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Top End Rebuild: What do I need? Piston, Cams, Rockers, Springs, Time Machine?

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After picking up a used 2002 xr650l a few months ago, I noticed that the power delivery was very low compared with other xr650l owners reports here. I thought it was a jetting issue, so I rejetted the carb. No gains. I thought it was a clutch slippage issue, so I replaced the clutch. No gains. Upon a reco of some friends in my motorcycle garage, I ran a compression test last night. Bingo.

With the choke disengaged, the compression test showed 75 psi, less than half the stock setting. After shooting some wd40 into the cylinder, the next pass on the compression test netted about 78 psi. It didn't waver, so the prognosis is worn piston rings and/or worse scored piston and cylinder sleeve. Sigh :) Rather than get steamed, I look at this as an opportunity to really get to know the bike and this will be my first top end rebuild. Which is exciting in my wanna-be mechanic way.

I was able to remove the head last night. I plan on removing the cylinder block tonight, so I haven't seen the piston yet to comment on piston condition.

I did have fun riding the bike on 50% compression, so my initial thoughts on goal for the rebuild was to perform this at minimal cost to get back to near stock performance. The rationale was taht with full stock compression the engine should perform 100% better than what I was previously used to. Since I'm doing the majority of the labor, my estimate for cost will be $100 for machining, $50 for gaskets, $150 for piston for a total of $300.

3 questions:

First. Aside from new gasksets, cylinder (potentially), and machining (potentially) is there anything else I would need replace to get back to stock?

Second. I am curious to the cost for really turning this thing into a beast. Overbore, high compression, new cams, new rockers, etc. If I'm already into the engine, it appears to me that any of these mods would be a simple addition. Any estimates on cost? Any recommendations for a particularly well known setup? I'm a weekend warrior, so while I am concerned with reliability, I might be interested in the next level of performance as long as it involves minimal cost increase (and little expense to reliability.)

Third. Since I'm already in the cylinder, is there anything else I should replace? Valves? Springs? Rockers? Chain cam tensioner?

Thanks for the help. I hope to be back on the road and dirt soon!

Cheers.

-Scott

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1.) The ancillary parts that need to be replaced partially depends on how many miles on your bike.

2.) Did you make sure the valves were adjusted correctly? Did you hold the throttle open during the test? Did you disable the automatic compression release before doing the compression test? If not, then 78psi was actually a normal reading. If you did disable the compression release and oil on the rings didn't make the compression come up, then that points to the head.

3.) The cost for a performance engine varies a lot. A bigbore/high compression forged piston is ~$180-200 and a cam is about the same. However, there are only a handful, if that, of cams that can use the stock rockers. The rest need hardfaced rockers and that usually costs $60 per rocker. Porting, if you want it, will be at least several hundred. Full exhaust, $600+. An aftermarket rod with installation will there again, be a couple hundred.

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HeadTrauma,

The previous owner had apparently disconnected the speedometer, so an accurate mileage reading is indeterminable. The odometer reads 2,000 or so miles, but I suspect it's at least twice that based on visual condition of bike.

I'm new to this;) can you explain where the 75 psi in line with specs? The clymer manual indicates that with decompressor disengaged, psi should be 199 psi and with engaged 92 psi. The 75 psi was at read at idle through throttle blipping with the choke completely closed.

I did not check the valves as the compression reading didn't fluctate +/- during the test. I was told that flucations in compression reading would point to valve leak vs. head/piston.

So about another $500 for high compression piston, cams, hardened rockers. Maybe I'll keep it stock.

Thanks for the continued help.

-Scott

1.) The ancillary parts that need to be replaced partially depends on how many miles on your bike.

2.) Did you make sure the valves were adjusted correctly? Did you hold the throttle open during the test? Did you disable the automatic compression release before doing the compression test? If not, then 78psi was actually a normal reading. If you did disable the compression release and oil on the rings didn't make the compression come up, then that points to the head.

3.) The cost for a performance engine varies a lot. A bigbore/high compression forged piston is ~$180-200 and a cam is about the same. However, there are only a handful, if that, of cams that can use the stock rockers. The rest need hardfaced rockers and that usually costs $60 per rocker. Porting, if you want it, will be at least several hundred. Full exhaust, $600+. An aftermarket rod with installation will there again, be a couple hundred.

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So quick update.

I removed the cylinder head and cylinder block last evening.

The piston dome, valves, and bottom of the cylinder head were covered in scorched oil. Somehow oil was getting past the piston rings and into the combusion chamber. A few quick passes on the wire wheel on my bench grinder and all the scorched nastiness came right off these parts. Good news, the piston surface and cylinder wall are completely smooth. I might get away with simply replacing gaskets, valve seals, and piston rings.

Also, the gasket between the top of the engine case and the cylinder block was comletely worn out/destroyed. The gasket appeared to be made of paper and was baked onto the cylinder head. It took me a solid hour to remove this crud with a razor.

So next on my list is to measure the cylinder sleeve to ensure it's in spec and not ovalized.

A few questions:

As long as cylinder and piston measurements are in spec, I shouldn't have to replace these right? Rather, I'd just need replace the piston rings? (And gaskets/valve seals.)

What could be the culpret for oil getting past the piston? Previous owner dumped too much oil in the bike? Didn't change the oil? Ran the bike too hot?

Thanks.

