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Dec 06, 2013, 04:31 PM
bigrion
Discussion

How many head gaskets in pre-Surpass OS FS-120


I am enjoying rebuilding an old twin rocker cover head OS FS-120 I bought on e-bay. When I got it apart, it became apparent it had been rebuilt at least once before: vise grip marks on connecting rod, and missing rocker arm plastic spacers, among other clues.

It has two aluminum head gaskets. Outer diameter is the same on both, but one has a slightly smaller inner diameter. I have not been able to find an exploded view of this engine. The parts list is no help.

Is two head gaskets stock configuration for this model engine, or did a prior re-builder add a second head gasket, either accidentally or on purpose?

Is there any reason to lower the compression ratio by adding a second head gasket?

And as long as I have your attention, does anyone know why one rocker cover is stamped "1" and one rocker cover is stamped "2"? They look identical to me.

Thanks, bigrion
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Dec 09, 2013, 11:45 AM
Registered User
I have a NIB FS 120, and it has a single aluminum head gasket.
Regarding the numbers on the rocker cover, it might just be two different castings they came from, the part numbers are the same so the are identical.
I can check mine if interested?
Dec 09, 2013, 01:25 PM
bigrion
Thanks, Vigger. I am going to reinstall just one of the head gaskets, with confidence, and continue to wonder why OS used two different casting numbers on the identical rocker head covers.
Dec 09, 2013, 01:46 PM
Registered User
earlwb's Avatar
Well, there are two reasons one might have more head gaskets or shims on a engine. One is to lower the compression ratio so that the engine could run glow fuel with more nitromethane in it. If the engine was made for the European market for example, then it would have had a higher compression ratio so it could run zero nitromethane glow fuel. But if it was brought into the USA, then the compression would have been too high for those people using higher percentages of nitromethane in the glow fuel. Also if someone wanted to run higher nitro percentages than normal they could have stuck on the gasket too. Now then I forget if the early OS 1.20 engines came into the USA with higher compression ratios though, so it could have come that way from the start years ago.

Another reason is if the head or cylinder needed to have a little taken off the top, so to speak, to square up the mating surfaces so they fit better. Thus milling a few thousands off, would raise the compression ratio, so they needed to use a extra gasket or shim to compensate.

I do remember people going to buy a jug of 10% glow fuel and thinking that 15% nitro glow fuel ought to make the engine run even more good, so they get 15% nitro glow fuel instead. Then you have the folks that think that if 15% nitro glow fuel is good then 20% nitro glow fuel has to be even better still. Then there are those who decide to get 25% nitro glow fuel as it has to really bring the engine alive then.
Last edited by earlwb; Dec 09, 2013 at 01:51 PM. Reason: add more information
Dec 09, 2013, 04:59 PM
Registered User
The numbers on the rockers could be the number of cavity they came out of when stamping. When stamping several parts at a time they usually number the cavity that the parts come out of in case they have a run of parts that are not correct they can then go back to a particular cavity to inspect it for any problems.
Dec 09, 2013, 07:35 PM
Registered User
Quote:
Originally Posted by jgfan
The numbers on the rockers could be the number of cavity they came out of when stamping. When stamping several parts at a time they usually number the cavity that the parts come out of in case they have a run of parts that are not correct they can then go back to a particular cavity to inspect it for any problems.


???????
Dec 09, 2013, 08:17 PM
Registered User
Quote:
Originally Posted by datsunguy
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You don't think they make just one cover at a time do you? What they probably do is have a machine that stamps out several at a time. If they are in full production on them and as they are assembling them they notice that one has a flaw in it instead of just scrapping all that are made they look and see what number is on them. That traces them back to the specific mold that has a flaw in it. Then they can either take that mold out of service and only have to scrap the ones off of that one mold or fix that mold. But it basically is a way to track down which machine or mold made that particular cover and fix the problem.
Dec 09, 2013, 08:32 PM
bigrion
Earlwb:

