Modifying an OB

[TimD]

Nitrotoys,

Firstly, you need to measure the squish clearance. In other words, measure the distance between the head and the piston at TDC.

If you aim for about 10 thou clearance you should be OK. Use the appropriate thickness shim if you are under 10 thou. If you are over 12 thou then machining is required to the head or top of the sleeve to reduce the gap. A shim is not essential - you don't have to have one in there if the clearance is correct.

If your motor is a K&B with a stock rod, DO NOT KNIFE EDGE IT! it will fail. Porting is only needed on the liner.

Tim.

 

[Kris Flynn]

Billy, with your after run cleaing up...do you run some no-nitro fuel through it to clean it out, then spray WD-40 all over/in it then fill it with ATF and turn it over then more ATF? or just the ATF bit?

Tim, so because my squish clearance is .0197" (which is a bit over 19 thou) i will have to machine down the head button, or what? There are no head shims between the head button and the sleeve...I am only running 10% nitro at the moment but when i begin to mix my own fuel i will move up to 20-25% (should be a week or two before i move up) what are you recommendations on the clearance?

Also someone suggested to me that i should remove the wall in between the two holes in the sleeve, which effectivley creates one big hole, is this a good thing?

Kris

 

[TimD]

Nitro % should not effect where you run your squish clearance!!!!!

A tight head clearance gives more power, especially on 21's!!!!!

Too tight a head clearance makes piston hit head!!!!!! :'(

Overcompressed due to fuel bowl being too small causes detonation!!!! :'(

Using a head shim to prevent detonation is sacrificing power!!!! :

A head button with the correct volume bowl and a tight squish clearance is the way to go.

Kris,

I'm sure if you ask Ian (Nitrocrazed) nicely he can get the right head made for you at a good price. He only runs 25% nitro in his motor's and they haul!!!

Tim.

 

[TimD]

Also someone suggested to me that i should remove the wall in between the two holes in the sleeve, which effectivley creates one big hole, is this a good thing?

Kris

Do you mean in the exhaust port?

 

[Ian Inverarity]

Tim is right about the importance of low head clearances. The squish band can only work at small clearances, and the squish band increases the amount of compression you can run without detonation. I have had an engine which was detonating, where I took OUT shims to get the squishband down and the detonation stopped! This is very important, and often poorly understood.

Ian

 

[GTR]

Hi Kris,

You can cut the head clearance down using the wet & dry paper, oil and a piece of glass trick. JD described this recently in a post. Otherwise have someone skim a fraction of the head and add shims if required(???). I checked 5 other heads the other night and they all have the same depth into the cylinder - must be the way K&B made them (probably soft so people can't hurt them)

DO NOT remove the bridge in the exhaust port!!!!!! > It is there to stop the piston rocking into the exhaust on the up stroke and doing major damage.

Don't spend all your time chasing more power but trimming the hull to get the most from the power you've got. When you get that sorted push for more go. Reliability beats a rocket everytime - now if you can have both yea-ha. Hang on and enjoy the ride. ;D

GT 8)

 

[billr]

What I do for after run is to disconnect the fuel tubes and exhaust tubes. Turn it over with the starter and glow heater on. Run all the nitro out of the engine if any is left. Then I use the transmission fluid as described before.

Billy R.

 

[Kris Flynn]

Tim, you i mean on the exhaust port...but GT has now explained that it isnt really a good thing to do

I was just asking questions to get idea to do for later, i wasnt planning on doing any major mods yet, but rather do what you said and trip the bat with the power i have. I will have to look for that post about the wet &dry, il and glass trick, i must have missed it???

Bill, thanks, i wasnt sure what the first part of your clean up was...i will do it like you explained from now on...

Ian, thanks for clearing that up in my head...it is all a new world at the moment

 

[_laxmasta2039]

hey guys,

can someone explain to me what "knifing the rod" means? i assume it has something to do with the piston rod, but why knife it? from reading an old RCBM about port timing "porting the sleeve" sounds like something that's best left to the engine manufacturers...is there some way to adjust the ports that gaurantees better engine performance without possibly destroying the port timing? as far as after run, i just disconnect the fuel/exhaust tubes, take out the glow plug, and squirt some Trinity after-burn oil in the carb inlet and glow plug hole.

Joe

 

[Atious]

Knife edging the rod means basically cutting the leading edges down to a sharper edge to reduce windage. Knifing doesnt really cut the edge down to a props sharp edge but just reducing the drag by reducing the leading edges overall width to aproximately half its original width. In super high RPM engines this is a good benefit. On something like a Homie or Zenoah, its wasted effort.

 

[_laxmasta2039]

thanks Atious, sounds like something i should leave to the pros...

the KB website says that the head clearance for it's .21 engine [all .21s] is .018.... what thickness shim should i get?

[im assuming the old KBs have the same head clearance as the newer ones....does anyone know differently?]

Joe

 

[Jeff Torgalski]

There is one thing I would want to mention about using transmission fluid for anything.

It eats paint like nothing else! At least, that's my experience with automotive finishes.

Be careful!

Jeff T.

 

[duc]

Are you sure? You don't mean break fluid?

 

[billr]

It really has not eaten mine yet. Of course I wipe down the boat at the end of the day. But it works great as far as shaft lube, and after run treatment.

Billy Richardson

 

[Kris Flynn]

OK, Say someone had trimmed their boat to get the most out of what power they have from their stock motor...what can they then do (other than adding a pipe)?

I am thinking more along the lines of piston and sleeve mods...and what you can do with the water cooling to get the motor temperature (more specifically a W/C K&B 12mm crank) correct for optimum performance?

Maybe Grim can chime in with some suggestions, please? 8)

Kris

 

[Jeff Torgalski]

Hmmm, maybe I was thinking about brake fluid.

 

[Jeff Torgalski]

Have you considered prop mods?

Did you see the article "K&B 3.5: Great article for newbies"? That could give you ideas.

 

[Grimracer]

Kris

This is why i love OB racing..Tinker, tinker and tinker some more.

Your 12mm motor can be made to really rock and roll. It has a great head button and most likely have a larger piston and liner (closer to a .21cc). There is no substitute for cubic inches.

Leave the piston and liner stock. I would not cut up the liner. Not worth it.

Also pipes are great but can make your motor picky to set.

If you could, it would be fun to send your motor (power head and lower unit) to me to go over. I know that shipping is tons of money and you would be out a motor for some time but there is a good chance you would be happy with the motor when im done with it.

Grim

 

[Kris Flynn]

sent you mail Grim...could be a good thing to think about

Jeff, yeah good info will have to see if i can undertand it firs

Kris

 

[topfuel443]

Hey Grim, Ive seen your stock outboard run, and I just got finished with my stock K&B engine/tunnel, its pretty slow comparitively. Im not used to the way it sounds with a stock lower, so I was thinking it was running rich, but when I tried to lean on it, it would die out. It would sound like it was about to come "on the pipe" and then die. I had some modified sleeves that I tried and a couple of different carbs and props, I got a little more speed out of it, but it still has alot to be desired. whats the secret 8) I have the 13mm crank with new everything...bearings, P/L, rod, crank the works. The tunnel is a villian/JD scratch built knockoff and seems to handle great, with a perfect C/G balance. I have some decently fast mod tunnels but this "stock" thing has got me searching.

~ T/F