Dropped Valve Seat 3300A

[kiwiaviator]

After a couple hundred hours of trouble free motoring since a major rebuild, I have experienced the first issue with my 3300A engine (SNo 33A1240) I had completed the usual 25 hour oil/oil filter change along with leakdown tests (>78 on all cylinders) A ground run was completed. No issues were observed and after a visual leak check, the aircraft was put to bed. The next morning I decided to go for a few circuits to ensure all was well. During the preflight, I rotated the propeller to check for normal compression, noises, etc and I noticed a lack of compression on one of the cylinders. I narrowed this down to #6 cylinder exhaust and removed the head to find a dropped exhaust valve seat. Photo attached. It obviously happened during the ground run.  This cylinder has reasonable temps sitting around the average (130 deg C in the cruise)and runs a little rich (as does #5)  I predominantly use Mobil 95 mogas. There seems to have been minor contact with the piston so I intend on replacing valve, valve guide and seat. I am an engineer with a background in avionics and mechatronics and do not profess to be an engine expert, so anyone with ideas, similar experiences, questions or other comments very welcome. 

[facthunter]

Unless the motor suffered a serious overtemp the interference fit must have been less than needed. . I suggest the seat be installed using liquid  nitrogen. (Cryogenic.) This is done with some Porsche engines. Don't make the inserts OD excessively large either.  Nev

[kiwiaviator]

38 minutes ago, facthunter said:

Unless the motor suffered a serious overtemp the interference fit must have been less than needed. . I suggest the seat be installed using liquid  nitrogen. (Cryogenic.) This is done with some Porsche engines. Don't make the inserts OD excessively large either.  Nev

Good suggestion Nev. Used to dealing with cryogenics (mainly LHe in MRI scanners) and LN is readily available. Maybe I wouldn't need to heat the cylinder head if I used that method? My research so far indicates 7 thou OD oversize seat for aluminum heads.

[IBob]

Interesting (if unsettling) sequence of events, Kevin. I'm certainly no engine expert either, but can I ask the following questions:
1. Do you have hydraulic lifters?
2. Did you turn over the engine at all between emptying and refilling oil?

3. You mention a dropped valve seat, also minor contact with piston. Is it possible that the contact bent the valve, which then dislodged the seat? (Hard to tell from the pic.)

Also I fished up this, which I expect you have already seen, but if not you may find informative:

 

[facthunter]

That coil binding would not cause the valve head to be bent. Pumping up could and would also cause roughness and powerloss. I doubt if a bent valve will dislodge a properly fitting seat. Heads of valves can break off if the guides are worn over a period of time. Nothing I'm saying here contradicts anything in the video. Hydraulic lifters are something else that can fail and you have to run the extra strong springs with them and they will HIDE problems.   They run a different camshaft profile also..  Nev

[RFguy]

Looks like quite alot of recession there- or is that a high crust ? Alot of recession would indicate long term overheating.  
remember, once they get soft, essentially over tempered, its all over even if you keep the temperatures down....

 

[Thruster88]

Is that a crack in the top right corner? 

[RFguy]

time for a new head. Jabiru South Africa. AUD1250 I think

[kgwilson]

The serial No indicates it is a Gen 1 or 2 engine. Based on the contact comment, I assume the piston was the early flat head piston without the valve relief indents so if the valve became stuck and was pushed back by the piston, that may have been enough to drop an already weakened valve seat.

[Old Koreelah]

6 hours ago, facthunter said:
 

Hydraulic lifters are something else that can fail and you have to run the extra strong springs with them and they will HIDE problems…

Nev could you explain tne need for extra strong springs. When he had my hydraulic Jab 2.2 heads off about 100 hours ago, my LAME insisted on fitting only the new outer springs, leaving out the smaller, inner ones because he believes there would be too much spring pressure. Seemed like a good idea at the time and no problems so far. Your thoughts?

 

[RFguy]

flat top pistons were still used up to at least3300#2575. (mid gen 3)

the springs in jabs are weak. I went right through that investigation, and it is elsewhere on the site- jab valve springs are about 1/3 the tension that those lifters are usually used with.  Double springs are approx same overall tension as singles.

 

New head. simple fix. you might replace the rocker bushes while you are at it. (small arbor press required) - watch out for slithers of the material being extruded during the extraction and insertion).. 

[justinjsinclair]

Hi have 5 spare heads(well 11 if the LCH conversion goes well)  here in Brisbane, 5 of them have about 500 hours with some recession and the other 6 have 125hrs also with a little recession pm me if you want - Justin.

Edited March 25, 2023 by justinjsinclair

[facthunter]

There's little chance of a good insert going soft. Its well over 50% nickel and chrome +a few other exotic materials so stays hard at refractory temps. They can hammer into the aluminium or come loose  if the alloy is too  soft due to overheating or just being s bit suss on quality. It relies on the right interference fit being maintained and good finish to transfer the heat away. A thin insert has more "give" and has more chance of staying tight. Going deeper into the port helps also to give more area of grip.. Look up  well-tite inserts.  Beryllium copper is carcinogenic. Don't work with it.  Nev

[kiwiaviator]

16 hours ago, Thruster88 said:

Is that a crack in the top right corner? 

No. Just light crud.

[kiwiaviator]

16 hours ago, RFguy said:

Looks like quite alot of recession there- or is that a high crust ? Alot of recession would indicate long term overheating.  
remember, once they get soft, essentially over tempered, its all over even if you keep the temperatures down....

 

No recession Glen.

 

A bit more info. Double springs installed. Non Jabiru forged pistons were installed at the last rebuild hence the lack of valve relief. This was due to the problems we had with the then supplied Jab pistons (that's another story/thread)

I'm going to remove at least one more head (#2 which runs the highest temp)

Thanks for the comments so far.

[RFguy]

So, I see Justin has some coarse finned heads,  they might be good.  
there are a few from time to time as you know on jab parts facebook. 

another option is ask on jabcamit forum.

I have some but they've got plenty of recession . Need milling down 0.9mm  (to take the recession out to get square again ) and dropping copper shims in (I have the shims- got them water jet cut) 

these guys have the new heads - recommended : 
Philip Gibbs  Shadow Lite CC  t/a Jabiru Aircraft SA CAA AMO L14 & 909 George 
Airport, Aviation Area, Hangar 22, National Rd, George, Western 
Cape, South Africa PO Box 2540, George, 6530, South Africa
Ph: +27 44 - 8769991 / 2            Fax: +27 44 - 8769993
Email: [email protected] <mailto:[email protected]>   Website:
www.jabiru.co.za <http://www.jabiru.co.za/> Office Hours: Mon-Thurs
08h00 - 16h45
                            Friday         08h00 - 15h45
Click on this link for a view into our factory:
https://youtu.be/vPYUwuFTKSM