Underhead (Head Button) for S-25

[BlueRidge1]

I stripped out my underhead last weekend. Went to the LHS. They had an underhead that was not in a package - but matched it up and said - "It's the one."

Well, the engine won't start. No wonder - I can't even get it to flood if I try. Went back to the LHS. Guy still says it's the right underhead and sold me some cylinder gaskets to help the underhead mate to the head. Well, it will start now, but runs like crap - takes a lot of nitro too.

I think I may have the s-21 underhead. There's only 33.8 vs 33mm diameter difference in the two. Mine seems closer to 34mm. Anybody ever have this happen?

 

[pissant]

wish i can help you, but i do not think .8 mm is going to make your engine run like crap! What I would do is reset all your needles to factory and restart again. hope that helps, sry could not be of further help

 

[drummerjamesh]

i dunno these are some pretty percise engines .8mm could kill ya i think...does it need to like rebreak in or be retuned or something..outside of that i dunno

 

[Digital Liquid]

It sounds like your shimming is way off, and have totaly changed your combustion area. I wouldn't bother using that underhead. You should try to get your money back & get the right part. You will end up wasting more time trying to get the shims to work & can ruin your engine in the process.

Here is a link to the underhead for the .25:
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXFRD9&P=7

Later....

 

[SokuShi]

you could always try going from a medium hot plug to a short hot plug, that should clear up some room, take the shim out of the plug too to clear up more room if needed, i bet the piston is hitting hte glow plug.

 

[BlueRidge1]

I got the right underhead today after I proved to myself that I had the wrong one - man, Tower's sight has such useful info. The main difference is in the center of the button where it mates to the head. The S-21 button had a lot of slop in it where the s-25 was a snug fit (of course). It's not obvious unless you really stare hard at the two.

I used 3 shims between the underhead and the cylinder as recommended in the s-25 manual. Everything appears to be running ok. I did leak some nitro and compression until I got the whole thing tightened down real well.