This thread contains multiple sections about porting horizontal Minarelli two-stroke (1E40QMB, Yamaha Jog/Zuma, Polaris 50-90cc youth ATVs) crankcases / carters. If you are more interested in port matching the cases to your cylinder, check out THIS THREAD instead.
190mech's First Trench Cut Cases
The first trench cut modifications made by a forum member, including the theory behind trench cutting. Large, aggressive, trenches. CLICK HERE to go directly to the post.
90GTVert's "Basic" Case Porting
This was the first case porting I tried. At the time I didn't want to break through the cases, so I kept it very mild. Having done more cases since then, I find it better to fill the cases and expect to cut through in spots. Then you can cut the trenches however you think they'll flow best, without worrying about case thickness. CLICK HERE to skip to the "basic" porting post.
90GTVert's Larger Case Trenches
This section has lots of pics of the process and also shows a milder and a more open style of trenching around the cylinder stud area. CLICK HERE to skip to it.
Pics And Snippets
This section contains more pics from forum members and notes for additional information or inspiration. CLICK HERE to navigate to it.
Lucass's Case Work
Very well done case porting and smoothing. CLICK HERE to navigate to it.
90GTVert's Trenching And Port Matching Video CLICK HERE to check it out.
Last Edit: Jul 21, 2019 8:12:53 GMT -5 by 90GTVert
I did a web search about case(carter) tuning and found quite a few tuners did a trench cut from the reed area to the main transfer entry,guess the theory is when the reeds open the second time during pipe suction,the mixture flows smoothly to the main transfers..Here is a pic I scrounged to illustrate the mod;
You will find that cutting the case like i showed you there will make a massive difference Dont worry about the lowering of crankcase compression its not an issue with the modern pipes we have they have such good scavenging they will actually open the reed block as well. While opening the reed block is a good thing if you didnt need to then you would get more gain again. Tightly compressed cases have less lowe end torque than a more open case and a very narrow power band all good for a cvt but the they dont actually make any more up top than a normal case so its a case of no gain what so ever When you consider the weight of air which is huge you see the need to keep the flow headed towards and up the transfers. Every time you try and change the direction of air it loses some of its momentum and since you have already paid for that momentum it doesn't make sense to do this
Beadblasted and epoxied the outer cases today so when I begin to cut the transfer and 'trench' port there is no worry about going through to the other side.
Down loaded pics that Wax posted elsewhere and started roughing out his 'trench cut' mod;
A few more" Wax trench cut" pics;
As you can see a bit more epoxy and finish work is needed!Plan on fabing a wedge shaped reed spacer to direct the charge towards the transfer tunnels..
Here are some pics of the 103 project,cyl to case fit;
Not sure if it is a good idea or if it will even do anything, but I cut a baby trench here. I'm not looking to go wild and have to fill. I measured the depth of the bolt hole, and only went this far to leave plenty of the floor intact there. I figured it may have some small effect, not really sure though.
Since it's hard to see... The green areas were cut down to match the boost port.The red area was cut down to be level with the reed inlet area.The blue area was cut away and rounded.
A couple of things I was considering...
I might cut this area away a bit to hopefully allow more flow to the boost port.
I might cut this area of the cylinder just slightly to help flow to the transfer through my mini-ditch. I would mock it up with the piston in place just to take a look first, although I'd imagine that little bit would be safe from what I've seen others do to their cylinders.
I think I'm going to round this area to smooth flow up to the transfer.
This side is pretty much done. There are some scratches that should be sanded out, but I'm tired of filing and sanding right now.
Untouched vs matched cases. I had to remove a good bit from the transfer. It looks like I removed nothing from the boost port, but I removed a bit there to match it up to the cylinder.
Here are views looking toward the transfers on the modded and unmodded sides. Nothing great, but you can see a bit of difference.
I didn't end up doing much to the other case half. I matched the ports, but I couldn't cut even a mini-trench in this half. It would have to be filled because it would cut through. It's a shame they weren't both constructed the same way. I still need to sand both cases a bit, but they should be about done otherwise.
I cleaned up the cases so I could apply JB Weld to the areas where trenches will be at some point and let it cure. If you decide to go with JB Weld, get the standard JB Weld and not the JB Kwik quick-setting stuff.
Random tip. If you grab a pack of plastic forks, knives, and spoons at a dollar store, you've got all the mixing and application tools you'll need for a very long time. You can break some of the prongs off of forks to get into tight spots if needed too.
Filled the large half.
Then cleaned and filled the small case half as well. When filling the cases for trench cuts, you should be able to tell which areas are directly behind the areas you'll be working in. Be careful not to get epoxy into any bolt holes. Also remember that the epoxy may run (JB Weld does), so leave the cases level or in a position where filler would move in the direction you'd like as it cures.
I started making the trench cut in the large case half. I wanted the trench at least partially in there before any attempt at filling my previous blunders while port matching would be made. (See the port matching HERE.) Here are a few looks at the cases before any trenching.
If you want to stay on the safer/milder side and not break into the cylinder stud passages, measure the depth of the hole and then mark that plus a couple mm onto the case so you know where to stop cutting.
I started out with the milder cut like I normally do at first, and here's a look at it after. I went fairly deep without ever breaking through to the JB Weld on this side.
Then I turned my attention to the small case half. Shots of this side pre-trenching.
And some pics after the more mild version of trenching, keeping the cylinder stud area intact. I broke through into the JB Weld pretty quickly on this side.
Later on, 190mech answered my question about sealing broken-through stud passages. If you wish to cut wide trenches as you can see in his post, you have to cut into the area where the cylinder studs are a bit. This means you would create a potential case leak past the threads. I was told to use Permatex #2 or #3 to seal the studs in these passages. (I have since used Permatex #2 and it sealed.) I went ahead and tried the wider trenches for the first time.
If you've seen the huge tunnels that 190mech cuts in, these probably still don't look that big. I looked at the reed block and I wasn't so sure there would be much benefit going farther with how the block sits. Perhaps with a spacer under the reed block to aim it at the transfers better it would do more?
I also checked and the stud comes through the hole, so cutting farther may not be great for that reason either. Pics of that soon enough.
You can see here that I had a bit of a bubble or air gap in this spot where I filled the cases. That will need to be filled.
Next, I moved back to the large case half to enlarge the trench.
Then I applied a little JB Weld to the pinhole in the small case half. I started looking at the cylinder stud hole and thinking about filling it partially so it wouldn't create any sort of turbulence or dead area. As it is, the stud comes out through the opening though, so without cutting it down or stopping it from getting that far it would want to push the patch out anyway and the stud itself is not smooth with the trench. I don't really think it would make enough difference to be worth the effort, and on top of that I don't have an M7x1.0 tap or chase so I'd rather not work that close to the threads with JB.
After the JB Weld cured, the area was sanded down. Repeat if you don't fix the holes the first time, and try to push the filler into the hole. Make sure the cases are sitting so filler will run into the area.
Later I rounded off the corners that lead into the boost port area on each case half as well.
I may smooth some of it out a little, but basically the case stuff should be done.
Today finished trenching the cases. Tried to copy 5BM cases and I am quite happy with the result. Used almost a whole package of JB weld. The case halves are quite symetric to the eye (haven't measured things though, don't think thats necessary).