Screamin Demon .67..
 

With winter closing in, I decided to pull the OPS .65 out of the Demon and strip it to see if everything was in good shape. It was, the crank spun like a top, and it was certainly well run in, in fact there was no perceptible pinch despite the cold-soak in my 55 degree basement shop. Tight piston seal and heaps of pop and bounce at room temp. So perhaps I have a good but not 100% fresh runner. So I thought I'd freshen it up with a new marine 7-port .67 piston/liner set, and tuck the .65 parts away as spares. I've seen comments that the .67 has a noticeable increase in top end over the .65.

All back together, 8.25x10 prop and open header for break in runs on bench. Loud time. Bet it hand-starts.

I may toss in a shim or two and fly on 25/30%, I don't think the engine really cares. May as well burn out the hand launcher's eyes. I recall launching RVman's Picco .45DF delta on I think 30%.. gack cough! The pipe fires it right at you, and with the open "nitro" pipes they are launched all fat and smoky. Mmmmmm.

http://www.rcuniverse.com/forum/atta...mentid=1941708http://www.rcuniverse.com/forum/atta...mentid=1941709

Is the piston side notched to help uncover any of the ports and the bottom end of the stroke...? Some engines are laid out to make this idea look like a good feature.

The piston is not cut, no. The .67 liner has.. hmm, I should learn the terminology for all the ports.. the same 5 ports as the .65 liner, but the three opposite the exhaust are milled on the exterior of the liner for more area whereas the .65 are not, and if the two on the outside of those are boost ports, the boost ports are milled to angle upwards but not on the .65. Then there are two extra ports below all these. Should have taken a picture. But I've seen it written that the .67 has about 10% more top end, I presume that is expressed in hp. We'll see.

That makes sense if it is able to provide a noticeable difference you ought to be able to see how they went about it.

If nothing else it can't hurt acceleration out of a hand launch..

Another thing I noticed in a quick visual, was that the exhaust port is slightly taller, cut in both directions so opens earlier and closes later. I guess I should get my act together and learn a bit more about porting and timing than the meager set of basics I think I sort of know. I'd love to have a go at the .65 P/L and see if I can reproduce some of the changes and gain some top end. It ain't the extra .02 ci that does it all.

Is Harry Higley's book the best reference for two stroke theory? Or.....?

I can not imagine that a Higley book would be much use except as a brief over view about piped engines in general.
I'd rather put the $25 that a Higley book costs towards a fine carbide tooth rotary file, a Bosch die grinder, degree wheel, etc.
If a guy like Bill Wisniewski or Bob Fogg wrote a book about tuned pipe speed engines, then I'd be eager to buy it.

I'd like to have a go at the Conquest mods, and hopping up a Moki 2.10, but I should get my act together on more expendable projects first I think. Maybe converting an OS .61 clone to sporty pipe timing or something like that.

Massaging ports on the .65 liner is probably a good practice project.

You can easily figure where the 180 degree mark is by taking depth readings at TDC, then BDC...then figuring out where the exact center of those 2 events ocurrs on the liner. This mark will become the top of the exhaust port for your "high timed" engine. If you do not wish to take the full plunge, you can just raise the port to look like a pyramid, or an igloo, then try that.
180 degrees was found to be starting point for the miraculous explosive power of resonance to occur in an engine that is propped to take advantage of it. If you have too much prop, you completely miss the boat. Too little prop is much better than just a hair too much prop.
BTW, most sport timed engines that I've ever raised the exhaust timing had the aluminum casting of the case already at the 180 mark. All I had to do was grind the exhaust port in the liner until it matched the roof of the cast aluminum exhaust port. Just make sure you make a nice square edged cut with no burrs that will scuff the piston. You do not want a rounded or chamfered edge, it just needs to be clean. Get the finest tooth carbide rotary file and a real die grinder instead of a Dremel. Do not let the cutting tool contact more than one surface and use duct tape to help protect areas that are not involved with the grinding mods.

I pulled out my Higley engine book last night and it's kind of worthless. He talks a little about hop-ups but his overall message is if you want to know how to do hop-ups you'll need a different book.

Then I won't waste any money on it.

Here's a good start for hopping up weed whackers, chainsaws and go-karts.. http://www.amrca.com/tech/tuners.pdf

Actually looks very interesting, will print it out and read through tonight.

You can always shim the liner to test things. At least you can go back to stock. Or buy a cheap ASP or something at a swap meet to experiment with.

I have enough engines lying around I could host an engine hop-up course, materials included, if I knew what I was doing.

"Machining" liners, crank shafts and crank cases free hand can be done. The ideal tool would be like what the jeweler's have, a stationary high rpm motor with a flex shaft that is about 3 feet long. Probably really expensive. A cheap pneumatic die grinder that can accept 1/4" shaft tool bits would be plenty if you have a good air supply.
You must figure that little mishaps like losing your grip on the piece is bound to happen, so duct tape everything that isn't going to be modified. Avoid clamping parts unless you can make cylindrical holding fixtures to protect them and to avoid distortion. Scribing cut lines on liners can be challenging on the nickle or chrome, so paint the area with a magic marker to high light your scribe marks.
Be light handed with the cuts and maintain the same level of patience and force as if you were doing dental work on your daughter.
opening up the crank shaft bore can be done with a wood dowel and 40 or 50 grit sandpaper. Take a hacksaw and cut a lengthwise slit in the dowel, then insert a scrap of sandpaper into the slit. Now you can hog out the crankshaft. This will test your patience, so just tackle the project in shifts and it will eventually get done. Trying to do it with a long rotary file is tricky and much harder to keep the hole concentric.
A real machinist might detemper the crank then drill it, then retemper it...or else drill the crank in it's natural state with a very expensive drill bit at just the right speed and amount of pressure.

The OS .25 VF - not VF-DF - that I have is a good example - there must be .060-.070" exposed liner below the top of the casting.

We were poking fun at the size of my OPS .15 elsewhere, so I pulled it apart and snapped a couple of pics. It reminds me of one of the favorite rifles I ever had in my collection some years ago - a Martini-Henry .577-.450 sleeved down to .303 and then sleeved to .22 caliber. It was almost comical firing .22 rounds out of the thing with its enormous barrel. The little piston centered in this enormous crankcase brought the image to mind. I'm curious how much bore it could handle - guess it depends on the minimum practical liner wall. There's probably enough material there to add 33% more port area by milling the liner walls to match the case ports.

I thought the rod was pretty beefy, but it's not much different than the CV-A or Conquest (billet) rods.

http://www.rcuniverse.com/forum/atta...mentid=1942334http://www.rcuniverse.com/forum/atta...mentid=1942335http://www.rcuniverse.com/forum/atta...mentid=1942336http://www.rcuniverse.com/forum/atta...mentid=1942337