Modifying an OB

[_laxmasta2039]

Can anyone explain to me what the diff is between a stock and modded 3.5cc OB? or what exactly a head shim is and does?

the only type of modding i know of outside of elecs is adding a tuned pipe, bigger bored carb, and maybe some new bearings....

from what i've read a head shim adjusts the hight of the engine head, but what the heck this does to performance is beyond me....SOS??

Joe

 

[billr]

A stock engine is an engine that comes out of the box with powerhead and foot from the factory(no external mods like carb bore and a stock muffler). On a stock engine with the IMPBA you can do internal mods but none to the muffler or carb. Usually a modded engine can be changing timing, porting, tuned pipes, different foot, really modifying anything on the engine. Removing the head shim decreases the head clearence, which will increase performance to a point. It also gives more compression. It can be taken to far though. Ask around for your particular engine. Hope this helps.

Billy R.

 

[_laxmasta2039]

thanks Billy it does, once the ice melts and i've got the engine running some i'll ask around about messing with the shims on my engine.

thanks for the help,

Joe

 

[billr]

What engine are you running? I have a CMB greenhead and the clearence is .0065.

 

[_laxmasta2039]

an ancient-but-unused KickButt....it's the one that comes standard with a godawful black plastic carb and a bottom half that appears to be two peices put together around the flexshaft. [i noticed the newer ones dont have that either] do you know of any starting points for head height on this engine?

Joe

 

[billr]

Ok... the proverbial can of worms....... measuring the head clearance varies from person to person. I have been told by Jim Irwin, Hopper and some of the other big guys to measure it as follows: Take a piece of thin solder and bend it in an L shape, insert in to the plug hole and make sure that the solder touches the cylinder wall, with the other hand turn the engine over till it has squished the solder, carefully take out the solder and measure it with a good set of calipers of a micrometer. That is your head clearance. Some call that measuring the squish band and say that is not head clearance, but that is the only way I have been told to do it if you have a non-flat piston or domed head. Now K&B I belive has a flat piston, but anyway, some people also sell feller guages to insert for the same thing. The solder method has always worked for me. As far as K&B questions... I really do not know, never owned one... now OS and CMB, that's a different story!!!

Billy R.

 

[_laxmasta2039]

okay thanks Billy i'll do that, though i've got to either sand down my collet wrench or get a new plug wrench b/c the cooling head's opening is too narrow for the one i've got....

hey, ya cant run 'em all!! [well, maybe you could but still!]

Joe

 

[billr]

My cmb is that way. I use a regular socket that you put on the end of a driver. I am not sure of the size but it works.

Billy R.

 

[_laxmasta2039]

Billy-the one that's too small IS a regular socket!!

i know, very wierd that they would make it that tight....the socket i have almost fits, and i've sanded it with 600 grit paper for about 20 minutes with visual progress so i'll just keep doing that i think....good exercise for the hands!

Joe

 

[Kris Flynn]

well i just did the solder method and measured it with my micrometer and it says my head clearance is .5mm or .019685" and it is a brand new motor (K&B, 12mm crank) that i havnt done any mods to yet...just finished breaking it in really.

Anyway does this sound right for 10 -20% nitro content on a stock motor? is is something wrong and i have to change it?

BTW Joe i think most K&B motors are like that, i hade to grind down the thinkness of my socket wrench for it to fit...

Kris

 

[topfuel443]

Joe, get a Craftsman 1/4 drive deep well 8mm socket it will fit.

~ James

 

[TimD]

Kris,

Does your motor have a head shim in it between the head and the sleeve? If so measure the thickness of it. I would aim for a max. of .012" on a .21 motor - especially with a stock head button and low nitro. Where you have it now is very conservative and won't cause any problems but you can get more performance for little to no expense by reducing the head clearance.

Tim.

 

[_laxmasta2039]

James,

is 8mm the dia. of the entire socket head or the length across from one side of the hexagon to the other?

[i would measure but i dont have a metric ruler handy]

the one i've got says 8 mm on it and it just barely doesnt fit, i've been sanding it down some and the very end goes in now but not enough.....

Joe

 

[TimD]

8mm is the glow plug size.

 

[_laxmasta2039]

i guess that would mean it's the length between opposite edges of the hexagon then, because this is just a standard socket head and it's labeled 8mm. i'll try to shop around at Lowe's and find an 8mm socket that has a dia. that is less than that of the head button's glow-plug hole bore. thanks all.

Joe

 

[billr]

You tend to run a higher nitro with a lower head clearence. But with a stock engine, you could run 20-40%, easy. I am running 70% in my stock os, but I race it. It also has a head clearance of .007. It really likes the nitro. One of the guys here has his os running about 43 mph on a hopper 260(HTB) tunnel. He also has a low head clearance, a few other mods, and runs 65 - 70%. Again he is racing all the time though. I would say, take the shim out and then measure it. Use would 30 - 40% if it were me playing around at the lake, but do a thorough after run cleaning at the end of the day.

Billy R.

 

[Kris Flynn]

There are no Head shims between the head and the sleeve( i dont think), there is just a gap where i can see the sleeve through...and it is just under 1/32" wide (see pic)

Billy, my main reason for not wanting to run 70% nitro is because of the costs involved with the nitro content and the fact that they probably need repairing more often?

Kris

 

[waterbug]

Kris,

Try a S.A.E. 5/16 ths. deep wall socket . Thats what I've used for years on small airplane engines, as a matter of fact that's all I've used for 25 years.

 

[billr]

High nitro does cause more corrosion if left in the engine. I have been using automatic transmission fluid as after run oil. I flush everything out of the engine, take the plug out, disconnect fuel lines, fill if with ATF, and turn the hell out of it. Then do it al over again. It lubes the engine up really good, gets the water out, and ATF has detergent in it that will aid in cleaning. I did that and looked at one of my engines recently, and it looks like it just came off the factory line. Also add some to your flex shaft lube, never broke a shaft with it!

I can agree on the cost!!!! It is really high here to. When you compare it to airplane fuel, well 20% wins hands down.

Billy R.

 

[nitrotoys]

HI all, I Just checked my head shims, and with head off, I DO NOT HAVE A SHIM, I bought this engine used, it fired up on 25% for a test for a few seconds. It has great compression. WHAT SHIM Should i use? Am going to run 40% nitro.

Next question, When you port it, do you do the sleeve only? Or the case too? Please any info would be great. I know about knifing the rod, am used to nitro trucks,

thanks all

Tom