How to attach an Ofna Pull start to a RB C4 motor.

Updated Version 0.

--- Pull start components needed: -----

(All pull start parts are from an Ofna HoBao .21 Hyper engine.)
 
 $30     53025  Pull start Body/Rope/T-handle/screws
 $11     53024  Hex Axle pull start
 $3      53023  Rear Cover Screws (M3x8, 4pcs)
 $15     53022  Rear Cover, Pull starter base and seal
 $26     53021  Turbo Fan and Washer (one-way bearing)

Engine:
 $270     RBC1004-B Worlds Buggy.  aka. C4 Off-Road
      (C4 discontinued replaced by the C5. 5-1-2002)

McMaster Carr Supply Drill Bits:
 $10  8944A77 Solid Carbide Straight-Flute Drill, Wire Gauge Size #38.
 $10  8944A78 Solid Carbide Straight-Flute Drill, Wire Gauge Size #39.

Tools:
     Mill/Drill or Drill Press. (Accuracy is essential for success)

Measurement equipment:  (Accuracy is essential for success.  See Notes)
      Dial or Digital Calipers (Optional)
      Square (Optional)
      Micrometer  (Optional)
      Dial gauge  (Optional)

Measurements found on RB C4:
  The most important measurement is:
     Center of the crankshaft to the center of hole to be put in the connecting rod journal.
     The center of the crankshaft to the exact place to where the hole should be drilled is .327.
     Hole depth to be drilled is .310 - .330  (About 8mm would do it)
     You will notice that the C4 journal has an indent where the hole is to be drilled. 
     This is most fortunate, for this is a pre-centered hole from RB. 
     I say this because I measured from center of crank to center of journal .327 (Score!)
 
Measurements found on the Hyper Pull start components:
     The pin on the turbo-fan is .098 or 2.5mm diameter.
     This hole, in which this pin is to fit needs to be .0995 or .1015. (see notes).
     The hole in the Ofna crankshaft fit snug the .1015 bit,  loose the .0995.
     And the rotational center on the pull start pin (the one I used) was .327 
     Hole depth as measured on the Ofna Crankshaft is .330 
     The pin on the Turbo fan measures .310 length from turbo-fan.  

------ Make the Hole  -----

     Read all the way through this before you do anything and please understand the concept.

     Remove the Crankshaft from the RB motor per RB Concepts instructions.

    Place the crankshaft in to a secure mill vise or jig and ENSURE the hole to be drilled is 90 
degrees "square" with the crank, in-line with the pin which will fill it. Take your straight flute
wire gauge number 39 and drill a 8.38mm deep hole right down the center of the securely mounted
and squared RB Crankshaft journal.  
 
    That's it, you should be done.
The next step is to test fit all the parts and check crankshaft rotation for free movement.
If it binds you can re-jig the crankshaft and drill it open another .002 by using your number 38 
wire gauge bit. Test fit the pull start parts and check the free movement again. This time you
should be able to rotate without any binding. If not, something went extremely wrong, This is not
good, check all your measurements, and check your square settings. Find your mistake..
 

-------- Notes ---------

          The idea is to have the closest fitting hole possible to prevent the pin from "slapping"
around inside the hole, but a pin that binds is far worse then a pin that is a couple thousandths loose.

 Also, If possible use my measurements as a mere reference. Ofna and RB might not adhere to strict
tolerance disciplines in their production lines thus making my measurement incorrect for your parts. 

     Be careful not to scratch the RB crankshaft when mounting it for drilling, remember when using
a solid carbide drill bit that the work piece (the crankshaft) must be held RIGID or the brittle carbide
bit could break. The use of solid carbide was necessary because the crankshaft is extremely hard. Also 
be sure that the crankshaft is 90 degrees square to your drill bit, any offset in the hole will cause
the pin to bind and you will have just ruined an expensive RB crankshaft. 

     Also I found a .030 play in the depth of the RB (being longer) then that of the Ofna, nothing
was done about this play. (An idea was to add a shim between back plate and turbo fan)

     Also there were worries about the turbo/fan hitting the piston skirt on BDC (bottom dead center)
I did not remove any media from the turbo/fan I felt it unnecessary, my belief was that if contact wasn't
being made at 1 RPM, it shouldn't be hitting at 35,000. Let's hope I am correct.

     I do not recommend anybody blindly following these instructions, and you can almost guarantee that
the life of the RB motor will be greatly reduced if not instantly destroyed by the above actions.
Yet, Here they are.

------- Done --------

 Pictures and updates to follow: (see top of page)
 Your Experience is greatly valued, please keep me informed  Kosak42@hotmail.com
 If you can improve on this, or your measurements varied PLEASE SHARE!  
 I will eagerly update this with your technical data.. We all will benefit!

------ Follow Ups ------

 The RB engine is extremely tight, even without compression (plug removed), simply cycling the engine
with the pullstart is extremely difficult at first.  
[This is most likely the reason Rb didn't put a Pullstarter on it from the factory]

========== FEED BACK, Success Stories [Send in yours] =========

MiTMaxx provides his measurements.  Thanks 7-16-02
 .303  = pin length on hyper fan
 .0985 = diameter of fan pin
 .2390 = width of drill blank (mo239) snugged in one-way
 .4990 = fan pin to drill blank outside measurement
 .1687 = 1/2 diameter of fan pin + drill blank
 .4990 - .1687 = .3303 rotational center
 .3303 Rotational Center  ** [Most important!]

This C4 had run 25 tanks at the time of this post and is running strong.


        SubIngenious  
                    *Well, the "Big secret" is now public. Have fun.
                                                                                js April 2002