BobbyGee

My Feedback: (3)

Wonder if anyone can help as to what might have caused a broken rocker arm on a Saito 72.

I have an engine that sat for a couple of years. I loosened everything up with some after run oil and some squirts of WD40. It turned over very smoothly.

When I started it up I was very pleasantly surprised that it fired up after a second or two with the starter. It ran sweet with the glow plug on it for a minute. With the glow plug off it cut out at idle - nothing new, nothing unusual.

I started it at least half a dozen times with the starter and each time like the first - it fired right up. It was so sweet that i decided to go at it with a chicken stick. Twice it took one flick and it was running.

It tached at about 8600 rpm with a 13 x 6 prop. It held idle with a couple of notches of throttle trim. Each time I tried to get it to idle slow it cut out.

After running about half a tank and some 10-12 starts, it wouldn't start again. We saw bubbles in the fuel line and noticed it wasn't drawing. Compression was very weak, too. We suspected a valve needed calibrating.

On bringing a it home and taking off the valve covers I saw that rocker arm on the intake valve was broken clean in half on the valve side of the rocker arm support.

What can cause such a break? I've got parts on the way and don't want to do the same thing again.

Thanks for any insights.

BobbyG

DSTIEBS

Maybe it was metal fatigue and just snapped or it jumped timing allowing the piston to hit the valve when the valve should be closed.

Star7

Is the valve stuck?

BobbyGee

My Feedback: (3)

Thanks for the replies! It never takes long on RCU.

I don't know what jumping timing is - how does it happen? What can cause it? Is it preventable?

No - valve isn't stuck. Everything inside is fine. I broke the engine down some to look inside, removing the cylinder, cam cover, cam assembly, lifters and tappets. Valves operate just fine, too. Both springs compress with the same amount of pressure.

Thanks again,

BobbyG

XJet

If the tapered end of the pushrod jumps out of the divot in the rocker arm, that can cause the valve to remain open and there's a fairly good chance that in such a condition, the piston will hit it placing *huge* stress on the rocker arm.

Star7

I suspect that dstiebs had it right...if you jumped timing for whatever reason and the piston hit the valve you would have a valve head that no longer seats. There would be a poor seal, low compression and no fuel draw...this is kinda a wag and I have no idea why it would have jumped time in the first place. Can you see if the intake valve looks straight or if the head is a little cocked? sorry you got trouble!

w8ye

My Feedback: (16)

Saitos don't jump timing unless the cam gears break or the cam box breaks.

A valve can stick and the pushrod can jump out of the socket as was described and then do the resulting damage as has already been described

Lifters can stick and break rocker arms and bend pushrods.

Most of the time if there is a valve-piston strike it bends the valve guide and ruins the jug. On the smaller Saitos the rocker arm stand is part of the jug or cylinder.

Did the rocker arm break or did one side of the rocker arm stand break off the head?

BobbyGee

My Feedback: (3)

WOW! Thanks, everybody!

It was the rocker arm itself that broke. Rocker arm stand is fine.

XJet may have hit on it. When I popped the valve cover the tapered end of the pushrod was out of the "dimple" in the rocker arm. It may have happened as I tried and tried to get a start, or it may have broken during the last run.

Valve shows no damage. I pulled the piston and got a good look at it in the head. No metal fragments inside the cylinder - it's clean as a whistle. Pushrods, tappets, cams are all fine.

Valve sticking may well have been the culprit - or a stuck tappet at the other end down by the cam. Any way to guard against that?

Best,

BobbyG

DSTIEBS

After looking at the parts listing at Horizon Hobby http://www.horizonhobby.com/Products...prodDetailTabs I cannot see how it could jump time. Roll the push-rods on a flat serface and see if they are bent. If they roll flat and are not bent I don't think the piston hit the valve. When you put it back together be sure to line up the marks on the cam gear with the crank gear. That sets your timing for the valves to open and close at the right positions to the piston. There is also a gap of .002 between the valve end and rocker arm. Without that gap it won't run when it gets hot. Horizon has the manual for .95 cents plus all the parts to rebuild it.

