nesikachad

My SD50 with a Jett 60 has been giving me fits lately and I'm curious to see if anyone with a Jett motor has experienced this.


First off, this should in NO WAY be interpretted as a black mark on Jett. I love the engine and I think I am the one who goofed up here.

I've had this thing go lean on me a couple times. The high end needle was vibrating/rotating in flight. A couple smoked glow plugs taught me that lesson. The last time I flew it it just seemed to be down on power and no ammount of fiddling with the needle would resolve it.

What I discovered is that the HEAD BOLTS where backing out causing a loss of compression and a lean condition.

My question is has anyone else had this happen and if so how did you resolve it? Loc tite seems extreme to me considering its an aluminum cylinder case and aluminum threads are usually pretty good about mitigating a bolt from vibrating loose.

Thanks.

C

ChrisAttebery

My Feedback: (15)

You need to retorque the head using a torque driver. I'd use around 10in/lbs. Use a star pattern to ensure that they are torqued evenly.

It's not uncommon to have the head bolts loosen up after a lean run or two. If it's a new motor you should check them before running the first time and then every gallon or two.

vasek

My Feedback: (4)

Chad,
i hope Bob27 is still around.... haven't seen him in a while

I'm sure he would tell you if the head bolts got loose to check the cylinder liner alignment!

Check the exhaust port opening and make sure it's not "sideways" a bit compared to the cylinder opening.

It's normal during lean run(s) that the head bolts can get loose on ya

If the engine does not experience lean run(s) the head bolts should not loosen once tightened properly

still4given

The head bolts loosening up on a lean run is normal. The first time it happened to me, I didn't notice until I changed the blown plug and spun the cylinder when I tightened the new plug. (The head is two piece and the button will turn if the head is not tight.) The engine ran terrible so I put another plug in and this time I turned the cylinder so far it  wouldn't run at all. Freaked me out. Bob from Jett engines nailed it right on the head the first time I told him what I was experiencing.

I have not had a problem with the head coming loose as long as I don't get the engine hot. Make sure that you back off the needle at least 500 rpm from peak before launch and I bet your problems go away.

Oh and make sure you align the sleeve before you tighten the head down. Look through the exhaust port and center the sleeve.

Blessings, Terry

combatpigg

My Feedback: (3)

The expanding aluminum could cause the bolts to stretch enough to make it look like they loosened themselves. A properly tight screw is already microscopically stretched.

nesikachad

Didn't realize a little 60 would make a fastener yield like that.

I know all about tensile loading (stretching) a fastener. Built Comp Eliminator engines years ago and we measured all rod bolts instead of using a torque wrench for precisely that reason.

I'll just make it a normal part of my PM to put a t wrench on them.

Cheers and thanks to all.

Chad

freakingfast

My Feedback: (3)

What still4given and vasek said.

bob27s

My Feedback: (19)

Good call guys, and thanks for the help !!

Head bolts coming loose on the engine are not uncommon, and are an indication that the engine gotveryhot somewhere along the way. It happens often enough, more often with the BSEengines, but not uncommon for the 60L

Do NOTuse loctite.It will actually cause the bolts to come loose easier next time - or - will tempt you to over-torque the screws and strip the aluminum threads - or - will lock the threads a bit too well and you will never get the screws out again.

First, the packing nut on the needle assembly should be tight so you have to use a little bit of effort to turn the needle.

Second, if the plug blew, and the head came loose, and you changed the plug, there is a good chance the sleeve is now misaligned.

Take the muffler off of the engine, look in the exhaust port with a flashlight. Visually inspect the sleeve port, and be certain it is lined up (side to side) with the crankcase.

If it is not lined up, loosen the head screws, use a glow plug wrench to turn the head and sleeve (both will turn together) until it lines up. Then tighten the screws back down.

Start with a fresh glow plug.

After the next engine run (ground) verify the engine is performing properly. Isuggest approaching this run as if it were break-in - use a 9x7 prop. After the run, while the engine is still slightly warm, check the screws again - ensure they are snug.

I typically only use a screwdriver handle wrech on the screws. A T-wrench can be used if you are careful - do not get carried way. This is not steel-on-steel. It is EASYto rip the threads out of the crankcase with a t-handle.

As was noted, periodically check the screws at the end of your flying session. They typically are fine. But if you find them loose, chances are the engine did get hot during a flight so an indicator to watch the needle setting next time out.

Bob

nesikachad

Bob,

Thanks for the guidance.

Assuming the engine got hot, which I'm certain it did, leads me to ask a few things regarding the 50 Sundowner.  I built a duct from thin balsa to help direct air over the head and it dumps out the back where the cowl cheeks catch up the wing.

Should the exit be enlarged?  On my 42% Kelly F-1 the exit is quite a bit larger than the entrance.  I'm learning a bit more about this negative pressure thing to pull air through the cowl.

On the Jett site where the 50SD is featured the photos/narration don't mention much about cooling.  Any help would be greatly appreciated.

Thanks.

Chad

mk1spitfire

cool, Bobs back

bob27s

My Feedback: (19)

Cooling is important in two areas.....

you want a bit of cooling exit area on the bottom of the cowl...

you want a bit of cooling exit area around where the exhaust extender exits (a slot works ok).

A few folks provided some vent holes along the belly to vent out the muffkler area just a bit more.

Also...... close off the air "inlet"opposite of the engine cylinder.

use a light prop ...... 9x7, and allow the engine to run at least 800 rpm off-peak for launch. When it unloads in flight...... it will need the fuel.

bob27s

My Feedback: (19)

Yeah..... when Ican

Trying to find a new job in this environment is a full time job in itself. Here in Ohio, unemployment is up over 10%, and locally here its probably closer to 11-12%.

Networking is critical to finding opportunity.

Bob