jymz

I have several OS 120AX engines. I was thinking of switching the thinner head gasket from the 120 Special in to the stock 120s I have. I was looking to increase compression in order to run FAI type fuel for next season. I can mix my own fuel and would like to eliminate the nitro all together. Would the change from the stock .39mm gasket to the thinner .10 gasket work? Will I need to change glow plugs?

Thanks

tigrejohn

My Feedback: (7)

It may work. But effectively reducing your squish band clearance by . 011 inches in one step may be too drastic. Unless your capable of measuring the actual squish clearance, suggest starting with 3 .10mm gaskets at first and then work down to one if you can. This way, you will probably avoid causing detonation damage (I've seen collapsed pistons, bent rods, and snapped cranks) if you go too far. If you can measure, shoot for .008-.010 inches clearance for FAI fuel. With your engine, it may be possibly less or more depending on the style of the band.

jymz

I can try the three gaskets on the old one first.

Looking at the parts numbers for the Special it looks like the only change they made was the gasket. Head, cylinder and connecting rod etc are all the same. Thats why I was thinking it should work.

Not sure how to measure for the squish clearance.

j.duncker

My Feedback: (2)

Use a piece of dentsl wax in the chamber turn engine over once then remove and measure.

Sometimes you need to sandwich the wax in kitchen foil.

airraptor

My Feedback: (66)

Dont worry about the gasket I have ran mine 120 with no gasket at all no issues. also the main change that makes the extra 300 rpm on the special is the crank mod. just open up the crank a little just below the carb like they did on the SP and then put a radius on the crank and square up the corner toward the back of the engine in this same hole. usually goo for 500 rpm ... 200 more than the SP.

Now if you want another 800-1,000 rpm just run a Stock Jett 120 muffler (Red) if in the 8,800-9,300 range or the (Gold) one for the 10,000-11,000 range

jymz

I'll try it out on the old one first and get some data and see how it goes. Thanks for the help

jymz

Finally got around to testing the gaskets.

I have a well used 120ax, APC 16x8 prop and #8 plug.

With Wildcat 10% Premium Extra it ran at about 8500rpm with the stock .39mm gasket.
With Omega FAI it ran at about 8600rpm with the .10mm gasket.


One question is should I try a different glow plug? Its rough running till warm then its OK.

airraptor

My Feedback: (66)

The best I got on a stock un touched engine was 8,400 with the 16x8 APC with baffle and 8,600 with it removed. This was on 10% nitro. Now try it with no gasket and see what happens.

I would try as many plugs as you can to see if it improves anything.

I just run the OS F plug though in mine.