ST G51
Improving the stunt performance of the Supertigre G51

Page 3
My Mo-Best

Further ideas for development
Now that I have one engine performing reasonably well I am thinking of what to try to improve it further. I still have some speeding up when the wind blows and I would like to get better control of that. One thing that seems to work on a number of other engines is to provide a shield to the plug. I don't like the Retro version as I have doubts that I could fit it firmly enough to be sure that it wouldn't drop out. Brian Turner has used a different method to provide the plug shield and has found some improvements from its use. Brians method is more foolproof but it does rely on the head plugging into the cylinder for enough distance to allow it to be fitted.
The reasons for this modification come from the experience Brian had whilst developing the Stalker 61 LT LS prior to building his own engine. What he found was that when he set the engine sufficiently rich to get the lap speed he wanted, its running became noticably rougher and the engine vibrated more than usual. After adding the shield the engine was significantly happier. as part of the testing the engine was run without the shield and with the plug connected. The needle opened until a point was reached when the revs would drop when the plug was disconnected and the revs that this happend at was noted. With the shield in place the same test was made and it was found that the point where the engine noticed the disconnection of the plug was at a much richer setting. and the engine was sounding happy at a RPM setting that, without the shield, would have been horrible.
Adding a Plug Shield
head before machining skimmed to squish
The head before machining starts. Only the chamfer has been done.
the sealing face is about 0.4mm above the squish face
The sealing face has been machined until flush with the squish band. Only a clean-up skim has been taken from the squish band.
head in miller groove cut
The head is set up in the milling machine. I took a lot of care to get the face for the slot truly parallel to ensure that the slot was the same depth at both sides. The slot has been machined exactly 1.15 deep to match with the 1.15mm thick mild steel sheet that I plan to make the shield from.

shield fitted finished plug shield
The plug shield is made from 1.15mm mild steel and is 3mm wide by 26mm long. Ideally I would have used 1mm thick but didn't have any to hand. I don't believe that this extra few thou of thickness will make any difference. It is very important that the groove is exactly the right depth for the material used to ensure that there are no leaks. I painted a little bit of Loctite 601 into the slot before pressing the shield bar into it. The head modifications are finished off by fitting a 0.6mm thick aluminium shim over the steel strip. I coated the mating face with a very thin skim of JB Weld to ensure that the resulting assembly was fully sealed. I chose 0.6mm as I had removed 0.4mm already and needed to put that back and on my other engines I had 0.5mm of shims in addition to the head gasket. I expect to need another couple of shims possibly up to 0.4mm to get the compression back to an ideal setting.

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This description has been put together by Mike Nelson and is a description of his findings
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