Piston Skirt Relief
10-01-2003, 05:19 PM
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Piston Skirt Relief
I'm not trying to stir up anything, but I have a feeling that there may be a few differing opinions on this topic. 
I haven't been running "real 424" engines until recently. I've mostly been running JETT's (428 and Super Sport .40), but I've also been running .46's on the front of Quickies. The .46's I've been using require a piston skirt relief in the backplate. If you order an OS .46 FX backplate from Dub and specify that it has the relief, everything works out fine.
But I've heard of some groups that (at least in the past) allow the piston to be modified so that the non-relieved backplate mounts can be used.
On the Thunder Tiger Pro .40, the backplate has a piston skirt relief. Even though it looks like the piston would clear a non-relieved backplate, the backplate provided with the engine has the relief.
So ... the question is, does a replacement backplate have to have the relief? Using a non-relieved backplate would decrease the crankcase volume, unless the replacement backplate has a corresponding amount of material removed, regardless of the location.
Also, the only spring I see in the drawings is the carb barrel spring. Is this the one that gets ingested occiasionally?
I haven't been running "real 424" engines until recently. I've mostly been running JETT's (428 and Super Sport .40), but I've also been running .46's on the front of Quickies. The .46's I've been using require a piston skirt relief in the backplate. If you order an OS .46 FX backplate from Dub and specify that it has the relief, everything works out fine.
But I've heard of some groups that (at least in the past) allow the piston to be modified so that the non-relieved backplate mounts can be used.
On the Thunder Tiger Pro .40, the backplate has a piston skirt relief. Even though it looks like the piston would clear a non-relieved backplate, the backplate provided with the engine has the relief.
So ... the question is, does a replacement backplate have to have the relief? Using a non-relieved backplate would decrease the crankcase volume, unless the replacement backplate has a corresponding amount of material removed, regardless of the location.
Also, the only spring I see in the drawings is the carb barrel spring. Is this the one that gets ingested occiasionally?
10-02-2003, 06:58 AM
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RE: Piston Skirt Relief
In the field there is no practical way of measuring the crank case volume. So the only way to be sure is to compare the original backplate with the replacement. If the replacement was not identical to the original I would claim it was illegal.
I realise that we are splitting hairs here and this discussion is somewhat academic. I have never seen a backplate challenged.
Is it necessary to modify the piston? Most side exhaust engines I have seen have the piston skirt cut-away anyway to allow for better flow to the front and rear inlet port.
Ed S
I realise that we are splitting hairs here and this discussion is somewhat academic. I have never seen a backplate challenged.
Is it necessary to modify the piston? Most side exhaust engines I have seen have the piston skirt cut-away anyway to allow for better flow to the front and rear inlet port.
Ed S
10-02-2003, 09:56 AM
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RE: Piston Skirt Relief
Ed,
The OS 46 FX does have the piston skirt cut out out for better fuel flow.
The Tower Hobbies .46 BB ABC does not have cutouts for the front/rear ports and does not look like it would hit, but I get the backplate mounts with the relief anyway. This engine is consistantly much stronger than the OS, at least for me.
The engine I heard you had to modify the piston on was the Thunder Tiger Pro .46. Since I haven't had one, I don't know for sure. And it's not a 424 engine anyway.
The main reason for asking the question is that since I'm going to race in some actual 424 races, I want to be legal.
I just got one of Lewis Schwabs backplate mount where you use the original engine backplate in conjuction with the mount. This is probably what I'll run on the TT Pro .40 just so there's no question.
The OS 46 FX does have the piston skirt cut out out for better fuel flow.
The Tower Hobbies .46 BB ABC does not have cutouts for the front/rear ports and does not look like it would hit, but I get the backplate mounts with the relief anyway. This engine is consistantly much stronger than the OS, at least for me.
The engine I heard you had to modify the piston on was the Thunder Tiger Pro .46. Since I haven't had one, I don't know for sure. And it's not a 424 engine anyway.
The main reason for asking the question is that since I'm going to race in some actual 424 races, I want to be legal.
I just got one of Lewis Schwabs backplate mount where you use the original engine backplate in conjuction with the mount. This is probably what I'll run on the TT Pro .40 just so there's no question.
