Hi msavitt, i've been following your thread with a lot of interest since it discusses so many useful technical issues. Sounds like you're well on your way to finishing your resto and entering the exciting 'comissioning phase'. One thing I was wondering though is your use of a piston utilising a dykes top ring. I might be wrong but believe this was an old design (albeit very clever too) that compensated for mid-late 20th century machining tolerances and materials composition. The all important combustion chamber seal was achived by using the compression of the stroke to 'push' on the inner edge of the dykes ring and hence force the outer edge on to the cylinder bore. Talking of bores i've gone on for too long already...Zzzz !! All i'm saying is maybe give some consideration to a modern piston and rings installation with all round better engineering qualities. Wishing you well with your resto. Andy L:)

Dykes rings were used in many racing engines, including Formula 1... and are still used in racing engines today. They are relatively expensive compared to conventional designs which might be their biggest drawback.

Are they really necessary in this application? No. But there isn't anything "wrong" with the design.

Hi Andy - i am getting close for sure
re the piston, the only alternatives that would fit would either 1. required modification, or 2. fit, but with one ring so right or wrong i chose to go the stock route with 1st O/S, which I would have preferred not to do at 120 euros. The real issue and risk as I see it is that aluminum melts at 1,221F so whether old original design, or even a forged Wiseco were it to exist, they all melt at the same temp. So my mission as I see it is to control heat and detonation. My biggest toe-stub originally getting this running was not closely inspecting all jets in carb when I rebuilt and then cruising with a main jet that was 3 sizes to small (e.g. 150 vs 180), perhaps exacerbated by new single lip bogus seals.
Today I am going to put the top end on and then begin to rig up my vacuum and pressure test system and complete this before putting the clutch cover or magneto back on so I can visualize deals which i hope isn't necessary !

The factory dual lip seal is unique. Other than these Yamaha engines I have not seen any other two stroke that utilized this design.

While going through my 360 trying to resolve the jetting issues I replaced what was probably a perfectly good dual lip seal with one of the "bogus" aftermarket seals. Made no difference at all. The aftermarket seals are no different than what most two strokes use anyway.

I'm also not a huge fan of Dykes rings. They solve a problem that was solved even better by making rings thinner and lighter. The commonly used .5mm thick rings in modern two strokes do not flutter even at extremely high RPM. They are easy on the bore and very ductile and durable. The best part about using an alternative, modern piston is cost and availability of parts to freshen it up down the road. Paying scarce NOS prices for 1970's parts is still an option for now. Probably not for much longer.

All that said, Dykes rings work perfectly fine as long as port widths are kept reasonable and detonation is kept under control.

I've seen other two stroke engines other than Yamaha with true dual spring-loaded lip "SW" type seals. Honda and Kawasaki used them also. It was a Honda engine I was racing where I learned how vacuum testing is important... even with new SW seals.

That said, there are thousands if not millions of two stroke engines out there with other design seals... Some of the European engines like KTM/Rotax use a special dual-lip crank seal design without any springs at all! However, those seals are specially designed for the application and to seal both vacuum and pressure.

The original seals Yamaha used on many bikes were better design and materials than "garden variety" seals other companies used... Yamaha wanted them to work well for a long time. Same goes for their pistons. The Yamaha piston alloy used is very high in silicon which is hell on machining tools during manufacture but produces a better, stronger, long-wearing piston.

Many years ago Yamaha had a slogan... "It's a better machine".... That statement wasn't just advertising fluff.

It's interesting to read about your thoughts on the likely causes of your meltdown and I guess we'll never know if it was the wrong jetting, a leaking seal or a combination of both. I'm at roughly the same stage on my '75 DT250B resto and so look forward to hearing how you get on with your pressure test method/kit and likewise for the vacuum test. Sure would be handy to know how well your set-up works and any pitfalls you encounter and resolve, plus of course your actual test results. Thanks in advance Andy L

I am a mechanical engineer who started his career 35 yrs ago in aerospace fluid systems so all this talk of seals and rings is very interesting to me!

I think my meltdown was due to a main jet 3 sizes too small, exacerbated by a B8ES vs B9ES plug. I could have had leaking seals but am starting to doubt it based how good these looked abs ignoring that they were not proper SW that likely resist pressures that probably alternate like crazy. Also thing base gasket and inlet stuff was probably okay too, but I will never know for sure.

So this time I listen to MarkT and pressure and vacuum test, hopefully no leak down.

I made progress this afternoon after pleasing wife with a shopping trip. Used yamabond on both sides of base and inlet gaskets using the small brush method I saw Allen Millar incredible motorcycle builder - you got to check his insane stuff out on YouTube! Used modified - as in bent - ring compressor from my bug build and then had wife help me hold jug with piston sticking out bottom while I installed pin and circlip. Waiting on a 29mm socket so I can properly torque clutch nut so time for cocktails!

Getting set to do pressure followed by vacuum test.

Thinking about decompression valve which I cleaned. If this leaks then it will impact leak down.

When engine is operating could a slightly leaking decompression valve contribute to lean running and damage? I have no reason to believe it would since it would be sucking exhaust eg no oxygen? Don’t know for sure so need to ask experts.

For the pressure test should I put some grease on it?

Got a rubber cork for intake spigot and made a steel backed 1/4” rubber plate with a Schroeder valve for exhaust that will hook up to brake bleeder that has a low pressure gauge with hose from tire pump installed. Feel pretty confident it might work but it’s primitive.

Any and all advice massively appreciated.

Thank you.

Don't forget to block off the oil injection port.

Oh thx I didn’t think of that, will do.

Any thoughts on decompression valve questions?