My new little GEM.
This one cries out for a Venturi!

can you bench mount that little jewel and post us a video of it screaming? nice pickup there.

That's a great start to a speed project!
What I like about the smaller engines is the planes that they go in are twice as easy to build and twice as easy to build lightly with common materials.

Does it have a spot cast into the backplate where you can easily add a pressure tap? Add a 6oz Jett or Tettra tank and away you go.

I'd be all fired up to build something if I had that engine. See how well it turns a 6x6 prop.

So that's what it is, I wondered what that was on the backplate.

Someone else will know better than I what drill size to use, but the usual deal is to backfill a barb fitting with solder, then run it through with a small number drill. I think you have the machining needs covered.

You don't need to back fill with solder for good running. I think the back filling helps slow down accidental flooding due to pit handling errors. I use 3/32" ID brass tubing for Norvel and Cox .049 taps with no check valve either. Fox sells a steel fitting that has #4 threads that would work well here.

So in that case I wouldn't need a pressure feed from the pipe?

Your right CP, I'm just waiting on some smaller Screamin' Demon parts.

Nope, this is higher pressure than pipe pressure - crankcase pressure or a bladder is how you can run the big venturis. You'll need a decent NV assembly. Pipe/muffler pressure is modest in comparison, but can be used with RC carbs whereas crankcase or bladder pressure cannot.

The crankcase pressure tap is if you want to run without a pipe. If you run a pipe, you want the tap on the pipe. The pressure in the pipe ramps up proportional to what's happening with the engine better, according to Nelson and Shadel. I kind of doubt that it matters that much, but I don't question those guys very much.
If the engine is timed 180 degrees or more, it'll most likely be a dog without a tuned pipe...so you won't have much say in the matter. If it is lower timed, then it would be much simpler to run it open exhaust and one of the biggest benefits of that is cooler running.

I thought crankcase pressure was also cool with pipes. You learn something new every day! Well, some days anyhow.

Here is something I found on the topic:

I then end up wondering if the pressure tap orifice diameter would play a role in the smoothness/consistency. Anyhow.. if pipe pressure is enough to feed a F3D or Q40 engine then there you go.

Not sure this is all that high timed of an engine - with HP rating given at 19k for the RC aero version I suspect it is like my "Speed" .15 RCA (R/C Aero) - peppy but not radical. SRV, break out the degree wheel..

I run my piped and high timed Webra 50 with a backplate pressure tap and it works perfectly...so I'm not entirely sure if there is any hard fast rules. My other post was mostly about strong advice I got from the Nelson gurus...but my Webra contradicts their advice. It doesn't put out nearly as much power as a FAI pylon engine though. I'd prefer to tap the backplate and try that before punching a hole in a perfectly good pipe.

With a degree wheel, exhaust duration timing seems to be 170 degrees.

If you want to get to the magic 180degree number without butchering the liner, make a liner shim out of .005" brass so you can raise it. You can use the liner as a template and score the shim stock until it slices through. Cuticle scissors can also be used for cutting them out.
The guys at the NASS would be a lot of help with this size engine as well as the guys at those German RC forums. I'm just guessing that a 6x6 prop would be pretty close, but the guys who've actually run this engine for RC speed could save you time and money.

I think that bears it out - again, OPS uses the word "Speed" much like Webra does, or like salespeople use the word "friend". It does not automatically mean high strung timing for balls-to-the-wall applications. I would call this a peppy engine but not radical. That said I have yet to see a 2-stroke glow OPS that disappoints. The box for this engine likely has a label that says 3.5 Speed RCA SLA = RC Air, SL = side exhaust, A = anterior (front) intake. OPS rates that engine at 0.7php at 19k, whereas the marine version (aka ducted fan timing) at 1.3hp at 27k. My carbed OPS .15 RCA SLA is turning a 6x5.5 prop at close to 23k static (so I guess around 24k in the air) and throttles as well as can be expected on an open pipe. This 3.5 ought to spin a 6-6 with authority. My .15 is built like a tank internally, I have no concerns about running it all day at that speed. I would imagine the two engines are set up similarly.

OPS's designation for a speed timed aero engine is VAE, hence the infamous .60 SPP VAE - SP = posterior (rear) exhaust, P = posterior intake, VAE = aero speed (perhaps velocitÃ* aria estrema?).

BTW OPS carbs are pretty, aren't they?

Those specs suggest it could turn a 6x6 prop at 29,000 rpm, developing 1.3HP. The resulting speed would be in the 160s. Just arm chair racing numbers, it's probably got more potential than that if it can be tuned / propped just right. The limiting factor with most engines in this size range is the hole going lengthwise through the crank. It is a crap shoot to know how far that hole can safely be opened up, but I'll bet the C/L speed or German speed freaks know.

The box says ...3.5cc speed fan

Fan? Cool.. that doesn't show up on my OPS literature, but it's probably too old. I wonder if it is indeed timed for the mid 20's then? If so then that's a nice score! If it follows OPS's other displacements in that sense, then it would share the p/l set with the marine version and be a snotty little devil. Pretty slick engine for the SJ-2! Or.. you know..

how much does it weigh?
what the size of the crank hole?

Here's some spec's

Weight w/o carb: 6.7oz's
Weight w/ carb: 8.2oz's
Crank opening width: .315" (8.1mm)
Carb throat dia.: .278 (7.08mm)