Longboy's "OVERTIME" I/C Twin!

[90LX_Notch]

Longboy-

This is an interesting build.

-Bob

[Longboy]

Thanks Bob and others who follow!...... The deck is bored oversize to the O.D of the cylinders, squared up to the crank centerline and a trio of 4-40 screws tie them together. Though it ended up not necessary, the deck overbore when the crank is close to the underside and a longer stroke displacement or just short rods can aid in clearance with rod angles. I had enough clearance with a 1 inch stroke without shaving the deck underside or narrowing the rod beam.





Clamped together, scribing a perimeter line around radiators to help locate the screws in deck.

[Longboy]

With the cylinders now located on the frame work begins on the cyl. heads. A 3/16in. deep combustion chamber bored to the O.D. of the cylinder here.



The cylinder rises above the radiator 1/8in and a 1/16in O-ring is laid over. No need to make gaskets, the head sits on the O-ring to make the seal.



The sides of each head is fly cut to match the radiator butt when sitting on top of the cylinders. Heads now ready for drilling of ports, sparkplug, valves guides, rocker pedestals and head bolts.

[Graham G]

  Hi there Longboy, now I'm really getting settled in, I do like your methods and I have a question, The cam gear assembly with the plastic idler gear, should it not be the same size as one or the other gears as you would loose the 2-1 ratio or is it maybe an optical delusion on my part ha ha.
As you may be able to tell, I am not that great with working out gears/ratios
Cheers
Graham
 

[90LX_Notch]

Graham-

 An idler has no effect on the final ratio.

-Bob

[Stuart]

Graham

To get your head round what Bob has said think about teeth not size one tooth one the driven will move one tooth in the gear train

The ratio is the number of teeth on the driven and driver all the idler does is to transmit one tooth and change direction

Stuart

[Longboy]

Valve guides from 3/8in brass rod test fitted here.



The horizontal V-groove in the machinist vise provides an arbitrary head angle for the spark plugs. Shown is a flat being cut in the chamber with a 3/16 end mill for a 90 deg. approach by the center drill, then tap drill for CM6 sparkies.



The head is milled & flycut across parallel to the plug angle to a point to expose the plug hex to get a socket around it.
From the top side of heads the CM6 hex will not clear the opening to engage the head threading. Since they don't make a reduced shank 13/16in end mill to fit my machine, the China special boring bar will broaden the space. 



The heads are cross milled & fly cut parallel to plug angle to a point exposing the hex to get a socket around them. 

[Graham G]

Thanx Bob and Stuart, it's dammed easy when explained simply
Cheers
Graham

[Longboy]

The valves are made form 1/4-20 long stem hardware bolts.   ....yep, they cut and work fine!

[90LX_Notch]

LB-

I assume the bolts are stainless.

-Bob

[Longboy]

They are Zinc plated grade fives Bob.

[90LX_Notch]

I know them well.  I use 5/16 to make shear pins for my snowblower. 

Why not use stainless for the valves?

[Longboy]

You know, I tried these out for valves previously. Thought they would chunk & gall up under the tool bit but getting close to the final diameter and taking light cuts under oil they clean up fine, even the valve face which they must. So I continue you use them!

[Longboy]

Five head bolts are used due to the porting, one intersecting the butt of the heads. Between the plugs, the exit ports for the intakes.



Outboard valves are the exhaust, inboard are intakes. A two step port drilling is needed to get between the plug boss, valve guide and head bolt. 1st step starting at the butt into the valve guide seat and then the 2nd.step drilling an exit straight outside along the butt seam. From this 1st. access point a transfer punch is used to mark the valve guide when placed. Removed and drilled separately, then epoxied into the head. Access point is then sealed off with short 10-32 set screws.





A diagonal is milled at the head corners and then arc filed for styling.

[Roger B]

Looking good    I have used stainless steel bolts/screws to make my valves so far.