Yamfan wrote: CT3 exhaust port is smaller than DT, but has a blowdown hole above the top edge of the port.
DT175 433 cylinders, fitted with YZ85 VForce reed block, are the best for power (outside of IT or YZ ones).

I'm not certain that the CT3 exhaust port was significantly smaller than the 443 DT175 cylinder.  The porting was slightly different though and I would agree a stock 443 is probably slightly better than a CT3.  There is not a huge difference in performance though.  For the time, the USA CT's were all very much "high performance" engines with fantastic power for their size.  1974 DT175 carried on that tradition but didn't raise the bar a whole lot in terms of performance. 

Compared to modern engines?  The CT or DT175 is low performance.  However, it would be just as ridiculous to compare the high performance for the time 1932 Ford V-8 to the 1969 Ford Boss 302 that came out 37 years later...  (sometimes it's easy for me to forget that.)

Anyway, I wanted to mention "factory performance" cylinder/head (swapped as a pair) choices for the CT and DT175.

For the earlier "long stud" model 175's (up until 1976 in the USA, possibly later in other countries) any CT1,2,3 or DT175 cylinder and matched head will fit...  of course the CT1 was piston port and the CT2/CT3/DT175 were reed valve.  But you can freely swap top ends around.  No rod length issues that you find on the 250's.

The only 175cc "factory performance" cylinder/head for this era is the 74-75 MX175. (Note that "MX175" is NOT the same meaning as used in the UK and other countries for the later "monoshock" models.  In the US, "MX" always meant "Motocross" not "Monoshock")   Not really recommended for most applications.  Somewhat "peaky" high rpm power.  You'd be much better off having a stock 175 cylinder ported professionally by an expert like Rich (Racer Clam here).

Yes, there is a 1976 YZ175C, rare one year model, does have the long studs...  but the stud pattern is different and a YZ175C cylinder will only fit on YZ175 cases. 

You could also use earlier YZ and MX 125 cylinders...  but you are giving up 50cc's which is a lot on a 175.  The MX and YZ 125 tend to be "peaky" high rpm engines and a stock 175 is nearly as powerful and much easier to ride...  many early YZ125's that survived to today have DT175 top ends.  Makes a very nice rideable bike compared to the "explosive" MX/YZ power in those days.

The IT175 did not come out until 1977 and is the first of the "short stud" 175's.  The 1978-81 DT175 (monoshock) shared this same "short stud" architecture.  Unfortunately a IT175 cylinder will not fit onto DT175 cases without extensive case mods.   And yes you could give up the 50cc's and go with a "short stud" YZ125 or IT125 cylinder on your monoshock DT175 but not a great idea in my opinion. 

Note that the US monoshock MX175 lost the DNA of the earlier twin shock MX175's...  The "short stud" cylinder of the monoshock MX175 was exactly the same cylinder as the 78-81 DT175...  the 175 motocross racing class had pretty much been phased out and the monoshock MX175 we got in the USA in about 78-79 to 81 was pretty much a DT175 with no lights...  a nice off-road only trail machine...  not a fire-breathing race bike like the 74-75 MX175 or 1976 YZ175.

 

One last "off topic" comment... the AT1/2/3 and DT125's can all be easily upgraded to a 175 by using the corresponding parts from a 175... following the guidelines in my previous post.

On the piston ports, the carb and jetting is even identical on the AT1 (125) and CT1 (175).

Jetting was different between 175 and 125 when you get into reed valves. The carbs are generally the same size (exception: 1980-81 DT125 had a smaller (VM22) carb than the DT175 VM24).

I honestly thought there was much more difference between the 433 cylinders and the CT2/3 ones!

But other than the DT exhaust port being a bit wider, the only major difference is the reed valve cavity size!

The 433 cylinder uses the same size reed cage as RD250/350, and can be modified to accept a YZ85 VForce cage. The CT cylinders I would guess can be modified to take the 433 cage, but the cavity is too small for the YZ85 ones, and will break through if opened that much.

