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os fs-48 Surpass my first 4 stroker tear down


Old 04-10-2011, 10:28 AM
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Default os fs-48 Surpass my first 4 stroker tear down

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 consistantly 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,


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Old 04-19-2011, 07:25 AM
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Default RE: os fs-48 Surpass my first 4 stroker tear down

A new ring and a bit of polishing of the liner with 600-1000 grit sandpaper will usually do the job.  You can touch up the valve seats with a bit of toothpaste and hand pressure, but be very careful, there's not much metal to work with.  Full replacement of piston, ring, and liner would be needed if you have bad scoring.

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