Tri Z Engine Build

[El Camexican]

YTZ drew- Is correct, it is called "Dwell"

I have heard that term before, with ignitions I think. Is the meaning the same?

[El Camexican]

So how many degrees ATDC is your exaust opening? according to your wheel.

86 degrees. The bottom is slightly lower than the crown at BDC

[El Camexican]

You have seen the right side intake pics of Z cylinders blown apart right El? They love to break around the cylinder nut area,,some very impressively!!

Bad things only happen to bad people. I honestly wonder if there doesn't need to be something else wrong before they let go, i.e a weak stud, missing or loose alignment rings OR... (and this concerns me a lot) ... a misaligned engine.

When I dropped my engine in the first time It looked like the upped head to frame mounts were missing a washer, but then I see that others are the same and tapper to the bottom. It's like the factory made them like that. Anyway, when I was removing mine this time I noticed that the head was pushing against the right side and that there was a gap on the left. So the question is why? Tweaked frame? I should tighten that connection before tightening the rest of the mounts?

Don't know, but I do know I'm not going to run it like that again. All that force transfers to the cylinder base. Add heat, vibration and maybe a bottomed suspension and perhaps that's what's causing some of the failures. I plan to shim the upper mount after the lowers are tight and eliminate as much stress as possible from that area. My recollection is that my right side boyesen port (learning the lingo eh) isn't too different than the stock hole. I'll be sure to post photos.

[onformula1]

Keep in mind frames do stretch, which is why Supercross & Motocross factory works bikes get a new frame every 3 races. Now think 1985-1986 trike there can be some movement.

Works bikes have the engine mounts and head stay mounts shimmed by hand by a factory mechanic. I always shim everything, some of my buddies would break the crap out of frames and when you would check them some of the welded in motor brackets would be 1/16" off, sometimes per side!

They would just tighten them up and not think about it. One race at a pro level and there's a crack starting. Check and shim if needed, start at the bottom mount and torque, go to the front mount and torque, then torque swing arm mount, last is the head stay and torque. (You don't shim the swing arm bolt)

Shimming also cuts down on vibration and on big bores using larger motor mount bolts helps with vibration too. (YZ490 trick was to drill out to 9/16" SAE) Worked on my CR500 sand drag/dune bike too.

[El Camexican]

Great points onformula1. The mounts on the Tri-Z are small and flexible compared what you see nowadays on some bikes. The mount bolts on my KTM are so tight it's like they reamed the frame and engine holes together and all hardware is 10mm on a bike built to be light.

Obviously the swing-arm bolt on the trike is the boss of who goes where as it relates to the rest of the mounts, but the front mounts are flexible and all the other bolts are 8mm. The more I think about it the more I'm convinced that a stressed upper mount could cause the cylinders to crack when weakened by the added hole. I may punch out my mounts to 10mm as well.

[3 Wheel Drive]

I'm pretty sure the Tri Z jugs cracked from that 2nd boost port. I've started a thread about the phase 2 wrench report mods a few years back and got a lot of replies from Raffa, Zmpolaristech, Mosh and many other Z guys.... You'll notice outside the stock boost port that its thicker than the other side without the boost port.

Before drilling out the 2nd boost port it is suggested that you lay down a good amount of reinforcing weld to strengthen the jug. In one of Moshe's builds he has large pics of the weld for this purpose. I added my own port without any weld, then off to get sandblasted and powder coated.... When it returned I noticed a very small pin hole in the thin area which was supposed to be welded/ strengthened.

The only Z guys that I know of who cracked jugs are racers, I accidently deleted that thread while editing my post... It had a lot of good info from the 3ww pros.

[onformula1]

I may get some flack for this but- A KTM is a hand build and/or assembled bike to a very high standard, when I raced for Team Green Kawasaki I only got support for the 250, 500, open four stoke, Open Pro classes. No 125's at GP's I liked to race a ton of class so I raced a KTM 125 with a little support from KTM through a dealer.

After a long Grand Prix the KTM 125 only needed a good wash and a clean air filter, the Kawasaki's needed much, much more work, but boy did they work great and were very fast!

[Billy Golightly]

Here is my .02 on things;

Ronnie ran high compression in the Zs and I think the main reason it worked right was because of his head dome design he used, and probably timing as he was always dickin around with it.

