RD350 Track Bike build

[5port]

Oooo, Itchy fingers time!   
Quite a bit happening in the background with this bike.  Anyway, I think we are about to move onto the Cylinders and heads! 
The thinking I have is based on reading all the articles and books that I have found with details of RD/TZ tuning.  Genuine details are difficult to find, partly because they are stock-in-trade
for tuners to make a living and also because they can be very specific to a particular set-up.  However, if you put the effort in, you can find considerable detail in online articles by Ed Erlenbach and Dale Alexander, for example.  Thanks to all who have posted info!   
The old AIRCOOLED Yamaha twins based on the DS7/YR5 cases were easily adapted to the inlet port Reed valve of the RD.  They also inherited the limited area for transfer ports but, that still allowed over twice the power of the standard road engines to be developed on full-on racing versions. Tuned to that degree, they do suffer from short component life and can be very fussy to use.  Fuel becomes critical due to detonation problems and, you soon have to use expensive racing fuel if you tune very highly.  Now, to make the extra power, you have to burn more air and fuel in the engine and create more heat.  That is great but, remember that over 3/4 of the heat from the fuel burning is wasted as heat into the engine and down the exhaust!  So, you have to think of the effects of the extra heat and revs that the engine will have to stand.  Yamaha are quoted as saying that the sensible limit for their Aircooled RD layout twin racing engine was 55bhp.  The TR3 (350) Aircooled racebike was 58bhp and so, that was why they went to liquid cooling with the TZ in 1973-and onwards.  Liquid cooling made the control of the cylinder/head temperature easy, lowered piston temps to a degree but, was heavier.  Now this doesn't mean that you cant build 70bhp Aircooled racers but, it is not easy.
For this Aircooled 350 track bike, I am aiming at 50bhp.  The RD350LC is quoted at 47bhp in standard form and running on Regular fuel so, I have looked carefully at that. 
The RD350 Aircooled cylinders can be ported to the 350LC spec-and beyond.  Knowing tuner Mick Abbey, I went straight to ask his opinion.  He agreed that he could do a reliable tune to give 50bhp at sensible revs (8500) on 28mm carbs and a good exhaust! 
We agreed to try 22cc heads that, with my engine config, will give 6.4:1 CR and used 8mm wide squish band that equals 40% area squish.  Overall, I hope to be able to use 98 Octane fuel, the TR3 had just over 7:1 CR and needed 100 Octane. 
OK, Back with pics and more soon! 

5port 

       
 

[5port]

So, I thought I will post these pics up and come back and insert with words after cooking and drinking!     

Top end.
Mmmm, Nice spare ribs in BBQ and our own Honey marianade!   
This top end is USA sourced RD350 cylinders and UK sourced 1A2 RD250 heads.  The cylinders had some rebore mistakes and were on +0.25.  Mick had to take them to +0.75 to get them OK, after the porting.  I am not going to list Micks porting figures precisely because that is his info. However, as I said before, we started with the RD350LC and went onwards.  The exhaust port is wider and shaped more like a rugby ball.  So, it gets earlier opening and greater time/area, closer to a 350YPVS or TZ750.  Now, of course, you can make an engine a high rev only monster if you go too high with the exhaust port but, the transfers are also matched and I do not expect a light-switch powerband.  In fact, these cylinders came with the Ex ports raised to about where they are now, and no other mods!  So, I think they will now be Hot, but not too hot to handle!  
You can see that the usual RD250/350 top gasket step has been machined back to a flat face.  This is to allow the use of a later RD type full gasket that has better reliability.




Cylinders.
Seen here, the liner is cut back to favour the main transfer ports.  The liner near the aux T.fers is not cutback much.  This is just because it works this way!  What you can't really see is how Mick has not just hogged-out the passages.  They are still shaped to work well.





Heads.
The Heads are important.  They control the squish and CR to give good combustion.  There are lots of options but, it should be possible to get a good performance with modified RD heads.  These started as 1976 onwards RD250 1A0 heads.  They have a greater thickness than earlier heads and that helps with strength and heat dissipation.  They are wider than the early 350 cylinders and I have cut them to match.  However, if you felt that you had a heat problem, you could run with the extra width fin.  Mick has done a great job recutting these heads.  With just a 0.25mm face cut, he has then cut an 8mm wide squish face and then a lovely hemi bowl to give exactly 22cc. 
The sparkplugs need a copper washer to give a precise fit with the slight change in thread depth. 
You can change a few things such as gasket thickness to get your squish/CR  modified.  I would likely get another set machined if I need different details.  These cost about £40 a set so, no need to buy fancy £600 heads IMO, Well not at 50bhp level anyway.     
BTW, The anti-ring fin dampers are made by me from 10mm dia Hi-temp silicone rubber rod.  Each one is 11mm and a strong press fit.
   




Pistons.

A great big Thank-you to Steve Moffatfor these pistons. They are heat-cycled TKRJ 4L0 pattern.  Now, they are sized by Mick at 2 Thou inch skirt clearance.  Bear in mind, they will run at a considerably higher heat-load than an Aircooled RD350, and that is why Mick recommends 2Thou inch in this track engine.
These pistons were designed for the 47bhp RD350LC and, I think they will good to start with in my 50bhp track bike.  Once again, thank you Steve!   




Cheers

5port

[Motty]

Some exciting work going on there
Have the pistons been modified at all?

Sent from my Moto E (4) Plus using Tapatalk

[5port]

Some exciting work going on there
Have the pistons been modified at all?

