Gents,
I have a spare set of stock RD 350 cylinders, still at stock bore, that I am considering having bored to +.25 or +.50 mm, as well as converting them to RD400 head gaskets and fitting dowels to locate heads.
Is it worthwhile having a local 2 stroke shop clean up the transfer ports first. Just removing any steps and casting flaws in the upper part of the transfer ducts and the ports. And maybe cutting the boost ports a bit deeper as well. No plans for more exhaust port duration, however.
If you have an air compressor you could buy a die grinder at Harbor Freight and clean up the ports yourself.
if you local shop has a porting tool, then yeah its not bad idea, the roofs of those ports shaped and cleaned up can def help.
I do have straight die grinder, a Dremel etc, but no right angle handpiece. And since those are pretty expensive, I am looking at having this done by a somewhat local shop. I can easily deal with intake and exhaust ports myself, but do not intend to change timing, maybe just improve the exhaust port finish to slow down carbon buildup.
Sand blast
Reducing and removing the ledge is basically the first thing you do. Matching the sleeve at the divider is common. I see a lot of knife edged cylinders there. Knife edging is easy but it's old school. Nice rounded and blended ports are preferred. Then clean up and set port timing.
I picked up a Chinese Vevor Foredom knockoff flex-shaft grinder and a right angle attachment on Amazon for ~$150. That's really the setup you want for port grinding cylinders. Dremels don't have enough power. I've used air die grinders on cast iron ports and they're so noisy and really require a big compressor tank and 2 cylinder compressor head. My 20 Gallon 5hp Sears compressor won't keep up so I have 2 compressors feeding the same air line. Super noisy with all that running in my shop.
A real Foredom is almost $300 without the right angle handpiece which is almost as much as the tool. I'm pretty happy with the Vevor even though I'm not fond of sending my money to China. My only complaint is that the foot switch is not variable speed. Just on/off.
IR8D8R
John Ritter worked some magic on my 400 cylinders. RZ size reeds.
RE: Vevor, I read a recent article in business pages Vevor will be opening multiple brick and mortar stores in US this year to compete with the likes of Home Depot, Lowes, Harbor Freight, Tractor Supply all under 1 roof. I bought a Digital readout Torque wrench off eBay that looks to be good quality and currently waiting for delivery of their branded air hose. The website that tracks your purchase looks really sophisticated too like all set for the big launch...
Quote from: 1976RD400C on March 15, 2026, 08:28:13 AMJohn Ritter worked some magic on my 400 cylinders. RZ size reeds.
very nice :thumbs:
Quote from: Dvsrd on March 05, 2026, 03:12:14 PMGents,
I have a spare set of stock RD 350 cylinders, still at stock bore, that I am considering having bored to +.25 or +.50 mm, as well as converting them to RD400 head gaskets and fitting dowels to locate heads.
Is it worthwhile having a local 2 stroke shop clean up the transfer ports first. Just removing any steps and casting flaws in the upper part of the transfer ducts and the ports. And maybe cutting the boost ports a bit deeper as well. No plans for more exhaust port duration, however.
Quote from: Dvsrd on March 05, 2026, 03:12:14 PMGents,
I have a spare set of stock RD 350 cylinders, still at stock bore, that I am considering having bored to +.25 or +.50 mm, as well as converting them to RD400 head gaskets and fitting dowels to locate heads.
Is it worthwhile having a local 2 stroke shop clean up the transfer ports first. Just removing any steps and casting flaws in the upper part of the transfer ducts and the ports. And maybe cutting the boost ports a bit deeper as well. No plans for more exhaust port duration, however.
Promise not to hi-jack the thread here. I'm 6 months into owning an RD and cleaning up the aesthetics, and I'm finally starting to think a little more about the engine itself.
I understand the concept of over-boring and port cleanup. Can someone elaborate on the RD400 head gasket and "dowels" to locate the heads? Are we simply discussing altering squish/compression? Going slightly more towards a race tune? Much appreciate y'all's insight.
doweling a cyl head and cleaning up squish, really, is an efficiency thing. centering the dome makes for a better combustion chamber as does tightening the squish.
on my tzr, with everything in the motor stock, the head was cut to .8-.9mm squish. what this does is actually center the mixture for an optimal burn, and allows it to run cooler. to the point, the harder you flog it the cooler it runs due to it being so efficient. its weird watching the temp gauge drop some at long sustained high rpm runs. but it does it. this of course helps when adding performance mods. you can do one e out the other. my t500 w the billet heads has dead stock porting as well, but an actual squish band and o ring head and centering made a big difference alone. Chuck q used to recommend centering even stock heads on 400 builds, and changing over to 400 type head gaskets in 350 builds & correcting the squish.
Wow. Reducing squish makes sense to me from the standpoint of raising compression, but I NEVER would have considered that a stock head isn't truly centered. It all makes sense though. Talk about some delicate machining work with a little wizardry!! I've tried reading 2T Tuners Handbook, but I feel like you need an engineering degree to understand half of what's being said. If you have any other resources, please point me in that direction! Thanks, Marc. Very very cool stuff.
The technology is a little outdated, but A. Graham Bell's tuning book is excellent and an easier read.
