Basic Porting - What To Cut

A Close Look Shows Rough Stuff

This 1985 ATC 250R barrel has been bored a few times (it's 1.0 mm oversize) since it was new but it was never ported. Several interesting things are easy to see in these photos - I was lucky to be able to find all of these things in one place.

This is what I call a poor match between the iron liner and the aluminum casting. It needs a tremendous amount of correction.

On the other side of the cylinder, the opposing transfer ports look about the same. They need vertical & horizontal correction.

Piston Scraper

Also what I notice here is the very poor (almost nonexistent) bevel around these ports - even the transfer ports need a bevel to help the rings live longer. Without bevels, the sharp edge of port becomes a scraper. This will remove oil from the piston - not a good thing. The problem gets worse as the piston heats up and the pressure starts to build.

Looking through the exhaust port I can see the rear walls of the rear transfers (5th ports) are very uneven. That's quite an alignment problem.

The cylinder wall here shows grooves. The lower ring (left wall) caused the deepest groove - but both sides of the port have them. This is where the ring end gap rides.


The grooves in the cylinder wall here can be caused by a worn piston, rings - excessive ring end gap, loose T.E. (top end - rod) or B.E. (big end - rod) bearings and even crank main bearings. When these parts (all or just one or two) are worn out there will be excessive clearance. A loose fitting piston can lead to this condition. Worn rings cause the end gap to grow and allow the ring to walk - or rotate a few degrees. Worn bearings also let the piston and rings rotate more and allow for more movement from side to side.

Ports that are beveled incorrectly can cause the ring to suddenly shift to a different lateral position when it encounters a bad spot. Port openings with bumps or ridges can cause a ring to "wear in" and "follow" the groove. This is more of a concern with the exhaust port on this motor since the outside port window is angled about 15 degrees outward. It could drag a ring quite a few degrees this way. At 8000 RPM the piston is going up and down 133 times per second - wear can accumulate fast.

This engines rings were coming out of their grooves at this position and eventually would have broken off - details. There's such a small amount of cylinder wall for the end gap to slide on - keeping it right in the middle of the wall requires a tight top end and good bevels. More modern porting designs use 2 additional (smaller) ports in addition to this engines rear ports and let the end gap cross the open port window. With such a small port to cross it can not bulge into it - or get caught on it - enough to matter, though a bevel is still necessary. This allows for even more transfer port area than would be available without such a design.

This transfer port tunnel has quite a number of casting flaws. The auxiliary port - the feed from the intake - is squared off and sharp and the liner is sharp and uneven.

It's hard to see but the front wall of the main transfer port is very rough - I already discovered the other alignment problem. Porting tools are necessary to get these trouble areas.

The exhaust port - it's in about the same condition as the others - poor. Such a large port is easy to get to and correct.

Looking at it this way - everything LOOKS fine (kind of). The porting can be discovered by making a port map (details).

Index It

It would be easy to "index" this motor by blueprinting its ports after all the worn parts are replaced. With the engine together (mock-up) - degree it in using a piston stop and a degree wheel (details). Using Dykem (metal ink) or a permanent marker (not that permanent on metal), paint the areas around the ports. Using the installed piston (without rings installed) as an indicator, lightly scribe lines (using a pick or scribe) so the transfer port windows will open at the same time at 120 degrees ATDC (stock setting). The boost port opens 2 degrees later. The exhaust port will open at 90 to 91 degrees ATDC. Scribe a line to mark the top of the port. This line will be the "cut to" or "match to" line to use. This barrel needs quite a bit of material removed to make its tunnels match the liner. It would be a good idea to pay particular attention to the roof angles (details).

In the 2 very top photos in this series (and almost every other photo) I can see that the aluminum is cast lower than the iron. It is doubtful that the liner is too high. If this is so I would cut the aluminum to match the liner. Epoxy can be used to fill voids and to correct tunnel shapes. After measuring each ports width it is easy to determine which ports need to be corrected. Matching port widths to its corresponding (opposite wall) companion is most common - match the width of the right main transfer to the width of the left main transfer - and so on.

For many people the time to perform this blueprinting procedure is before the bore job is completed. This can be difficult because everything in this engine was loose fitting and it would be difficult to degree in such an devise with any amount of accuracy. Why not let the shop performing the bore job bore the cylinder and leave the barrel honed tight - to say .0005" to .001" - and may be given to you without the port bevels. This will allow accurate degree wheel indexing and porting modifications to be done and give a safely margin as well - wouldn't want to scratch the new bore. When everything is done it can be brought back to the shop for the final piston fitting.

I can tell by the depth of the grooves in the rear wall that this bore will not clean up with a .25 mm oversize piston since it measures on the large size already. It will be necessary to have a .50 mm oversize piston fit to it. The ports should have nice smooth bevels to them when the cylinder gets its new bore job. It is always a good idea to go over the port bevels with 400 or 600 wet/dry sandpaper (with WD40 or similar on it) in case there is a tiny burr. There is very little chance of damage from doing this and it always lets the rings live longer.

It's easy to see there's quite a difference in the size, shape and aim of the ports between this stock barrel and the modified 270 cc version (details) on my other page. They are both ATC250R barrels (shhhh) except that the 270 cc version (now at 275 cc) is a 1986 model and this one is a 1985.

For those who are serious about doing this stuff and have burned out their Dremels and need a right angle hand piece to get to the transfer ports there's only one place I can recommend - C.C. Specialty Tools (web site is not complete but it has most of their products). They are the experts when it comes to cylinder porting. They have a serious supply of cutting tools for our special needs. The Foredom stuff is really first rate. Their tools make it as easy as it can get. I keep a spare set of brushes and an internal cable around just in case. They are located at 6035 C.C.Lane, Lawrenceberg TN. Their phone number are 931-762-6995 (technical), 931-762-5165 (fax) and 800-762-6995 (orders). Tell them I sent you. They'll get a kick out of it.


A Past Experience Using C.C. Specialty

Funny thing about CC Specialty, the last time I called them up for parts, the guy was angry with me for waiting too long before rebuilding my big right angle hand piece - it seems he takes it personally if "his" products are not kept up to par. I guess I described in too much detail the amount of wobble that hand piece had. It must have struck a chord in him. So he balled me out for a few minutes (in red neck language) and wouldn't let me off the phone without ordering extra parts. I had to laugh. That hand piece worked just fine if you only had to work on one side of the ports - if you know what I mean.

Don't mention to him that you can get the bearings for the hand pieces at a local bearing house - order them from him - it's worth it just getting him on the phone, the guys a hoot. Oh yeah - never, NEVER order parts "on his time" without the correct part numbers or names of the parts - don't throw away the parts list/diagram when you get your tools. You'd think you were calling his family bad names. The guy is a perfectionist. I admire his loyalty.
Rick - Porting is a process of corrections and mistakes...

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