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Author Topic: LA 25 back plate gasket  (Read 1085 times)

Offline Neil Campbell

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LA 25 back plate gasket
« on: November 13, 2011, 06:44:37 PM »
 


I  just recently purchased a LA 25,, I have a os needle valve assembly and a thunder tiger back plate......Question is Do I need a gasket for the metal back plate?  the os with the plastic does not have a gasket.    which one from thunder tiger would I need?     Part Number?
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Neil C Campbell

Offline Steve Helmick

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Re: LA 25 back plate gasket
« Reply #1 on: November 13, 2011, 10:54:19 PM »
I just made one from a piece of a manilla envelope a week or two ago. Cut the hole first, and slide it onto the backplate. Then punch the 4 screw holes. Cut the outside with a #11 Xacto last. Wet the gasket with oil after putting it on the backplate, but before permanently installing the backplate to the case.

Yes, you could skip the gasket and just use silicone seal, if you wish. 

Do yourself a favor and replace the JIC screws with socket heads. 3mm x .5m x 12mm lg. for the head, 2.5mm x .6m x 8mm lg. for the backplate. You should be able to get both sizes at the local R/C car toy store.  H^^ Steve
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Offline kenneth cook

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Re: LA 25 back plate gasket
« Reply #2 on: November 14, 2011, 01:37:55 PM »
          Neil, the first thing I would check is to lay the plastic backplate and also the new metal one side by side. I had a brand new LA .25 that I replaced the backplate with a FP.25 backplate. I discovered there was a sizeable difference and the newer metal one wasn't protruding into the case as far as the plastic one was. Eventually the rod during running would work itself towards the back of the motor where it would start to slow the engine down due to it binding. I lapped the case for quite a while on a flat plate to remedy this. I never had the problem again. Side by side though you could see a noticeable difference by eye in the two backplates. This was a hard problem for me to figure out initially. I would put the plane away in the house nose down on the countertop. I believe it would fall forward and not have any noticeable bind. I would run the engine and it would start binding. It would get stuck. I would take it apart and reassemble and all would be good, then it would bind again. I got home after a fellow club member told me to compare them and sure enough I noticed the difference. Ken
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Offline Geoff Goodworth

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Re: LA 25 back plate gasket
« Reply #3 on: November 14, 2011, 06:34:55 PM »
Re Ken Cook's comment and not to question him, as I don't have any LA 25s but be careful about simply sitting the two backplates side by side. The flange on the plastic backplate of the LA 46 is approx 0.020" thicker than the one from the FP 40. The length of the piece that goes inside the crankcase is the critical dimension.

And Neil, you can still get GP 25 gasket sets which include a head gasket and a thrust washer. Otherwise, make a gasket as Steve suggests.
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Offline Bill Little

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Re: LA 25 back plate gasket
« Reply #4 on: November 14, 2011, 08:35:53 PM »
I usually use automotive silicone gasket maker on back plates.  No problem over the past 20 or so years......

Big Bear
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Offline Chad Hill

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Re: LA 25 back plate gasket
« Reply #5 on: November 16, 2011, 08:30:13 PM »
RSM makes round backplate gaskets for the LA25 (no dog ears). They also make delrin venturis for them.
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