One of my old engines has an exhaust port with a lower edge that is about 1mm higher-up than the surface of the piston at BDC. It's the only engine I have that's like this. Does the lip below the exhaust port at BDC serve a useful purpose, or is it just the result of low-cost machining?

It is a dam for the last bit of fuel comming in, if you grind it away you will not gain any performance but will notice more fuel consumption.

As Roelof says. It acts as an anti-reversion device.

Interesting. I thought the last bit of fuel came in as the piston is starting to travel upwards again.

Why wouldn't all engines have this feature?

I’ve never heard of a anti-reversion device employed in a Schnürle scavenged (ported) engine. That’s not to say it wouldn’t serve that function, but the reality is it likely doesn’t serve that purpose well at all considering the Schnürle port arrangement. The port arrangement and timing sequence pretty much causes the A/F charge to make a loop going up towards the glow plug, bouncing off the top of the chamber, and basically shooting towards the exhaust port. I suspect it’s one of those things where they use the same piston and liner in many engines and with certain stroke lengths and/or crankcases, the piston ends up being below the bottom of the exhaust port in the liner at BDC. I have many engines that have this “feature”, but none of them have “pipe timing”.

The old cross scavenged engines had a baffle machined into the top of the piston that, when the transfer ports opened, the fuel mixture hit that baffle and went up toward the glow plug(s). (Some engines employed dual glow plugs)

I've heard of that old design with ramped pistons and I've seen diagrams before, but I didn't know it was ever used in RC engines. Pics?

I have 6 different types of engines in my fleet, and only one of them has a lip under the exhaust port. That's why it seems so odd to me. It honestly looks more like the sleeve is just being used in an engine with longer stroke than the sleeve was originally designed for.

You want to see pics of a baffled piston? I have an HB PDP Blitz piston and liner out on my bench. I’ll get some pics of it for you.

According to a friend of mine, the ideal location for the top of the piston at BDC is flush with the bottom of the exhaust port. Below the exhaust port can mess with scavenging; in his words.

Here’s a few pics of a cross flow scavenging setup. The ports with the black around them are the exhaust ports, the other square ports are induction ports and the little thin slit ports are called PDP ports. (Perry Directional Porting). This is from a German HB .40 PDP Blitz. Liner is chromed and it uses a narrow piston ring on an aluminum piston; conrod is forged aluminum; not cast and not machined from billet.

Old school cross flow engines used a Meehanite piston in a (usually) hardened steel liner. Some guys used steel pistons in steel liners too.

o_O

How old is that engine? It looks like it's from the 60's or 70's.

The PDP Blitz is from the very latest of the 1970s into the early 80s. The PDP debuted in 1977 and the Chromed PDP “Blitz” a few years after. It’s listed at .84hp@16,500rpm, although it would likely do much better if it had a Dykes piston ring instead (as most other HB engines were so equipped).

You wanna see old.. I have an old OS Max-III .15 from the late ‘50s to Early ‘60s. It would wear out the crankshaft bushing if ran real hard. Mine has been beaten like a red headed step child. It’s a bitch to hand start.

I have the HB Grand Prix .21 engine over here, it has a same look a like piston with piston rings and a cam on the piston.

I still need to build it up as an ideal rain engine for my MRX6

Oh, look here for parts and manuals:
http://www.mecoa.com/hb/parts.htm

The owner of Mecoa is a jerk. Be cautious with how you deal with him when ordering parts - especially from overseas.

I bought an old Toki engine from MECOA a few years ago. I don't recall having problems.

He accused me of stealing from the United States government when I told him a friend of mine (who was active military) gave me a couple K&B Sportster .65 engines that came from RCMAT target drones and the engines didn’t run right. (I was posting on the forum at Mecoa about it). I checked with my friend to ensure there were no legal repercussions to me having those engines, and he assured me there was no issue. I went back to Randy asking questions about the carburetors and he called me a thief and that he wouldn’t sell me any parts to get the engines running correctly. At that point, I decided I wouldn’t do business with him and found parts from a friend of mine and fellow Sportster enthusiast.

It takes some balls to accuse someone of being a thief without knowing all the facts.