Longboy's "OVERTIME" I/C Twin!

[Longboy]

About 3 yrs. since I built my first I/C twin engine. My "GEMINI" is a good start but I want another crack at an inline twin to address a couple of shortcomings in its build. Can I make improvements in service, durability and add a new feature using basic machine shop know how and NO PLANS in the same scale? scratch.gif .....The story of "OVERTIME" starts this weekend!

 

[Longboy]

With this engine being an open crank style, a 4 piece frame is the backbone of Overtime. Two end plates, a deck plate and base plate. The deck is overlapped to the endplates and the endplates are overlapped to the base. Eight, 6-32 socketheads holds it together. For servicing you can remove the deck with all top side components as a unit and either end plate for access and removal of the crankshaft. A couple of brass bearing holders for each end plate will be the crankshaft support. I'll be using Oilite bushings here, 3/4 inch long ( 1/2 in. shown). More bearing area than a set of roller bearings that also will work fine, just builders choice here!

 

[Longboy]

A brass flywheel is secured to the crankshaft with a taper collet made from Delrin. A 3 screw retainer makes the press fit or actually draws the flywheel into the collet. This 1st collet was too short, didn't get the grip and a longer one was made without the split and tried out...the right length worked out better! The Delrin seems to compress enough without having the split down its side.

 

[Longboy]

A fabricated 5 piece crank starts with 12L14 steel. A generous length of each web 5/8in. long for heft and support of a 5/16in main shaft and 1/4in throw pin of stainless round stock. Drilled & reamed as one piece then parted to length.

 

[Longboy]

The webs are internally balanced to the piston/rod assembly. Two half inch through holes at 5/8 inch yields a hair under 1 oz. of removed material.

 

[Longboy]

The shaft is joined to the web by press fit. A straight knurl gets the bite into the web. Some cold bonding agent, either thread locker or Epoxy, supplements the primary grip of the knurl when driven in. After pressing, the assembly is chucked in the lathe. The shaft is paralleled to the web with some light taps and the assembly is then set aside and repeated for the other side.

 

[Longboy]

The weak link in a 360 Deg. crankshaft for two bearing inline twin is the throw pin. Its long and narrow and joins the heaviest parts of the crank. I used 1/4in. stainless round here, upsized from my GEMINI engine.

The pin is locked to the web with a 10-32 set screw instead of being pined or silver soldered to the web. The pin has to be removable as I am using roller bearings on the rods.

 

[Longboy]

In the big end here I'm using FR168ZZ flanged bearings. The rods are beefier than need to be for scale to accommodate the 1/4in. stainless throw pin and hold the bearing. The flange makes the bearing support slightly broader around the pin and also shows more rigidness in the 90deg. relation of rod to crank. You want some lateral float for the rod on the pin between the webs.

In the lathe the throw pin is reduced in diameter a couple thousands by file between the lines here where the rod runs on the pin. Area is cleaned up with 400grit carbide paper and now the rod will "walk" between these lines.

 

[Longboy]

The con rod starts out as a piece of 3/8in x 3/4in aluminum bar for the big end and 3/8in AL. hex stock for the piston end. The beam is hollowed by a tap drill for 6-32 threads and a stud joins the hex to beam. With two pieces the final beam length of the assembly will be determined later. R144ZZ bearings in the hex join the piston with a 1/8in pin and locked in position with 4-40 set screws. In this mock up I have a "too" long rod that would make a Diesel cry out. The beam is unscrewed from the hex end and shortened to where the piston rises to the top of the cylinder at this time.

Components are then scaled in to aid in the counter weighting.

 

[Longboy]

Doing the static balancing here, you can see a 3rd crank web member between the con rods. Saw this on an Italian modelers twin engine. Don't know why its there but I had to have it and esthetically keeps the continuity of the crank webs the whole length of the crankshaft now. Internally lightened about 50% and is locked to the throw pin with 2 set screws. For balance note the neo magnets on top of the outboard webs. The weight of a couple of nuts completes the balance. I weigh the nuts (do not weigh neo magnets) and take the weight reduction externally at the nose damper. Looks like I need a near half OZ.

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And ready to roll.

 

[gus]

Hi LongBoy,
Gus will be following this thread. Your engine is on my KIV build list.Am about to jump in to build the Howell V-2 Engine.

 

[Longboy]

Thanks Gus. Even though I build without plans there will be many photos of Overtime presented so others who want to tackle their own version should be able to get a good start.Thm:

 

[Longboy]

I make camshafts by pieces. Starting out with 5/8in CR round in the lathe with one jaw shimmed (or a jaw offset in chuck) and drill for a 3/16in. shaft. Your amount of offset will determine the baseline lobe lift. I'll be using a prototype lobe with the engine and will need to make 3 new lobes. Dye the end of shaft and I put my lobe on a short length of shaft, insert into new stock and trace the profile to new stock.

With the stock overhanging in the machinists vise, in 5 steps ( each side of lobe nose) mill to the scribe line your rough profile.

This what it looks like. The shaft is cut back between the lobes for part lines and to drill & tap for a 6-32 set screw to lock the lobes to its shaft.

 

[Longboy]

In the bench vice file over the milled facets to blend into a traditional shape finished lobe. You can get them perpendicular flat to their shaft by filing without being machine cut perfect and they will work fine.

Without a degree wheel the only thing I can check is the lobe lift running the profile on the lathe. Shows .180in lift here but for its duration, do I have a race profile.....or a RV profile?

Looking for a smooth idler I will have to wait till assembly of Overtime and see the relations of valve open duration to the crank degree position.

 

[Longboy]

I had two pairs of timing gears available for Overtime. Using the larger dia. set of the two, I set the meshing using a transfer punch into the bore of the cam drive and finger spin. Then make the mark and drill both end plates for the camshaft bearings......it was shortly later I saw reasons that my cam to crank centerline would be too close together. My "added feature"( whats Longboy up to !) would interfere with the nose damper. To set the cam farther from the crank I ordered some Delrin idlers and then used my smaller diameter pair of gears.

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The deck plate then had to be shifted over toward the cam side so the lifters would be over the camshaft . Since I didn't have the deck bored for the cylinders yet, it was a minor operation.

 

[Longboy]

On top of the cyl. heads go a pair of rocker pedestals. 3/8 x 3/4in brass bar is free style cut to something that looks like it will work with a roughing mill and a clean finished mounting tab that screws into the side of the heads. Its pattern is traced onto the bar stock and the duplicate is cut and drilled.

 

[Longboy]

This is called "rocker arms on a stick'. Not to be confused with a county fair treat. CR steel here a pair at a time, their counterparts are a mirror image by a staggered cut. Brass bushings for the pivot and stainless 4-40 socket caps for the pushrod with homegrown lock nuts.

A 3/8in brass rod is threaded down about 3/4in and a straight knurl is run across the threads for a superior finger grip in setting the pushrod length. Drilled and threaded for the stainless screws.

Finally the rockers are flame colored to a plum/ violet hue for contrast. I like the effect and here a matched pair are done!

 

[Longboy]

The radiator is bored for a DOM tube that takes 7/8in. dia. pistons. Separate block style heads will sit on top.

To reduce the bore centerline distance, the radiators are milled & fly cut.... and butted together on the deck.

The shinny arc inside of the radiator is a slight overbore to a 1/4in. depth from the bottom. A film of Epoxy here bonds the cylinder. to the radiator as a unit.

 

[Longboy]

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 shorter 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.

 

[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 side of each head is fly cut to match the radiators when sitting on top of the cylinders. The heads are now ready for drilling ports, spark plug, valve guides, rocker pedestals and head bolts.