glydrjocky

I have been running a used FS-48 in my 4 Star Fourty for quite some time. It flew fantastic on an 11-7 prop, F plug and either Omega or Cool power 15%. Recently I was beginning to have issues with it running inconsistantly and the compression seemed to be low, a new plug or fresh fuel has fixed the issues in the past but she finally said I'm done. I pressure checked my fuel tank, always run a fuel filter and didnt find anything out of the ordinary on tearing down the carb and manifold.

Decided to pull the engine down and see what was going on inside. Here is a list of a few things of interest.

1. The cylinder looks smooth but no real noticable cross hatching. The piston has some varnish overall but no scoring. The piston ring gap measured out at .014 which seemed a bit excessive to me.

2. The exhaust valve has some carbon buildup and a few black marks on the valve seat which made me wonder if the valve is seating as it should. I had checked the valve lash some time ago and I was able to detect some lash in each rocker before I tore the engine down so I'm guessing that wasnt the problem

3. The valve tappets (lifters) were gummy and required a bit of up and down manipulation with some marvel mystery oil to get them up high enough to remove the camshaft.

4. The camshaft looked fine but the inner bearing is very rough when the cam is inserted and turned. The outer bearing feels like new.

5. Both main bearings were rusted and rough so they will be replaced as will the cam bearings.

I guess my main question is, should I just replace the ring to freshen up the bore or will I need to replace the piston, ring and sleeve to get back to the fine running engine I miss so much? Can I freshen up the valve seats with some fine lapping compound and call it good?

I've done this work on auto and full size aircraft engines but the miniature size seems somehow intimidating to me.

Any thoughts would be of great help,

Tony

Motorboy

The piston ring gap in model four stroke engine is bigger than in the two stroke engine due the moving parts below the piston need lube. Cross hatching in the model engine is much smaller than the big brother of the engine such as in the auto engines since the tools to grind the cylinder is difference in the model engine and auto engine.

Also the ring gap at .014in your engine is normal so long the engine is easy to start and nice idling.

Clean up the engine free for dirty and residue and replace all bearings if they are worned/rusted. Lap the valves with chrome polish paste or extra fine lapping paste.

After ended flight time , give the engine ATF oil in the crankcase to prevent rusting of the ball bearings via breathing nipple and some drops into the combustion camber and rotate some turns to spread the oil overall.

controlliner

Seize the day, as little as possible trust in tomorrow

Campgems

1 That does seem excessive. 2 or 3 thou sounds better. I wouldn't worry about cross hatching, I just checked a sleeve for a O.S FS-52 and it looks like it was ground. No indication of wear including to ring ridge at the top.

2. The valves always have the black from burnt castor. That's normal

3. Again, the buildup of oil and gum on the lifters and the bore of the crank is normal. Iuse a small Allen wrench to reach in and push them out of the way.Just be careful not to scratch the bore in the crankcase they fit into.

4. I would replace boththe bearings. When the cam goes bad, it is really noticeable. I've only had to replace one, on a friends 48. It was really beat. They usually look brown from the Jastrow and just a fine wear line from the lifters. Getting the cam bearings out is a tedious job. I've tried making removers with mixed success. What always works though is my red sprue wax. A short length of 5MM drill rod that is polished down to a slip fit in the IDof the bearing is needed. Make sure the end is machined square and the corner is beveled slightly. Wad up about 5mm balls of the sprue wax and drop them into the ID of the bearing until no more fit. and then use the rod and a very small hammer to tap them down. Add more wax and tap down until the bearing is out of the case. Don't hammer hard as there is not much meat behind the bearings and you could easily blow out the cap or crank case. It is the same old trick for getting pilot bearings out of a crank, but the cam bearings don't have a shield or seal, so grease just squirts around the balls. The wax is just right to be able to pack in and push out the bearing. The nice thing about the sprue wax is that it comes right out of the case after the bearing is clear, and you can re-use it if you like.

