The Pro-X 69 mm cast piston makes a great 269.2 cc sleeper motor. With the right pipe/porting it will make about 60 rear wheel HP and will rev quickly because the piston weight is very near stock at 300.9 grams (piston/rings/pin/clips - before any kind of modifications) vs. the stock TRX parts at 288.3 grams. For 12 grams you get about 23 more cc's and it bores right in. You can use a Wiseco forged piston to make this set up if you prefer. They also make a 69.5 mm replacement piston which this barrel is going to get next. Then it's time to re-sleeve because the iron will be too thin to bore it any more.
This barrel has been around. It was on one of my TRX motors for over 8 years. I can't remember how many cast pistons its been through. It's ready for a bore job - finally. This time I'll use a Wiseco 69.5 mm forged (305.2 grams) piston. This barrel never even saw a stock sized piston. I took it out to 69 mm right after I opened the box it came in.	Yup - that's epoxy. It is smoother than it looks in this picture. The stock casting is very uneven. I built up that area in layers then removed aluminum from the opposite wall to create a tunnel with a greater arc. This makes for a wide powerband. I almost never knife edge port dividers - I prefer to radius everything instead. Note: this modification takes serious time.
The aluminum of the front transfer port has been cut and aimed so it will release its charge toward the rear of the cylinder - with extreme prejudice. Note how the tunnel is ahead of the port window. This will help to keep that mixture from going out the exhaust port.	A couple of extra holes here help the incoming mixture loop its way around and cool the piston. They are at the same angle as the main boost port and are aimed at the spark plug. It takes time (and nerve) to drill them.
Looking in the exhaust port - smooth but not polished. This motor was not taken apart for over 2 years. Not bad looking for a couple hundred gallons of gas and a bunch of AmsOil. The top of the port has a large radius - instead of being raised. This keeps the port open less and makes the power stroke longer.	Looking out the exhaust port. It is wide, but the duration is not that radical - 188 degrees open. The bridge has been narrowed a bit and clearanced too. Note the port probe plug and steel O-ring on top of barrel - it works well with a copper head gasket. I use a 69mm dome with the Cool Head now though.
Not as big as it could be, but it's big enough. You can see where the small boost ports feed from. The auxiliary ports are opened up a bit but not hogged out. Note the intake port probe plug - it was used to feed a mechanical fuel pump for the nitrous oxide. That was fun while the bottle lasted!	The roof of the main transfer ports are flat for 1/2" into the port and carefully blended into the port arc. The rear ports are aimed slightly upwards - 2 degrees. The transfers open at the same time except the boost which opens 2 degrees later. The transfer port duration is 130° open.
The exhaust port edges hang over the transfer ports slightly. Keep a careful eye on port symmetry - incoming and outgoing. The extra cc's of this barrel let it chug down very low and still have good power and it revs out to 9300 with ease. Much less vibration than a Pro-X barrel and almost the same output - within a couple HP at the peak and it revs faster and higher. It looks stock if you use a stock head and a copper gasket. It is one of my most requested engine builds.	My first Pro-X 310cc barrel bit the dust. If those cracks had been an inch longer the barrel would have broken off completely - it was this way on both sides. I was fool enough to take it to a welder who did a crappy looking job. These barrels are fast when ported correctly. They had some design problems when I bought this one (very early). It lasted a couple years though at 73.5 mm. Dollar for dollar the 69 mm kit is a much better way to go.
