Question on numbers stamped on piston

[7SPIDER7]

My 2004 GTI top end had been rebuilt before I got it, about 4 years ago. I want to ensure I get the right rings for a refresh. I have low compression now and pistons and cylinder walls look good. The pistons are stamped with 010-817-05. I believe the 05 at the end dictates the .5mm over bore. But what about the 817? It seems like standard pistons are 010-917. What is the difference between the 817 and 917 numbers?

I have ordered 82.5mm rings, I assume that is correct.

Attached is an image of the stamp on the piston head.

 

[mikidymac]

First off if your compression is low you have something else besides just rings. I haven't seen one that just needed rings. If your compression is low you need to have your cylinders and pistons measured professionally to make sure they are within spec. If it is within spec new rings for those pistons are WSM #010-917-05 and PWC Muscle can get them for you @hfgreg.

 

[GGuillot]

You should measure to be sure, but if the piston and cylinder walls still look good you might have a worn cylinder bore and an increased ring gap causing the low compression. Just putting new rings back in that are the same size with the same pistons might not raise compression. What was your compression before?

 

[7SPIDER7]

Thanks for the quick replies!!!
Compression before was 90 psi on both. There was considerable ring gap on the rings that came out. I measured the bore with a caliper and it measured 82.5mm, which is what it should be with .5mm over bore. Granted the caliper is not the best method, it's pretty darn close. Ive ordered the new 82.5mm rings and look forward to seeing what kind of gap they have. All I can think is that the previous owner put wrong size rings in.

I'm still curious what the difference between 010-917 and 010-817 is???

 

[Matt Braley]

Sounds like another bad compression tester.

 

[7SPIDER7]

I don't think so. First test was with rented set from autozone when I first realized there was a problem. Then as I started diagnosing the problem I went out and bought a new set and got the same 90 lbs.

 

[Cheflen]

Did the engine start? Did you try water testing the engine?

 

[Cheflen]

Auto zone testers only read 90, they are junk I have tried multiple ones. I would try a water test if the ski is all together to see if it has power

 

[7SPIDER7]

I did water test. BTW it cranks and runs fine out of water. Water testing proved hard to start and when I gave it gas in open water it would not go on plane. It felt like it was running at 1/4 throttle.

 

[mikidymac]

Thanks for the quick replies!!!
Compression before was 90 psi on both. There was considerable ring gap on the rings that came out. I measured the bore with a caliper and it measured 82.5mm, which is what it should be with .5mm over bore. Granted the caliper is not the best method, it's pretty darn close. Ive ordered the new 82.5mm rings and look forward to seeing what kind of gap they have. All I can think is that the previous owner put wrong size rings in.

I'm still curious what the difference between 010-917 and 010-817 is???

010-917 are the rings and 010-817 is the piston.

 

[7SPIDER7]

010-917 are the rings and 010-817 is the piston.

Thank you sir!!

 

[7SPIDER7]

OK, got the new rings, which looked much better and new gap was in spec. However I'm still at 90 psi
I think I'm off to get new cylinders and matching pistons now.

 

[mikidymac]

Hate to say I told you so but, well nevermind.

Don't "get new cylinders" just have them bored oversized and matched to new oversized pistons. I suggest Group K and Full Bore for the machine work but keep in mind you are now in the busiest season for any shop so it might not be a quick turnaround.

 

[7SPIDER7]

Yeah, I ran my cylinders by the local machine shop today and they confirmed I have a warp in my cylinders. I'll keep all posted on next steps with new cylinders. I think I will go new as one of mine was cracked too. When I say new I mean new to me. If anyone has a set of .5mm over they want to sell at a good price, let me know.

 

[mikidymac]

Contact Nick or Jess at Westside Powersports they are great and honest. Westside Link

 

[7SPIDER7]

OK, I got new cyinders, pistons and rings from the good folks at Westside and just got finished installing everything back. I'm still at 90 psi on both cylinders. UGH!!!!!! So what do I do now? I did check the timing earlier and it's right. What else could it be?

 

[GGuillot]

OK, I got new cyinders, pistons and rings from the good folks at Westside and just got finished installing everything back. I'm still at 90 psi on both cylinders. UGH!!!!!! So what do I do now? I did check the timing earlier and it's right. What else could it be?

Are you sure it's only at 90? try another tester.

 

[7SPIDER7]

You know, I had a few choice words when I saw your response. I've used 2 testers and the one I was currently using was brand new. But I said to myself, you have to follow protocol. So I rode down to Oreily's and rented one. Guess what.....150 psi....shocker right. Thank goodness because after replacing the cylinders and not improvement I was about to pull my hair out.
Thanks to everyone for you ear and expertise, especially for the into to the guys at Westside.
Now to enjoy the water!!!

 

[GGuillot]

You know, I had a few choice words when I saw your response. I've used 2 testers and the one I was currently using was brand new. But I said to myself, you have to follow protocol. So I rode down to Oreily's and rented one. Guess what.....150 psi....shocker right. Thank goodness because after replacing the cylinders and not improvement I was about to pull my hair out.
Thanks to everyone for you ear and expertise, especially for the into to the guys at Westside.
Now to enjoy the water!!!

I hear ya, I don't understand what it is with these compression testers lately, Doing my car hobby stuff, I bought one at Autozone over 20 years ago, and it hasn't let me down. Probably been 10 years since I've pulled it out a drawer. Tried in on the GTX rebuild I just did, new cylinders, pistons, rings, etc. It read 150 per cylinder.

Even the cheap stuff back then seems much better than what's being produced today. Glad it worked out for you.

 

[7SPIDER7]

I just back from water testing at the lake. She fired right up and idled strong at the dock and through the no wake zone. When I gave it gas it wanted to bog down. It acted like it wanted to take off a time or two but basically just did a little over idle. After a bit longer it died and would not crank again. Got it back home and it would crank, but not very smooth. I'm thinking carb. What is the best trouble shooting there?

 

[GGuillot]

787 or 720? I'd get a genuine carb rebuild kit and do it by the book, pressure test, return OEM settings, etc. You can follow the 'how to carb rebuild' link in mikidymac's signature on this thread. If that doesn't work look at the Rotary Valve clearance, I'd at least check it if you got the carb off. Fuel selector could be sucking air, cheap to replace that. Also cheap to replace fuel lines with clear so you can see fuel delivery to the carb while it's running.

 

[7SPIDER7]

It's a 720 and the fuel lines have already been replaced with clear ones.
Checking out the video now.

 

[7SPIDER7]

Wanted to give a update. I rebuilt the carbs with genuine Mikuni rebuild kit. Took it out on the water and it ran fantastic!! Only problem now is, it appears to be running too lean. It's difficult to start once it's been running and the spark plugs are grey. I'll start a new thread for that issue if needed.

 

[Sportster-2001-951C-Stock]

It's a long thread, sorry I didn't read it all but piston ring lands also wear and stacking up the tolerances a little here and there will drop compression..

Also don't forget thickness of cylinder base gasket and measuring squish.

 

[Sportster-2001-951C-Stock]

What is the best trouble shooting there?

Sounds like your carb(s) aren't flowing off-idle, low speed fuel circuit and pilot holes are likely restricted and need cleaning (THOROUGH, not cursory cleaning). Don't forget to confirm metering needle valve pop is correct and these carbs aren't forgiving of off-brand parts or the wrong parts.