Building the Little Dragon, Page 7

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Cylinder Head

Roy Clough omits the dimension of the part of the head that fits inside the cylinder, simply saying to machine it so there is a 1/32" clearance to the piston at TDC. By the time we allow for the deflector volume and the recess of the glowplug, this will deliver a geometric compression ratio of a little over 10:1 which is about what is expected for a small glow motor designed to run with little to no nitro in the fuel.

This is an easy job which takes a while because we have to change chucks and center some square barstock again. Life would be simpler if we could have made this part when we were making the front bearing as the number of set-ups would be reduced. But we need to have almost finished the engine to make the TDC measurement, so this is not possible. Although I did not do it, I now have an idea of how the head could be machined at the same time as the front bearing. I'll describe it after describing the "proper way".

Head 1. Head 2. Head 3.
Head 4.

  1. Start by assembling the engine and rotating the crank to place the piston at Top Dead Center. Now accurately measure the distance from the top of the liner to the top of the piston using the depthing probe of your Vernier calipers. The piston should be very tight at TDC, so make doubly sure that it is not lifting the liner off its seat in the crankcase and giving you a false reading. Subtract 0.031" from the reading obtained to get the length of the circular section that protrudes into the liner. You can allow for the brown paper gasket thickness if you wish, but it's not that critical.

    With some 3/4" square stock centered, turn the stock to be a close sliding fit into the top of the cylinder. Aim for a fine finish on the top of the flange as this is where we get the compression seal, not around the circular section. But you can see that is vitally important that the flange be perfectly at right-angles to the circular section is the head is to seal. Our set-up assures this.

    Part off the head a little over size and we are done with this setup. Note that we did not drill and tap for the plug at this time for a very good reason.

  2. We now reverse the head and face it to length, then drill and tap for the 1/4-32 plug thread. This ensures that the plug seat and the tapped plug hole are at right angles to each other. It's a good idea follow the tapping drill size with the thread clearance size (1/4" in this case) before tapping. The depth for counter-drilling is to the point the full diameter is just reached. Tapping always raises a burr at the entry point. Counter-drilling ensures that the burr does not raise the surface, thus providing a good, flat seat for the glow plug gasket washer.

    If the hole is not precisely in the axis, this does not matter greatly. The other advantage of this sequence is that having parted off the head, we can just dtill and that through. This is much easier than drilling and tapping a blind hole.

    Obviously I'm using a collet to grip the head for this operation. The collet gives excellent clamping with no localized pressure such as would be imparted by a 3 jaw chuck. If you use a chuck, be sure to use a shim of aluminium drink can stock wrapped around the head to protect the finish.

  3. This shot shows the Myford headstock spindle hand crank that I frequently use when tapping larger threads like that for the glow plug.
  4. The last operation is to clamp up, then drill and tap the head screw holes. The position of these can be measured and center-popped, or done by coordinate drilling once a reference point has been established.

    Drill #50 through the head and top fin. If you measured right, the hole will just clear the circular OD under the top fin. Next, open out the hole in the head (but not the top fin) with a #44 drill. Follow with a 2-56 bottoming tap. The head hole will act as a guide to align the tap vertical. Be careful as the tap emerges from the hole in the top fin. We want the thread to be fully cut, but the tap tip may hit the second fin and act like a jack-screw, resulting in a bent fin. A light grind to remove the pointed tip of the bottoming (#3) tap may be necessary.


Gratutious Tip:

If you wanted to make the head at the same time as the front bearing housing, you could try this.

Turn the section the fits into the head to a length of 0.15". This should be about 0.025" oversize. Later, when you can assemble the engine and measure how long it should be, screw an old, preferably dead, glowplug into the hole and grip this in the 3 jaw chuck, pushing the top of the head back against the chuck jaw faces to square it up. Now you can face the protrusion back to size. The "roof" will almost certainly not be normal to the axis, but it will be close enough and it does not matter all that much. As long as you are not tempted to bring the tool anywhere near the previously turned under-face of the head, all should be well. Take light facing cuts of no more than 0.005" at a time as this is a rather precarious, delicate set-up. I stress again that I've not tried this on this engine, but have used the trick before.