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G-mark .061 prop question

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Old 04-15-2019, 04:24 AM
  #51  
aspeed
 
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Yes that is as measured. I used a #5 centre drill which was broken before, and handy. It is 7/16" diameter, so 7/32" rad or so which is what I copied from the TTiger .07. As mentioned, I have been using 100 degree countersinks for the combustion chamber shape with good luck. That is what Merlin plugs were, and the Picco P zero had a slight radius, but really matched a countersink quite well. The .049 motor's depth to the base of the plug is .060" so with the G spot I made it .070. It turned out a bit deeper. In either case, I have prepared the threaded end, and countersink, or ball end mill to the 3/16" hole leaving about .010" extra. It seems that after a few tightenings that the plug protrudes from wear, as it is 60 degrees. The Nelson plugs don't seem to do that. They are 118 degrees from what I have measured. Probably just a regular drill bottom shape. They are 1mm larger diameter so I don't use them on the small motors. A .15 or up is OK but then the tapered end plugs don't help as much as the little guys. I did an LA .25 and there was very little gain, but the .049s with regular plugs really were night and day. I picked up a Nelson tap, but have not used it yet.
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Old 04-15-2019, 10:23 PM
  #52  
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I think that the construction with the conical surface will always make the head expand a little. This is very noticable on ABC engine where the head sits into the cylinder a bit. On the CS .061 engine it is even stated in the instructions that one should always loosen the Nelson plug before trying to remove the head from the engine. The expansion could perhaps be used as a "feature" in order to really seal off any dead space between the head and the cylinder wall.

I have never permanently deformed any head though, when I get the fit for the Turbo plug just right, there is very little force needed in order for them to seal. After that I rarely ever change the plug, but perhaps with frequent plug changes it would be different.

I have intended to make a Turbo head for the G-mark .061 for quite some time, but never got around to do it, will be interesting to see how it works out.

Last edited by Mr Cox; 04-15-2019 at 10:26 PM.
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Old 04-16-2019, 06:37 AM
  #53  
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Yes it is good to get in the shop and play a bit. It is a bit cool and windy to fly now, but the shop is a good temperature to work. We can't get on the field because of the moisture/standing water yet. It's coming very soon. The Gmark seemed like a good candidate for the turbo plug. I have just picked up a gold head Taipan .15 that is begging for a head. I had one before, but don't like running orphan engines often unless I have a spare.
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Old 04-16-2019, 09:49 AM
  #54  
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Originally Posted by ffkiwi View Post
I'm guessing they're probably M2.3-which is a genuine metric thread-but a very uncommon one! To make matters worse there is a fine thread and a coarse thread variant: M2.3x 0.4 and M2.3x 0.45...so the chances of being able to source slightly longer machine screws (the originals are 5mm thread length) in those thread variants are virtually zero....leaving you the option of either keeping the originals or tapping out the crankcase holes to take a US size-2.56 for example is fairly close in size [and the thread pitch is .456mm....so if the originals are M2.3x0.45, your 2-56 is almost going to fit if you need a longer screw to secure the radial mount....I guess it becomes your call depending on how thick you make the radial mount....and whether you counterbore for the screw heads, like in the original...

ChrisM
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I get a thread pitch of .40 on the backplate screws. Would I be able to run a 2-56 tap into it to re-do the threads or would i have to try to drill the holes to the proper size for SAE then run it in. I don't want to bugger up the case and have a paper weight, but I do have a spare .061 that has never been run. It is the one in the pic with the radial mount mounted. It is just making me nervous the little amount of thread actually grabbing the case. I will get a measurement of how much actually protrudes from the motor backplate with the radial mount on.
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