5 Use a premium quality bearing for both. An open bearing for the main and shielded for the nose. I pull the rear shied off on the nose bearings. W8YE states that he leaves the shields in place on both and that is OKalso, just a matter of what you like.

Ireally don't like the idea of lapping the valves on these small engines. There isn't very much spring pressure there and any increase in the valve to seat seal width will reduce the effectiveness of the valve vs helping it. Iwould soak the valves and head in some Dawn Power Dissolver (Look in the grocery store in kitchen cleaning stuff. A blue hand pump spray bottle, Keep it off your hands, it really drys them out). Let them set for an hour or so , brushing them every now and then, and that should take a lot if not all of the baked on junk off. The hard carbon will still be a problem. Once you have them clean, Imight take a scotch brite and do a quick burnish in the valve and seat, but be very careful not to over do it , epically on the seat. Once they are cleaned up, if you don't see obvious burn marks, leave them be. If you really think you have to touch up the seat, don't use anything more abrasive than tooth paste.

These engines are not difficult. Removing the old bearings are the hard part, and then main bearing is the issue. I just rebuild an old O.S 40 two stroke and to get the main bearing out. Iheated the case in the kiln at 250 degrees for about 20 minutes and then holding the case in a heavy gloved hand I whapped the back with a leather mallet. about three whack was enough to pop the bearing out. Others, have been a real challenge. Ijust did a Saito 150 that I had to turn a 15.5 mm rod that just fit the nose of the crank. The bearing IDwas 15mm so there wasn't much meat to drive on and that was the only way I got it out. The O.S 40didn't even give me the 1/2 MM to work with so I resorted to the leather mallet. Other than that, they are easy to work on. Just make sure you get all of the water from cleaning out of the screw holes and work some oil in the before putting things back together.

Don

glydrjocky

Thank you so much for the feed back guys. I was just out in the garage cleaning up some of the parts, soaking the valves in marvel mysetery oil really loosened up the carbon deposts but I will have to try the dawn product mentioned.

Campgems, I just tried to remove the cam bearing before I read this post. Tried the old heat method then tapped the case against a wood block. It didnt so much as budge. I'll try your method for sure. I was thinking of filling the area under the bearing with water and freezing it but not sure if that would work. Where does one get the red sprue wax you mentioned?

I agree that .014 seems excessive but I believe that motorboy is right about the 4 stroke needing some extra gap for blowby lubrication. I'd like to see no more than .010 or so. Thinking of purchasing a ring from Frank the Ringmaster as I have heard good things about his aftermarket rings. I have also heard that the ring for the OS .52 might work as the .48 ring seems to no longer be available.

Any thoughts on the ring situation?

I'd love to get this motor running right again. Love the sound, the torque and the fuel economy that it has given me.

w8ye

My Feedback: (16)

The OS 52 and the OS 48 use the same ring

Campgems

PM me with your address and I'll send you some of the wax. Iused to own a jewelry store and I also sold jewelery supplies. I have more that I'll ever use left over.

Don

earlwb

Since you have it apart, I would suggest going ahead and putting in a new ring too.
For a new piston ring just contact Frank Bowman. [email protected]
He makes rings for just about anything. His rings are better than the OEM rings.

gkamysz

My Feedback: (19)

OS actually has different part numbers for the two, but they are interchangeable.

.014" gap is not excessive, but OS usually sticks to the standard rule of .004" gap per inch of bore in the engines I've measured. The Chinese engines seem to be closer to what you observe right out of the box. The amount of blowby can be observed at the crankcase vent to see if you have a problem here, but it doesn't mean much.

If the face of the ring is not damaged I wouldn't replace it. The piston should also be closely inspected prior to installing a new ring. Worn ring lands will not allow the new ring to seal. In this case a new piston is required. I had an FS-48 where the wrist pin cap was missing and the pin gouged the cylinder. This gouged the ring a bit. I didn't do anything except use some 400 paper to smooth the cylinder and this engine ran very well for a while until I got a low time donor engine for parts. There was no obvious difference in performance.