So if I increase the compression ratio by using only one head gasket (stock US configuration), I should stick to 15% or less nitromethane (stock US 4-stroke mixtures) to decrease chance of backfires if I have a lean moment, correct?
Dec 09, 2013, 09:44 PM
Registered User
earlwb's Avatar
Yes that is correct, but do pay attention to any pre-ignition effects that could occur. I normally use FAI fuel or 5% nitromethane glow fuel. I only go with more nitro for those occasions where I need more nitro, such as for racing or a handful of engines that need more nitromethane in the glow fuel. High compression uses little to no nitromethane in the glow fuel and the compression gets less and less as the nitromethane content increases.
Dec 09, 2013, 09:50 PM
bigrion
I tried to measure the depth of both rocker covers with my digital caliper. As far as I could tell, at the spot between the casting number and the screws holes, # 1 is .1795 in deep, and #2 is .1770 in deep. It seems to me the depth is the only dimension that could matter. Assuming I measured the rocker depth at the spot where it would matter (this was not above the apex of either end of the rocker arm), and assuming I could accurately measure to a tolerance of 1/1000th of an inch on the bench, .0025 in is not much of a difference. The cam lobes on this engine are worn, so I wouldn't rely on measuring them to see if one cam lobe is taller than the other. You have to remove the covers to set the valve clearance. The instructions tell you to "Remove the rocker covers by unscrewing the Allen screws from the rocker boxes on top of the cylinder head." There is no mention of any difference in the rocker covers. There is no direction to be sure and put them back on the same side as removed. There are no wear marks above the rocker arm apexes on either rocker cover of this well worn engine. I would assume each rocker cover has covered both rocker arms of this engine at some point in its long life. One of them has impact marks on the outside, so I am assuming they are original. Lots of assumptions here, but I'm going with casting mold identification.
Dec 10, 2013, 12:41 AM
Registered User
Reginald's Avatar
These are all casting numbers indeed.
Dec 10, 2013, 05:56 PM
Registered User
They came with one head gasket. They run just fine with one.
Dec 17, 2013, 10:03 AM
bigrion

It lives!


New bearings, new piston ring, new O-rings, and a new glow plug have given this old war horse a new lease on life. But what a beast!

Tried to hand start it warm with a chicken tube and the kickback chipped the brand new Falcon 16x 6 wood prop. The electric starter drives it right through the kickback and it starts quickly at idle. Transitions well. Crackles if I lean it too far, but didn't throw the prop. I think it may be under propped. Ran it inverted on 15% nitro fuel on the straight pipe on a test stand. My guess is a previous rebuilder added the extra head gasket to mellow it out. The vise grip marks on the connecting rod are probably from straightening it. Query: is pre-ignition enough to bend a rod?

I could put the extra head gasket back in, or try 5% nitro fuel, but I hate giving up any of the power OS engineered into this engine. Think I'll try a CH Ignitions CDI on it.

Really like the way the engine sounds, and the looks of the twin rocker covers. What size and type prop have others had success with? I need a replacement. This is the original, pre-Surpass OS FS-120. I fly slow, heavy models.
Dec 17, 2013, 11:46 AM
Registered User
Reginald's Avatar
Quote:
Originally Posted by bigrion
New bearings, new piston ring, new O-rings, and a new glow plug have given this old war horse a new lease on life. But what a beast!

Tried to hand start it warm with a chicken tube and the kickback chipped the brand new Falcon 16x 6 wood prop. The electric starter drives it right through the kickback and it starts quickly at idle. Transitions well. Crackles if I lean it too far, but didn't throw the prop. I think it may be under propped. Ran it inverted on 15% nitro fuel on the straight pipe on a test stand. My guess is a previous rebuilder added the extra head gasket to mellow it out. The vise grip marks on the connecting rod are probably from straightening it. Query: is pre-ignition enough to bend a rod?

I could put the extra head gasket back in, or try 5% nitro fuel, but I hate giving up any of the power OS engineered into this engine. Think I'll try a CH Ignitions CDI on it.

Really like the way the engine sounds, and the looks of the twin rocker covers. What size and type prop have others had success with? I need a replacement. This is the original, pre-Surpass OS FS-120. I fly slow, heavy models.
I've been using a 16 x 6 on mine all the time on the one in my old Piper Cub.
Be carefull not to do a hard head-on crash since the casting is very fragile at the bottom end and NO replacement parts ! Like earl advised if you feel the compression is too high just put in another head shim.
Last edited by Reginald; Dec 17, 2013 at 12:32 PM.