Break-in (aka: wear-in). Is it necessary?

[Pete_Muller]

I'm peeling this topic of discussion out of Rainman's thread on the break-in cart he built, which is very clean, simple and effective.

I feel this topic should probably be on it's own, so that people can discuss what he built without intervention (by me, in this case).

I will continue to add to this thread with things I have learned over the last almost 50 years in the sport and building engines.

-- -- -- -- --

After I had been in karting for a good number of years, I came to realize that "break in" was essentially "wear in". My conclusion was that if I could figure out all the things that needed to be "worn in" before an engine would really run well, maybe I could make an engine run as good as it's going to run from the first moment it's started.

It took me a while to figure out what had to be done to accomplish this (my testing was primarily on a Yamaha, because they are cheap and it's one of the classes I was racing at the time). Essentially, understanding how to ...

- distort the bore of the cylinder for honing EXACTLYl the way it's distorted on an assembled engine
- understand the shape of the piston for a given application (how much cam and how much taper)
- how the ring needed to be prepared (flatness, fit to the bore, end gap, tension)
- how to assemble the engine exactly the same every time so all the above could apply

Once I was able to predictably and consistently do the above, I was able to go to a road race and the first time the engine would start was when the green flag dropped. Zero "break-in", no need to run it rich or easy. Note that this was not accomplished by running things "loose" -- quite the contrary: I ended up running less piston clearance than I had ever run.

to be continued ........

PM

 

[Pete_Muller]

There are just a few key things that make a significant difference. We can start with one item:

1 - Piston taper (or let's say: how much taper is required/ideal) is primarily determined by a couple things:
> How radical the exhaust pipe is
> How heavy the kart is, and what the gear ratio is (in other words, how fast it goes through the rev range)
> To a lesser extent, things like ignition timing, combustion chamber shape, and how the carb meters can also add to this

The more radical the exhaust pipe is, the more power is produced over a narrower rpm range (typically). More power means more heat, and more heat means the crown of the piston gets hotter, so the piston grows more at the top. Consequently, the piston really needs more taper (at least ideally).

Ditto on gear ratio and weight -- the heavier the kart is, and the taller the gear is... the slower the engine is going through the peak power range, which means, once again, more heat into the crown of the piston, so the piston needs more taper.

sidenote: The main reason for carb "triggers" (in road racing) isn't to lean the engine out at the top... it's to allow flood the engine with fuel through the mid-range to keep the piston alive, and THEN putting the mixture back to what it should be for over-rev / top end.

An example of the extremes:
- A very light, direct-drive go kart, with a mild/wide-range exhaust pipe, and a really low gear.
- A very heavy Yamaha enduro kart, with a crazy tall gear, running at Daytona, with an exhaust pipe that puts out a ton of power over a narrow range.

The general solution is to just measure the piston, and change the clearance until the engine doesn't seize.

The problem with that approach is, in the "high load, radical pipe" setup, the typical piston has to have a lot of clearance, and that beats the crap out of the piston skirt. That causes the piston to get "barrel shaped" fairly quickly, and the more the piston rocks, the quicker the ring wears. It goes downhill quick -- the more the piston rocks, the quicker it wears even more, then rocks more, etc.. That why a "fresh" piston is usually better/faster, but doesn't last very long (probably just one 45 minute road race).

to be continued......

 

[Bradley71]

I used to buy used leopard pistons and run in my 116cc motors.20$ rebuildsguys would run em 1 race i ran em 15.i had a epiphany when i looked in a kt100 that had a piston pin come out and grooved both sides of cylinder and the guy called his engine builder and he said do you know how fast that piston travels,it dont care.

 

[rainman]

Following on this topic, Pete, I have had builders recommending to break in an engine with non racing 2 stroke oil to produce that wear and some recommending the use of the same mix used on racing days. Your approach goes several steps further, but unless someone has your knowledge or experience, or someone sharing valuable info like you always do, it can be very difficult for most to be sure on how to build the engine to avoid the break in or wear in step. I also wonder how many pistons, cylinders, etc you have gone through on your karting career to get all that info. Not everyone would share what ave acquired with so much effort like you do. Thanks once more.

 

[Pete_Muller]

So back to this subject...

As many racers have probably done: seize a piston, pull the head and barrel off, and 9 times out of 10 (more like 49 times out of 50), the seizure will either be straddling the ring groove, or very near the top of the piston (and usually near the exhaust port). We have all experienced this.

I had gotten into building engines (for others) around that time the KT100 was introduced (late 1970s). On Yamaha engines, the galling of the aluminum that leads to the seizure appeared to typically start above the ring, and near the exhaust port. A band-aid to this issue "where the seize appeared to start" was to pick the piston up in the lathe, carefully shim it around in the chuck until the ring land (essentially the top portion of the piston) was indicating maybe 002" or .003" high right at the center of the exhaust port. Then use a tiny tool and just skim the diameter above the ring groove until it was round again. This would essentially remove .002/.003" right at the exhaust port, and nothing on the opposite side.

By the way, this is not my novel idea, as Steve O'Hara was "re-shaping" the top portion of McCulloch pistons before I was doing this on Yamahas.

Skimming the pistons above the ring groove (in the lathe) became a very standard part of my engine building, as it definitely seemed to reduce engine seizures. Some time around 1985-1987, I went to Australia to see some kart racing and see a few shops and engine builders. I connected with one very good engine builder, and as many of you probably know... the racing there (at least back then) was all direct drive. Very mild exhaust pipes, low gears, light karts. I spent some time in this engine builder's shop, and was surprised to see how little piston clearance he was able to run -- that was a major "aha" moment for me. Low load, and a mild exhaust pipe which produced less power but over a much wider range. As I recall, he was running .0015"-.002" clearance. Things were making more sense, and that setup undoubtedly caused the piston crown to run at a lower (and steadier) temperature.

Aha, the lightbulb came on, I realized that it wasn't simply that direct drive with a mild pipe could run less skirt clearance (which WAS the case), but the size of the piston at the top vs. the bottom was directly related to how much heat was being put into the crown of the piston. In other words, the TAPER of the piston was really important!

(as a sidenote, I also spent some time with the person in Australia who was making KSI pistons, so ended up working a deal to bring those pistons into the U.S.)

Since I started bringing in KSI pistons (and using them in the engines I was building for customers), I would measure every piston at the bottom of the skirt, and also right below the ring groove. I would write both those sizes on the box. There were (and are) slight variations in taper in most all brands/types of pistons.

I was building quite a few engines at that time, so enduro racers (more load, more time spent at peak torque/heat) got the pistons with the most taper, and sprint racers got less. That allowed me to set up most engine with about the same clearance at the skirt.

I experimented with trying to re-shape the top portion of the piston a bit (just fine sandpaper with the piston in the lathe), but this was not what I would call a repeatable process. In the late 80's, I bought my first CNC milling machine. Back then, high-end CAM software was not something I could afford, so most "code" ended up being hand-written. I played around a bit trying to machine (lightly skim) the top third or so of the piston by spiraling down around the outside using a lollipop cutter. The idea was of course to increase the taper in the piston (just slightly). I had limited success... the machine was not precise enough, and hand-writing code for a cam-shaped taper was painful, to say the least (it would be fairly simple to do now though).

OK... enough for now, and I'll continue in the next message on the learning process to be able to assemble an engine was minimal distortion, which was another key part in getting piston clearance down.

to be continued...

PM