I have engineering degrees.that stuff gets very boring to read even to me. :lol:
compression is one thing, but IMHO not nearly as important as to where the mixture is located/directed when burned. hence the shaping of the chambers etc. its all VERY different as to how a 4 stroke works overall. best way to view it for comparison, to me, is compression is dynamic on a 2t, the faster you rev it, the more it effectively makes. hence why its good to retard the timing higher in the rpm range
Regarding squish band, combustion chamber shape and head geometry in general: I follow a retired Swedish diesel tech, who is repairing and building 50-125 cc two strokes, and testing them on his home built dyno. IIRC, he tested a 50 cc engine with an 80 cc aftermarket higher power cylinder, power in the 11-13 hp range. He tested two different heads with the same volume, and found a significant power increase. If you are able to understand Swedish, check out "Tvåtaktsmeken" on YouTube.
Quote from: SmokeAddicted on March 18, 2026, 03:05:12 PMWow. Reducing squish makes sense to me from the standpoint of raising compression, but I NEVER would have considered that a stock head isn't truly centered. It all makes sense though. Talk about some delicate machining work with a little wizardry!! I've tried reading 2T Tuners Handbook, but I feel like you need an engineering degree to understand half of what's being said. If you have any other resources, please point me in that direction! Thanks, Marc. Very very cool stuff.
here's a picture of stock T500 from another Mark thread. Notice the off-center plug hole and oval dome shape.
20260221_112847.jpg
That's so cool!! The overall concept makes sense, but to achieve the optimal spark/head position must involve some intimate knowledge (and math/physics)of any specific 2T engine. Explains why I see "Chuck" mentioned so much in performance threads. Very cool stuff.
I found A Graham Bell's book for free online :vroom:
As the tread has wandered off into heads and squish, I wonder why many Suzuki two stroke heads had no squish area. Mark's T500, the GT750 and some PE and RM engines are of this design.
Not that Yamaha was much better, as the squish clearances mostly were too big to do much good.
Btw, has any of you any experience with redirecting the secondary transfers to a more forward direction on RD's? According to Kevin Cameron this worked well on TZ750, Jan Thiel is also promoting it (aka "the leaning tower")
The lack of squish on those models was because they weren't attempting performance. Or at least, weren't trying to compete necessarily with similar displacement engines on power.
mmm. i think it was an 'its good enough' design at the time. they were aiming for reliability, then performance. a reliable 2t 500 twin from them was a big deal at the time. i think that motor was designed around 65-66 with of course, heavy influence from early 60s MZ design. either way, it somhow works but its super dated. only bike ive ever had that pinged at idle. :whistle: :dawg:
Quote from: m in sc on March 20, 2026, 08:28:47 AMmmm. i think it was an 'its good enough' design at the time. they were aiming for reliability, then performance. a reliable 2t 500 twin from them was a big deal at the time. i think that motor was designed around 65-66 with of course, heavy influence from early 60s MZ design. either way, it somhow works but its super dated. only bike ive ever had that pinged at idle. :whistle: :dawg:
I seem to remember that the T500 was roadraced back in the 60s/70s, often in a Seeley frame?
I guess the engines were modified quite a bit for that use...Btw, is the T500 just like the 380 and 750 triples, just one pair of transfers?
(Although much better shaped transfer ducts, at least on the 750, than on any RDs, due to the pretty clever rotated cylinders)
it was.
I know I mentioned it before, but there was a Canadian that would show up at the club races in NH around 1980 with his tricked out T500. He could enter it in the RD modified class, where anything goes for engine and brake mods. He would blow the doors off all the RDs, and there were lots of them. I remember one weekend he showed up late and was pacing around in the pits, a bit upset, and in his French accent saying, "no practice, no practice". Well, it didn't matter, he won. :notworthy:
period ad.
Was it really capable of a 13.20 quarter mile? That couldn't have been much slower than a H1/ Mach III?
the 1st couple of years w the 34mm carbs, yup. later not quite but close. they're stronger than most people think. nowadays a lot of old guys fart around on them but they go pretty good. my previously stock 500 ran about the same as my 78 kz650. let's keep in mind , these were actually made to compete with British 650s & 750s, in particular, the Nortons. look at the lines of stockers proportions. its very Norton ish in stock trim.
Quote from: Dvsrd on March 20, 2026, 04:23:19 PMWas it really capable of a 13.10 quarter mile? That couldn't have been much slower than a H1/ Mach III?
Ha, I was curious as to how it compared to the 69' H1 and google AI gave me an answer that the H1 was quoted to be running 12.2 seconds at 118 mph in the quarter mile. So a second slower I guess. :whistle:
yup. the h1 def is a quicker bike, no doubt. but TBH ive had a few and really dont want another
Why is it quicker? I mean is it quicker revving due to a shorter stroke? Is the three piston design attributable?...seems like more rotating mass. Is it a better design in terms of combustion design?
Quote from: RDryan on March 20, 2026, 06:28:47 PMWhy is it quicker? I mean is it quicker revving due to a shorter stroke? Is the three piston design attributable?...seems like more rotating mass. Is it a better design in terms of combustion design?
The early H1 had 60 hp, vs the T500's 47 hp. Both manufacturer's numbers.
Being a triple the H1 had a shorter stroke, so could safetly rev higher. More revs gives more power. So the H1 was given more extreme port timing/ cylinder design, and an exhaust system designed for higher revs and more power. I believe the H1 exhaust actually was expansion Chambers, "hidden" inside more traditional looking mufflers.