-Scott

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If the cylinder is in spec, you could just deglaze it and do new rings. Make sure your valves are sealing while you have it apart too.

Are you sure it was burnt oil on top of the piston? Some carbon deposit on top of the piston is pretty typical.

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One of the guys in the shop said the deposits were beyond typical carbon residue, and from oil getting past the piston rings. The burnt on gunk was pretty thick.

I'll be sure to check the valve seating too.

What's the best appraoch for deglazing piston and honing the cylinder wall?

Thanks.

-Scott

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One of the guys in the shop said the deposits were beyond typical carbon residue, and from oil getting past the piston rings. The burnt on gunk was pretty thick.

I'll be sure to check the valve seating too.

What's the best appraoch for deglazing piston and honing the cylinder wall?

Thanks.

-Scott

Get a good 3 stone hone and some 10 wt oil. Do maybe 5 passes up and down, third to half speed on the drill. The desired effect is an approximate 40 degree cross hatch pattern.

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Since you saved the money on all of it, and you have it all apart, you should go ahead and put a hi-comp piston and hot cam in! I just did a cam in my XRR and it was a huge improvement in power over stock, well worth the money. You could do the piston and cam for $300 total if you buy the parts off ebay.

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Is it better to go to over-bored piston or hi-comp? A local machinst can do the cylinder block boring for about $50, so minimal additional cost.

If going the over-bored route, better for 1mm or 2mm? And what's the cost to reliability? Would I still be able to retain the stock connecting rod?

Since you saved the money on all of it, and you have it all apart, you should go ahead and put a hi-comp piston and hot cam in! I just did a cam in my XRR and it was a huge improvement in power over stock, well worth the money. You could do the piston and cam for $300 total if you buy the parts off ebay.

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1.) You can go with a high compression piston, a bigbore piston, or a high compression bigbore piston. The latter is a common choice. I prefer to leave room for another overbore in the future, so I personally would not go bigger than 101mm which is good for 657cc in a 650L. Another thing to remember is that raising the compression will yield more power than just increasing displacement by 13cc. 10.5:1 compression should still allow the use of premium(91 or better) pump fuel. 11:1 is getting a little high, especially when using the stock cam.

2.) Yes you can use the stock rod as long as it is within all measurable wear specifications.

3.) There is little or no cost to reliability as long as the machine shop gets the piston-to-cylinder clearance right.

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I think that most everything has been covered. Here are a few extra's.

A high comp piston will reduce life. The engine will run hotter and more forces will be exerted on the bearings and gears due to the larger pressures int eh combustion chamber. The margin of fuel or jetting will be less. Premium fuel will be required and in hot low altitude wide open throttle conditions pinging may be present. If done properly the bike will still be very reliable compared to say a CRF450.

With the cylinder off take a close look at the transmission. Make sure you use good lighting like the sun and/or a good flashlight. Look for gear pitting. When I last had my cylinder off I was planning on a new piston. However, the piston and bore measured good, but I had to spend my $$$ on new second and third gears.

Take a close look at the choke plate if you have the stock carb. It is known to break from metal fatigue. I caught mine just before it broke off. It can do a lot of damage to the top end.

I plan on keeping the stock 9:1 compression because I want the flexibility to run regular gas when I can't get premium and I want max reliability. I don't really need to go 100. The 85 that my bike does now is fast enough for me. My bike spends a lot of time on singletrack in second gear anyhow. Low end torque is what is needed then and the XR600 has plenty.

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Well, based on xr650rs and Headtraumas comments I made an executive decision to go all out on this one. I picked up a wiseco 1mm overbore high compression piston, hot cam, and an ex400 oil cooler to keep temps down. Cleonard, your comments are spot on regarding reliability. I think since I'm not a daily driver, this might not be such an issue.

The new parts should be here late next week. The next step will be to send the piston and cylinder off to the machinist for overboring. Then it's "simply" putting everything back together to spec.

I can't wait to see the power gains with these mods coupled with the new powerbomb header I already planned to install.

Is there a recommended break in process once it's all put back together? What are the steps?

Thanks!

-Scott

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Well, based on xr650rs and Headtraumas comments I made an executive decision to go all out on this one. I picked up a wiseco 1mm overbore high compression piston, hot cam, and an ex400 oil cooler to keep temps down. Cleonard, your comments are spot on regarding reliability. I think since I'm not a daily driver, this might not be such an issue.

The new parts should be here late next week. The next step will be to send the piston and cylinder off to the machinist for overboring. Then it's "simply" putting everything back together to spec.

I can't wait to see the power gains with these mods coupled with the new powerbomb header I already planned to install.

Is there a recommended break in process once it's all put back together? What are the steps?

Thanks!

-Scott

Break in....

My procedure has been tested and proven at work, and at home.

I ride/drive them as normal. I vary the throttle a lot, as to not ride at a set throttle setting for long. Use lots of engine braking, and moderate acceleration. This breaks the rings in evenly.

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Asking people about break-in procedure is like asking what the best oil is. Everyone has an opinion and none of them are the same.

In the automotive sector, piston rings sometimes actually come with break-in instructions.....and they sure as heck don't say to take it easy for 1000mi or whatever.

I personally would first make sure it's jetted well enough to not get hot and then break in the rings with moderate and varied throttle, including a little engine braking here and there.

Also be prepared to do a few oil and filter changes in the first several hundred miles and don't forget to check the frame filter at the same time.

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