Greg

glydrjocky

Got the cam bearing out!!!!! I was trying to think of a wax substitute and found some Mortite weather strip putty! It worked perfectly. I took a #8 drill bit, chucked in a drill press backwards and polished the shank to a 5mm fit. Balled up the Mortite, tapped it in place and the bearing magically moved out of the bore. I love physics!!!!! You guys ROCK!! I'll keep you posted on the progress. Gonna order parts next.

Tony

Motorboy

In case you has acess to a lathe, you can make the bearing puller as i did here post #23: http://www.rcuniverse.com/forum/fb.asp?m=7023803

glydrjocky

Silly question, but there was no gasket between the head and the intake manifold, should there be one? The carb O-ring is intact, present and accounted for. I guess I've seen stranger things but with no parts break down in front of me I can't be sure. The motor was used and someone removed the choke assembly long ago. Hmmmmm.

jimmyjames213

My Feedback: (2)

their usually is one but their doesnt have to be one as long as it is sealed

gkamysz

My Feedback: (19)

The 48 doesn't use a gasket at the head and manifold.

glydrjocky

New cam and main bearings on the way, including a fresh ring from Frank Bowman! Can't wait to start reassembly next week. Any advice on ccrosshatching the bore for a good ring break-in? The sleeve seems a bit too smooth to seat the ring.

Tony

w8ye

My Feedback: (16)

It will be OK. Just put the new ring in without cross hatching. You will find that the Bowman ring will seat in within the first couple runs

Some of these guys will have you cut your cylinder sleeve away with something harsh and it is just not necessary.

The sleeve never had much of a cross hatch in it from the beginning.

glydrjocky

Okay! Fresh Boca bearings installed for cam and mains!! Fresh Frank Bowman ring installed using Franks methods outlined in the paperwork he sends with the ring. Lapped valves gently with toothpaste then installed the springs. (Minty fresh exhaust gases)

The valve lash is .003 exhaust and .004 intake the way it sits, my copy of the manual states that it should be .0015-.004 thou.

Where should the idle bleed screw be set? I put the tip of the screw halfway over the port to start but the manual I have doesnt give a starting point.

Gonna try to fire it up on the bench this evening and hope to fly it if all goes as planed this weekend.

w8ye

My Feedback: (16)

Hlf way over the port is correct to begin with.

Scirocco14

An easy starting point that I use on the air bleed carbs is to insert a T-pin into the hole and turn the idle screw until it contacts the T-pin. Seems to be pretty close.

Mark

glydrjocky

Strapped her into the test stand and fired up the beast! Ran a full tank through it rich and it seemed to run strong. initial impression is its far tighter than it was and the bearings are sooooooooo much smoother. Had no idea it would run this quiet. igniter and power panel died before I could press on further. We'll see how things go. I put in a fresh F plug this morning as I was using a miricle plug for initial start up. Wanted to make sure it was running well before commiting to a expensive plug.

Gonna see how it runs this morning then mount it in the airplane if all goes well.

Thanks again,

Tony

glydrjocky

Got up Saturday morning and ran another full tank through the engine very rich. Satisfied with the performance I mounted the ole .48 back in my kit bashed Sig fourstar and headed for the field. Once there she started with a single flip of the prop, I ran it up to full throttle but kept the mixture purposely rich. It was taching just under 9000 rpm with a master airscrew 11-7 prop. Took off leaving a nice smoke trail in the sky, the engine ticking away like a fine swiss watch. The power was smooth but subdued due to the overrich setting. Did two full flights in this manner. On landing I was amazed at how much slower the idle was compared to what I was able to achieve before. It settled down to a nice 2800rpm tick, the landings were much smoother with far less float and a more controllable taxi speed than I ever had before. I'm stoked!!!!! Very happy with my first 4 stroke rebuild!

Thanks again for the help you all have given me on this little project, wishing I had done this a few years ago when I originally built this bird. Had no idea the bearings were so bad.

The dog has indeed learned a new trick

glydrjocky