As it was explained to me, the leading cause of preignition and detonation in cylinders is the piston done, and the arch in the bottom of the head dome being at uncommon angles. This is a little bit difficult to invision without a drawing, so if necessary, use some paper. You've got the squish band which can be anywhere from 10% of bore to 60%, that is the outside ring of the dome that has an upward slope, but not nearly as aggressive upward slope as the middle of the dome. What happens is, if the squish band is at an uncommon angle (Say, it has less slope than what the angle of the piston dome is) You'll actually create a choke point and trap hot gases on the outside edge of the dome in the head and around the piston edge.

At some point, the dome dimensions on most 250cc pistons Wiseco (Maybe others also) made changed - this I was told but I do not know from first hand measuring experience. Probably for manufacturing, or different design ideas, whatever. In any case, A LOT of old original head dome designs are setup for the old piston dome dimensions. So, when you throw in a new piston with a different dome dimensions, and a more aggressive arc, it makes that pinch point.

Here, I know this probably wasn't making sense if you didn't know already what I was talking about, so here is a diagram.

So anyways - Ronnie had a special head design he used that he said was a variation of a Paul Turner 250R dome pattern that he copied into a CNC program when he was machining Tri-Z heads, and that is what he ran on his and a couple other of the engines he built. I'm lead to believe that *most* issues people encountering with detonation when most other things are already in check, is caused by this issue. And piston domes probably vary from brand to brand, and even production run to production run. Has anyone here ever taken the time to actually check these angles when building a motor? I know I haven't.... I mean in theory, all you have to do is measure the dome angle on the piston, and machine your squish to match (or exceed, I don't know which would be better performance wise, I would think exceeding it).

I probably should have checked this on the rotax since I'm using an entirely different design of piston in it than what was intended originally.

[YTZ drew]

I have heard that term before, with ignitions I think. Is the meaning the same?

Same term, different meaning in the context of ignition systems.

In an inductive discharge (most battery-operated) ignition systems, dwell refers to the duration of primary circuit "on" time, measured in degrees of cam rotation (envision the 4/6/8-sided cam in an old points-type distributor). Nowadays nobody talks about dwell since we don't have to adjust point gap anymore. Dwell is now a function of rpm and the electronics in the ignition control module.

[fabiodriven]

William that is an exemplary explanation and I can totally see the knock, why it would happen, and where it would occur. That explanation of pre-ignition is something I will take into consideration henceforth when building a motor.

[Billy Golightly]

A kind of Ghetto way of testing the theory in an engine that has that problem (Without having to try and machine your own head or come up with a turning pogram for it, is to pull the piston, just check the up in a lathe, and cut a flat groove around the sides of it in the width of your squish band. Smooth off, and see. You'll be lowering your compression so its a double edged sword (and that alone will help with the detonation issue) but it'd be a method of trying it out.

I kind of did the opposite on a CR500 piston one time with a theory of balancing the engine by removing material off of the piston was easier than doing it to the crank and rod assembly. Worked too, smoothed it out a TON.

[ironchop]

THIS should be a sticky in Mad Scientist or something.

I just saved the link to my laptop for future reference.

[ironchop]

At some point, the dome dimensions on most 250cc pistons Wiseco (Maybe others also) made changed - this I was told but I do not know from first hand measuring experience. Probably for manufacturing, or different design ideas, whatever. In any case, A LOT of old original head dome designs are setup for the old piston dome dimensions. So, when you throw in a new piston with a different dome dimensions, and a more aggressive arc, it makes that pinch point.

So anyways - Ronnie had a special head design he used that he said was a variation of a Paul Turner 250R dome pattern that he copied into a CNC program when he was machining Tri-Z heads, and that is what he ran on his and a couple other of the engines he built. I'm lead to believe that *most* issues people encountering with detonation when most other things are already in check, is caused by this issue. And piston domes probably vary from brand to brand, and even production run to production run. Has anyone here ever taken the time to actually check these angles when building a motor? I know I haven't.... I mean in theory, all you have to do is measure the dome angle on the piston, and machine your squish to match (or exceed, I don't know which would be better performance wise, I would think exceeding it)..

A smart feller might be inclined to offer a matching domed piston with some of these new aftermarket heads out there.....as a matched set.

[MTS]

So billy peaked my curiosity, Dug out a old wiseco piston some lay out ink and valve grinding compound, Not exactly scientific but you get the point. Stock head vs a sprock dome, Both look like they match to me.

[NeverLift]

El I've got a stock cylinder with a cracked & rusted sleeve that I will give you for the price of shipping. If you want one to play with before you attack yours.