that an item (this includes engines) was "repaired" in the back room of Joe's hanger I take pause.
IMO if you can't hang an 8130 or yellow tag on a component, then you're probably not qualified to overhaul it.
A really excellent mechanic knows his limitations.
 
So, late yesterday afternoon after "Shop A" allegedly found the mag timing at 30 BTC, they reset the timing to 25 BTC despite the final alignment between timing mark on the flywheel and the casing splitline being off by about 5 teeth.

This morning "Shop B" checked the above reset timing and found it had been wrongly reset to 19 BTC. "Shop B" reset it to 25 BTC and now there is also good alignment with the flywheel timing mark. They also borescoped Cyl #1 and did not find any sign of damage.
B".

Sounds like all you?ve done is move the timing back to where you started. So it wasn?t mistimed in the first place. I hope one of the shops confirmed the actual TDC of the engine.

-Andy
 
Back
Top