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Old November 11th, 2018, 07:10 PM   #1
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Compression Numbers

Hey Ninjette!

Just finished a top end rebuild on a 2004 Ninja 250. After building oil pressure, it started right up almost immediately, then ran it in short sessions. After I was confident it was running well, I let it warm up and adjusted the pilot screws and idle, seemed to be running well.

After all that I shut it down, and did a compression check on each cylinder while it was still warm. I got 120 psi cylinder 1, and 125 psi cylinder 2, which seemed good until I looked in the manual, which said the usable range was 142-218 psi. Before I mounted the engine back in frame I did a leak down test as well and both were holding air well. Cylinder 1 had 30% leakage, and cylinder 2 had 20% leakage, both well in the green, tested multiple times and turned though to be sure.

Wondering what anyone else has seen as far as compression numbers.
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Old November 11th, 2018, 07:21 PM   #2
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That is fine. You need at least 90-100 psi to even get it to run. Stock compression will be below 150 psi. With 12.5 to one pistons I get about 180 psi. I can play with cam timing g and push it to over 200 psi. But that requires 110 octane fuel. I don’t know how they can get the numbers they say are normal. 120-125 is good.
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Old November 11th, 2018, 08:18 PM   #3
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Did you hold the throttle wide open for the test? No offense meant if you did, just checking.
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Old November 11th, 2018, 08:49 PM   #4
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After talking to my friend who blueprints Harleys and drag races Busas for a living, these numbers are definitely unacceptable. That being said, Triple Jim good catch, I learned everything I know for Aircraft, so I forgot to hold the throttle open for my warm compression test, so that should help. Also, the leak down test was a bench test, meaning it was at cold before the new piston rings had been seated (freshly built, no startup) so my leakage numbers should half at warm now. I'm going to try again tomorrow and do a warm leak down test as well, then I'll give my numbers again.

Also Racer X, I know what you're saying, having seen similar numbers from cars and my 2008 Ninja 250 racebike at cold, so I was surprised when the book said 142-218. That's why I made this post in the first place.
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Old November 11th, 2018, 11:08 PM   #5
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If it were my bike...

If it runs and rides well, I wouldn't worry about it.

If I were worried, I would run more viscous oil: 20W50 or so.
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Old November 12th, 2018, 06:43 PM   #6
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Well started right up and warmed up just fine today. Did my compression and leakdown checks after thoroughly warming it up. Not very different from before.

Cylinder 1: 125 psi warm (full open throttle), 20% leak with air past the rings, no leak on valves.
Cylinder 2: 135 psi warm (full open throttle), 25% leak with air past the rings, no leak on valves.

So, not too different. I'm going to finish putting the bike back together though and ride it, see how it does. All in all though for having bought it for $250 with a clean title, with a rusted out top end, partially seized pistons and fully seized rings, this is pretty good. I replaced everything except the pistons and the cylinder sleeves, just cleaned everything and made sure everything was in tolerance or close. Only spent probably about $150 in parts and materials for the top end rebuild.
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Old November 13th, 2018, 12:39 PM   #7
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Maybe after the rings break in the compression numbers will go up.
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Old November 13th, 2018, 04:16 PM   #8
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Maybe, but it's ran for about an hour total time now. That should be enough to seat the rings.
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Old November 13th, 2018, 04:40 PM   #9
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Have you ridden it yet?
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Old November 13th, 2018, 04:55 PM   #10
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No not yet, still a ways to go before it's ready to ride.
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Old November 13th, 2018, 06:00 PM   #11
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Maybe check the valve adjustment one more time?
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Old November 13th, 2018, 06:06 PM   #12
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Quote:
Originally Posted by NinjaBraap View Post
Maybe, but it's ran for about an hour total time now. That should be enough to seat the rings.
Running unloaded is probably not a good way to seat the rings, since there's very little combustion pressure.
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Old November 19th, 2018, 11:52 PM   #13
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Yeah you need to progressive load up engine to seat rings. After that 1st hour, you should to a 50% throttle run to 9k-rpm. Ride another 5-minutes, do a 75% throttle run to 11k-rpm. Ride around for 5-min, do 100% throttle run to 13k-rpms.

Some people advocate using single-weight non-synthetic break-in oil (made specifically without too many friction-reducing additives). After your hour of break-in and full-thottle runs, change oil to full-synthetic.
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Old November 20th, 2018, 12:14 AM   #14
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How long should each throttle run be?
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Old November 20th, 2018, 06:25 AM   #15
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There is no one correct procedure. If you research it, you'll get a different answer each place you look. Generally you need to vary the RPM and make sure you load the engine pretty hard intermittently for the first few rides. Running it an hour at no load may have made seating the rings a difficult task.
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Old November 20th, 2018, 08:01 AM   #16
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I like this way of breaking in a motor.
http://mototuneusa.com/break_in_secrets.htm
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Old November 20th, 2018, 10:01 AM   #17
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Quote:
Originally Posted by NinjaBraap View Post
How long should each throttle run be?
Up to target RPM: 9k, 11k, 13k with progressively larger throttle-openings. Use 3rd-gear and target throttle-opening. When you hit target RPM, stop accelerating and take break. Cruise around changing gears and RPMs. Then do next run, etc.