The main thing that seems to restrict the performance of these engines, though, is the rather small transfer ducts, which are dictated by the cylinder fixing studs, not allowing them to be any bigger.

Altering the transfer openings in the same way that has been popular on RD250/350 for many years, in conjunction with minor changes to the stock porting, works really well, in conjunction with a bigger reed cage, and slightly increased compression.

I am working on a 433 cylinder at the moment, which I have fitted a VForce Banshee reed cage to. It was a major big job, as the reed cavity broke through in 6 places, and had to be made good.

These little motors, can be made to breathe better, and make more power, which means that on a trials bike, it becomes possible to use 4th gear in the sections. Didn't find any of the mods made a lot of difference, until electronic ignition was fitted, though, but have found a producer in the far east, who can supply good quality systems for $130, so not all that costly.

Yamfan, I'm wondering if possibly the "CT" cylinders you have are actually the same as "AG" cylinders instead of USA-spec CT cylinders?

From my research the AG definitely had the compression release hole and reportedly had very mild porting which usually means a smaller exhaust port.

I'm pretty sure the US 72-73 CT had a smaller reed cage and opening than the 1974-76 DT175... even though bolt pattern is the same.

It's also definitely possible that the CT model in markets outside the USA came with something closer to the AG-spec cylinders? So they might not be mislabeled... just different from what we got in the USA?

Port charts were posted for the US CT models... (I've seen a variation that says stock CT2/3 exhaust port top edge is 30mm below top of cylinder, not the 31mm in the Service Manual diagram.) You could compare those specs to your CT cylinders.

Wait a second...  the port chart RT325 posted for the CT2 is not the same as the port chart in the US manuals.  His shows the exhaust port 34.5 mm below top of cylinder while the US spec shows the port much higher at 31mm (or even 30mm) from top of cylinder.  That's a significant difference! 

So that indicates to me that markets outside the USA very likely could have had different port specs. 

Here's the port chart from the USA manual...  I think you'll find it similar to your stock 443 specs?

 

 

34mm exhaust port cylinder sounds to me for a bigger cc motor than 125 or 175? My NOS CT2 cylinder came from a former Yamaha dealer, in original box with CT part number on it.

Your porting chart is very close to the ports on my new cylinder. New cylinder has got a Gas Gas reed block in it at the moment, which I will try it with after bit more work on the transfers and exhaust.

433 cylinders have 41mm wide exhaust ports, other than that, and smaller reed cavity the CT ones look very similar. Less finning on the CT ones though.

Do you have the Clymer manual which details modifications for these motors?

Yamfan, the port measurement on the charts is from top of cylinder down to top of exhaust port...  RT325's spec sheet says at 34.5mm the CT2 exhaust port is 3.5mm (or 4.5mm) smaller than the USA spec chart for the CT2/CT3 at 31mm (or 30mm) from the top of the cylinder. 

(That's much milder exhaust porting than US spec, not something from a bigger engine.)

Early 300 Fantic twin-shocks had low port cylinders, but on the very last ones, and the mono-shocks that followed, the port was raised 3.8mm. This made them rev much more freely and seemed to reduce detonation a bit?

I am surprised that Yamaha made 175 motors with 34mm ports, as thats not going to do much for the power of what is quite a small motor? All the stock 175 cylinders I have are around 30mm, which get modified to 29mm.

Most of the 175 motors seem to be around 30mm, and work fine, with only minor alterations. I did try welding up a 433 exhaust port, to improve the shape and that appeared to work well, but put quite a bit of heat into the cylinder, and I only tried it as it needed to be rebored anyway.

The Clymer performance manual, provides port timing specs for most of the smaller capacity Yams. Will have a look at exhaust port details tomorrow.

Blimey it's all happening while i sleep. Busy topic.
I stand by [in nz anyway] that AG's were the only 175 i ever saw with the hole. But that just me dealing with farm & trail similar models through the years of orange AG's.
Still things to learn i guess.

Does the AG have electric start? If so that might be a reason for the holes? My new CT cylinder, doesnt have the bike model number on as, the reed cavity has been heavily modified, but the other one does, and will check if it has an AG number.