Moderator adjusted the URL so it will link directly

w8ye

My Feedback: (16)

Here's the manual for free

http://www.horizonhobby.com/ProdInfo...der_Manual.pdf

w8ye

My Feedback: (16)

The rocker arms come two to a pack

Clearance is .002" to .004"

BobbyGee

My Feedback: (3)

Thanks, DSTIEBS & w8ye.

Already ordered the parts from Horizon. They're on the way.

In studying to prepare for the re-assembly I had doubts about the instructions in the manual about the timing mark on the cam gear relative to the crankshaft at top dead center. I had found an article online by FlyRC magazine's Andrew Coholic that was about a rebuild of a SAITO, I e-mailed him with a question about that and he was kind enough to reply within minutes! Really neat people in this hobby!

I will pay particular attention to the gap between the rocker arm and the valve end. Thanks for the pointer.

I must say that despite the pain in the neck and the expense of the repair, it is fun working on the little engine. Tinkering with it so far has been relaxing.

BobbyG

w8ye

My Feedback: (16)

Andrew Coholic is a very fine person. He used to be on here all the time and still visits on occasion.

DSTIEBS

Thanks for the manual. I saved it with my others. Never know when you will need one.

DSTIEBS

You are welcome. I love to rebuild them also. These guys on this forum are great. Good Luck on the repair job.

w8ye

My Feedback: (16)

A couple things

The small end of the pushrods goes towards the rocker arms

If you have the rod off the crank pin, The side of the rod bearing with the chamfer goes towards the counter weight

The engine crankshaft has to be at top dead center when the cam is set in place

The cam mark goes straight down. you can hold it there by using the small end of a pushrod inserted into the intake pushrod lifter guide hole with the lifter removed. There is a hole in the cam that the pushrod will engage and hold the cam mark at 6:00. You can also use a knife jammed in the cam gear teeth to hold the cam in place. You must use a gasket between the cam box and the crankcase to space the gears correctly.

Ram Jet

My guess is that it was simply a defective part that was destined to fail at no particular point in time. There is probably nothing wron with your engine overall. But a broken rocker arm is, based upon my life experience, very rare and usually happen when the piston crown comes into contact with the valve head and you have eliminated that probability. You would likely have bent the valve stem also.

Bill

Ram Jet

Possibly a good afterrun oiling.

Bill

Hobbsy

My Feedback: (102)

Bobby, WD 40 is a product that I keep in a separate room from my engines. I suspect that the sitting around for a time was the problem. The intake valve stem is gummed up, it can follow your finger up when you press the valve down but may be draggy enough to to be too slow to follow the cam at speed. The slow 8,600 rpm is a clue that one valve or the other was hanging up.

Ram Jet

Hobbsy, you gotta trust me on this one. Try one can of G96 Gun Treatment and you'll never buy another can of WD40 agian. It's wonderful lubricant and moisture displacer. Do yourself a favor and never spray WD40 into a tumbler lock. It gums up like castor after a short time. G96 is made in Patterson, NJ. They used to only sell by the case but now you can buy individual cans. You might also check a local gun shop, they should have it.

Bill

Hobbsy

My Feedback: (102)

Bill, I think I already shared that I'm sold on Corrosion X ans my choice of after run oil. Also I use STP power steering fluid as the lube for rocker arms etc., the stuff is tenacious and will still be visible in the rocker pivits after many flights. Thanks for the tip though. Its always good to know about another product that works. When I want to remove all traces of previous lubes I use Brakleen from NAPA, you just have to be careful and not freeze your fingers when using it.

BobbyGee

My Feedback: (3)

Thanks for your inputs Hobbsy and RamJet.

The after run oil I have is Evolution Blue Block.

I see that neither of you endorse WD40, I'll get some Brakleen at the local NAPA and give the valve stems and seats a good rubbing. I'll get some STP as a lube for the top end, too.

What range of RPM can I expect with a 13 x 6 prop? I'm using a Master Airscrew K Series.

Thanks again, everyone here has been a big help.

BobbyG