I've only got 50 engine-builds under me, nowhere nearly as much as the MotoTune guy in linked article. But have noticed exact same phenomenon with import autos and motos using harder break-in than traditional old-school methods.

Change in break-in routine has to do with more precise modern machinery. With old hot-rod V8s using severely out-of-round pistons & cylinders, tolerances were measured in thousandths of inch. Cylinder honing was very rough and deep. You need to use law of averages of many, many strokes to rock-tumble square pistons and bores into round shapes. And you have to be careful about removing too much material or you'll end up with huge piston-clearances and ring-gaps.

In contrast, tolerances on imported autos and motos are measured in 10-thousandths of a millimeter!. They're pretty much broken in and require just some high combustion-pressures to force rings against finely-honed bores to do the last bit of breaking-in.

Questions on your rebuild:

1. What was state of cross-hatching on cylinders? Did you ball-hone cylinders?

2. what was piston-to-cylinder clearance?

3. what ring-gap did you use?


This last part is important because it's difference between worn and new engine. If you have excessive gap with new rings, you've just created pre-worn engine. I'll usually install rings in progressively smaller and smaller gaps with multiple break-ins until I see shiny fretting from where ends butt up against each other when warmed up. Then replace rings with slightly larger gap for no fretting.. Might take 4 or 5 ring installations and tear-downs.

At that point, I consider leakdown to be "good" at 1-2% or less. My wife's Corolla with only 9.6:1 ratio gets 190-205psi on compression test. I think your'e leaving power on table here. As others said, if it runs smoothly, should be fine. You won't notice a difference of 2-3hp with seat-of-pants dyno anyway. But if this was competition engine, I'd do every last possible tweak to extract as much power as possible.

Last futzed with by DannoXYZ; November 20th, 2018 at 03:02 PM.
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Old November 21st, 2018, 11:27 AM   #18
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Interesting....when I rebuilt my 2009 (the last time ), I too was getting about 120 psi cold from each cylinder. The big difference for me though, was that my leak down was at about 4-8% per cylinder, and that was cold!!!

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Old November 21st, 2018, 11:37 AM   #19
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Questions on your rebuild:

1. What was state of cross-hatching on cylinders? Did you ball-hone cylinders?

2. what was piston-to-cylinder clearance?

3. what ring-gap did you use?
I used 45 degree cross hatch ball flex hone from Brush Research.

I just made sure the Top and Second rings were within the manual specs for clearance and gap, with new rings. Don't recall the exact numbers off the top of my head.

This is only my second motorcycle engine rebuild, so still learning. I may have mentioned it before but I learned what I know for aircraft, so having to do multiple tear-downs just to get to those numbers is a new concept to me.

That being said this is definitely not a competition engine by any means. The main goal of this rebuild was to revive the bike and learn, which this thread has definitely been helpful in that for future rebuilds.

A question so I can do a better job in the future though:

1. How do I reduce gap? I know how to increase, but not reduce.

I like your advice probably the most because of how detailed it is, which is very helpful since I enjoy engine work and would like to build a competition worthy engine in the near future. Thanks so much for the advice so far, I'm going to continue to post to this thread to learn more.



Also Triple Jim, your comment about there not being any "right way" and finding a different way everywhere you look is so true. The way I broke it in was based on two similar methods I found that were both with sport bikes. Needless to say after checking my numbers I wasn't too happy, which sparked this post. I'm still learning clearly, but I'm glad I posted since there's been so much help from the community. I must say, Ninjette is seriously the best forum so far I've ever used.
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Old November 21st, 2018, 12:38 PM   #20
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A question so I can do a better job in the future though:

1. How do I reduce gap? I know how to increase, but not reduce.
I worked with Wiseco direct to send me a ring set that matched my specifications.

Another interesting point: if you run forged pistons, if you use nitrous, if you have upped the performance significantly.....you will actually want to INCREASE the gap in order to handle the increased heat/expansion without failure. My gaps are larger.
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Old November 21st, 2018, 12:42 PM   #21
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I worked with Wiseco direct to send me a ring set that matched my specifications.

Another interesting point: if you run forged pistons, if you use nitrous, if you have upped the performance significantly.....you will actually want to INCREASE the gap in order to handle the increased heat/expansion without failure. My gaps are larger.
That's good to know wiseco will work with you. Yeah I knew that with performance builds you need to increase the gap, but the stock rings I used were within specification in the manual. Based on what DannoXYZ is saying though I was curious how one reduces gap.
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Old November 21st, 2018, 01:44 PM   #22
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No way to decrease gap other than plating the cylinder, getting a new cylinder, or new rings.
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Old November 21st, 2018, 02:01 PM   #23
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No way to decrease gap other than plating the cylinder, getting a new cylinder, or new rings.
That's what I thought. So he must be referring to Blue Printing.

That's definitely not what I was going for here, just a running but reliable revival on a thin budget, but it's fun to learn for future more in depth projects.
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