To 69 or not to 69 If you don't feel safe going out to 69mm right away, go to 68.75 and leave yourself an extra bore to work with. Using the cast piston, the piston to bore clearance can be set a little tighter (I set mine up initially at .0025" and ran it in well). It is as reliable as a rock. Replacing pistons (and checking them) often is a good idea with any high output motor that's used for racing or heavy recreational riding - it's a good idea to replace it when the piston to bore clearance is greater than .005". Having said that, it has been my experience that piston wear was minimal and never reached .005" before I changed the part out. The amount of hours and type of riding I do made me change it whether it was indicated by measuring it. I would change it about every 30 hours of riding/racing time. Have the cylinder bored and honed by someone who has/uses torque plates. A 1" thick aluminum plate torqued to 20 to 22 ft lb will simulate the head on and its torque on the head bolts (and corresponding bore distortion) pretty well during the procedure. This makes for a very round hole when the real head is bolted on again - much more round than you could achieve without using one. When boring any cylinder it is a good idea to do it this way. The pricey ($900 on up) Pro-X 310cc barrels use a Wiseco 72 to 73.5 (293.2 cc to 305.5 cc) mm forged piston. This piston has been through several (good) design changes over the years. The 73.5 mm piston weighs 318.9 grams with everything - that's why they rev a little (s)lower and vibrate a little more - power can be excellent though, when ported/piped correctly. This top end will bolt on without any modification to it. The 330cc version (76 to 77 mm piston) has more weight and vibration and a little more power - when dialed in properly. The block must be bored to accept the 3.255" sleeve. The 350cc version (78 to 79mm piston) is a massive piece of metal and requires you to bore the block to accept the 3.300" sleeve.   Using O-Rings to help seal or prevent leaks from the top of the motor When using the stock head with this piston/porting/head modification, a copper head gasket is probably best. Installing it with an O-Ring (stainless steel) installed in the top of the liner adds a nice touch and will keep the pressure where it belongs - 1 O-ring is enough. It's also a good idea to lap the head and barrel flat while you have it apart. Having the squish area dialed in and set properly will produce gains everywhere. O-rings are a permanent fix for the troublesome leaky head gasket big bore kits (295cc) that everyone sells too. I'm not talking about the Pro-X top ends - they are excellent pieces, especially when lapped flat - those pieces use rubber O-rings anyway. I'm talking about the resleeve/bore in your stock barrel type big bore kits that are available. Though they can be made to produce additional power, especially at midrange, they can be a handful when trying to keep their combustion and coolant separate. Try this to make that problem history. You need to have the tools to do this job or be able to find a machinist who can perform the necessary modification. The head and barrel both need to be taken off to be permanently fixed. The cure to the problem is to have a .030" stainless steel O-Ring installed in the top of the cylinder liner - in the iron, not the aluminum - far enough out to leave yourself room for over bores if necessary. You can use stainless steel safety wire for the O-Rings. Stainless is preferred to plain steel since it won't rust. Put the cylinder in a lathe and true it so there is virtually no run out - wobble. The O-Ring groove needs to be the same depth all the way around the cylinder. It is more important that the groove depth be the same all the way around than if the groove is not exactly centered to the bore. Using a carbide or high grade tool steel cutting tool (either a tool that is .030" wide with a flat nose or a tool that has a .015" radius on the end will work fine) make a cut into the iron liner to a depth of .025". Make this cut about 2.5 to 3 mm away from the bore itself (if your bore size is 72 mm your grooves will be at 77 to 78 mm center to center). Check the depth by fitting the .030" stainless wire into the groove - it should stay in the groove with little to no help. With the wire in the groove, lay a parallel block across the barrels raised ring. Run a feeler gauge under it while pressing lightly against the blocks. You are looking for the O-Ring to be about .004" to .005 above the rest of the surface of the cylinder liner. On the head, you'll be putting in two O-rings. One of them will be 1 mm inside the groove on the barrel. Measure the center to center distance of the barrel groove and subtract 1 mm from it. Make the cut in the head there (if your groove is on 77 mm centers on the barrel, then cut your head groove to be at 76 mm). Use the same depth as the barrel. It should also be raised by .004" to .005". Make the second groove near the outside of the head - about 3 to 4 mm in from the edge, but not so that it touches or crosses the water jacket. Use the same depth as the others. After the groves have been cut, size the stainless wire so it fits perfectly at its ends - this is very time consuming to do it well. To do this properly it is a good idea to cut it very close to the final length, then - using fine abrasive paper - sand it to the proper length. You'll be done when the ends just touch each other when the wire is pressed in the groove. Use a new copper head gasket with Gaskasinch on it to help it seal liquids. You can get copper head gaskets from any place that does custom 2 stroke engine building. Another source is Copper Gaskets Unlimited in Arizona. They'll want to know what thickness you want. Copper head gaskets can be made in thicknesses from .010" to .060" or more - depending on what you need your compression to be. Since O-rings press into copper gaskets, it is probably best to keep the gasket thickness to a minimum of .020" to avoid any problems with its thickness. It may be necessary to "fit" the gasket to the studs on the barrel. A small round file works well for opening up the holes. When everything is clean and its ready to go back together, coat the gasket with Gaskasinch and let it dry. After the barrel is put on the piston, put the head gasket on the cylinder, then the head and carefully torque it all down evenly. The O-Rings have to seat into the soft copper gasket. Torque the bolts to 22 lb ft in a triangle/criss-cross pattern in several steps. First to 10, then to 15, then to 18 then to 22 lb ft. Then check everything again at 22 lb ft. When it's put together the two inner O-Rings (one on the head and one on the barrel) will seal compression in and coolant out - they should not be on top of each other - but very close. The outer O-Ring on the head will seal in coolant. This set up will easily seal 200 PSI compression if done well. If you're running pump gas try to stay under 185 PSI static (180 would be better on pump gas - especially if you have an after market high output pipe) compression to avoid problems with too much pipe effect and/or detonation. This set up will seal the top of the motor - so if there are other problems, they might only now be noticed. The head gasket is reusable if you heat the part up until it red hot with a propane torch then dip it in water. This will make the copper soft again. You'll need to remove carbon from its surface with fine Scotch-Brite.