Sent from my Moto E (4) Plus using Tapatalk

Hi Richard!
The pistons have run 500 running-in miles in an RD400.  They are just nicely ready to go.  Weight is identical with no mods.  Steve was kind enough to donate them as he had other plans for his engine.  As Mick says, these are good to start with, and I agree.  Non standard engines usually need lots of changes before you get them right and that can cost £££.  Cheers

5port

[5port]

Thanks for all the views everyone.   
I thought that I might expand on the piston plan.  Now as many will be aware, I am keen to use standard Yamaha aircooled pistons in standard road engines, where they fall within the Yamaha availability, ie up to +1.00.  That is because the piston shape is correctly profiled for the temperature range and expansion characteristics of the aircooled road RD.  As we know, the price and availability of those Yamaha pistons is difficult, on both counts!   
Now, the aircooled race engines most similar to the Aircooled RD were the TD3 250 and TR3 350, the 350 TR3 making 58bhp @ 9500 compared to the road R5 36bhp @7000.  So, you can imagine the extra amount of fuel and air required to produce the extra power, bearing in mind the fact that about 80% of the fuel energy is lost and only 20% makes extra power!  So, it is no great surprise that racing engines give their pistons a very hard time, with all the extra rpm and temperature.  The late 1960's onwards Japanese two stroke road engines surprised European race tuners by the very high spec pistons in the road engines, using high-silicon Aluminium alloys that were better than the European Racing engines spec.  These high Silicon alloys are harder than the more basic alloy and there is a compromise of suitability for die-casting/hardness/toughness/and machining.  The happy result is that Japanese two stroke pistons were high quality!
Now, for racing the bottom line is performance and so, things like piston noise from skirt clearance when running cool can fade into insignificance when the throttle has been wide open at max revs for the length of the straight and the last thing you need is a seizure or a holed piston.       For my track engine, I think the pattern piston from the 47bhp @8500 4L0 engine is a reasonable start and the 2 thou inch skirt clearance is suggested by Mick from his experience.   
Mick has also said that he thinks the YPVS 31K piston will be a future upgrade for this engine.   Using my "compare with other engines" policy I can see where he is coming from. The 31K was claimed with 59bhp @9500 and so, the piston is up to the job.  It does have greater sized windows and cutaway inlet skirt length to assist intake capability and although the 31K cylinder has a bridged intake designed to use these, Mick says his porting on my RD cylinders will also benefit from this.  Overall, the later TZ750 had the inlet skirt virtually fully cut-away, with no restriction over the RD-like reed intake with single port.  However, the TZ750 ran backwards to take the thrust loads off the inlet skirt and onto the front skirt.  Running an RD with the inlet skirt cut away will have a very short life and possibly cause catastophic piston failure! 
That leaves us with the "tang" issue.  On the 31K YPVS, the inlet ports are bridged to allow the piston inlet skirt central tang to run on the bridge and stop the lower skirt edge catching on the bottom of the inlet port.  If used in an RD single inlet port engine, that central tang actually sticks further out on the downward stroke and can catch the bottom of the port.  This will rapidly cause piston failure.  The cure is to remove the central tang and, ensure that the bottom edge of the port has a well executed chamfer that "leads" the bottom of the skirt gently into the liner as it passes.
So, that is my plan.  Get the bike built and working.  Then, incorporate updates as the time and money allow!   

Cheers

5port     

[tony2stroke]

I would suggest putting a 7mm hole about 10mm down from piston crown in line with the middle of the windows, this will help with oiling of the little end bearings, it does work, there is lots written about it on tinternet.

[5port]

Thanks Tony,  I have read Frankie's write-ups on RD tuning and he reports that the effect was very good, with much reduced tendency to detonation due, he thinks, to reduced piston temperature.  I can imagine that the small bleed hole there might allow gasses under the piston to circulate.  Unfortunately, I do not have specific details of the reality of the flow that occurs with that mod.  However, I  am interested to try it!   

5port

[tony2stroke]

Every little helps as Tesco say.

I did this on my little 200 I tuned, the extra oiling of the bearings was enough for me to do it, let alone the cooling of piston you refer to.

[D Peacock]

Sorry Chris, don't know how i missed this one 

You look to be bolting together the correct bits - for info i'm using the following clutch configuration -

Original Ring gear - Mitaka outer basket / 350B inner basket / LC pressure plate. Plates are YPVS Steels with XS250 frictions (no rubber rings). 3 x LC springs / 3 x TZ springs. No slip and light lever action.

Heed my warnings on squish / head volume! It's one area I could improve on mine but after the issues I had i'm happier with the stock heads back on and it being safe. 

[5port]

Thanks Dan.  What head vol were you using with your problems.  I mean the actual "flat" head vol with a plug fitted ?
I am getting corrected CR @6.4:1 with 1mm squish, which seems pretty safe as standard LC listed as being 6.2:1.  Thanks for any detail!

5port 

[5port]

You might be interested that the early versions of the TZ350 have aggressive CR, listed in the Yamaha owners manual as CR 7.3:1 timing 2.0mm BTDC.  That is for 100 Octane.  That early head has 12mm wide squish face so, equates to about 60% squish area.  However, there is no detail of the squish clearance given by Yamaha.

5port 

[D Peacock]

Thanks Dan.  What head vol were you using with your problems.  I mean the actual "flat" head vol with a plug fitted ?
I am getting corrected CR @6.4:1 with 1mm squish, which seems pretty safe as standard LC listed as being 6.2:1.  Thanks for any detail!

5port

Hi Chris,
          I'm not 100% sure but the heads are currently off the bike so I can ask my mate to test them for me as he has the gear to do so being a 2T racer. Be interesting to find out.

[5port]

Thanks Dan!

5port

[[Mossi398]]

Hi interesting project do you have a link to Frankies tuning. Regards Phelim.

[german]

Hi interesting project do you have a link to Frankies tuning. Regards Phelim.

Hi Phelim,
here is the link. You have to change the language as it is pre-installed in german.
https://www.rd250tuned.de/

Regards Uwe