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piston question xl600

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I have a xl600r '86. The thing had a spark knock under load in upper gears so I started looking for the reason. I think I found it, the P.O. had put a weisco 11:1 piston in it. I want to go back to stock, or close. I was hoping someone out there knows how deep the recess in the stock piston is. My jug has been bored to 1mm over, finding a piston is tuff. I am not above turning my weisco piston but I want to know how deep. I know it is not the best answer, but if the piston has the meat, I think I can get pretty close I would go a little less than stock just so I don't lower comp. to much. Is the recess in stock piston flat or not? I have stock exhaust and stock air box, I just don't want to mess with jetting and breathing if I don't have to. Thanks for any help

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""I was hoping someone out there knows how deep the recess in the stock piston is. My jug has been bored to 1mm over, finding a piston is tuff. I am not above turning my weisco piston but I want to know how deep.""

I don't know what this means,,are you going to chop some of the Piston or something. New XL pistons aren't hard to get and neither's a new sleeve should you need one..been one for sale for ages down here for a hundred or so..sleeve that is..

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thanks Horri, yea I own many machine tools and have no problem with machining top of piston, I don't mind keeping the large bore I just want to lower the compression. I know the stock piston is not flat, but has a recess in the top,I just don't know how deep it is,I know it has a ring about 1/4 wide that is flush with top of jug then a recess, just have not seen a stock piston to know depth, or if convex, the depth at outside diameter and if center of piston is same plane as outer edge of piston. I am just trying to get rid of knock (detonation) I could just take .03 off top leaving outer 1/4 inch as is and probably be fine but if someone knows shape of piston top, that would be helpful.

Edited by thumpharder

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My stocker measures roughly .062" deep with .150" raised band aroung the perimeter. Rejetting is the better option IMO. I have 102.4mm Wiseco and XR500 small chamber head without any knock. Check your cdi too. May have melted somehing internal and f"ed up the spark curve.

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thanks valvesrule, yea I've tried a new cdi and an old one no diff. is that .062 flat across top ? no concave or convex shape? thnx alot for info

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Dead flat, though I'd be tempted to put a cone shape in. XR250 pistons have rounded domes to aid flame front propagation and improve quench. I.E reduce flame hitting the piston at direct 90 degree angle.

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Thanks valvesrule, that is a good idea, I think I'll run .062 deep for the first .25, then dome it up to center, ending about.015 below outer ring that is flush with deck

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Well, I am a retread newbie with motorcycles, but I have a lot of engine background. Add some Moroso octane booster, and if that cures your knock the compression is the issue. I don't think you will get there with just jetting, but I could be wrong. With todays gas, that shouldn't surprise anyone. Measure the volume at TDC and you can pretty easily figure out exactly how much you need to increase it to alleviate the knock. If you don't have a burette (graduated tube you fill with water and then see how much it takes to fill the combustion chamber with a piece of greased glass or plexiglass on the head) you can probably got to any community college and they will have one in a chemisty lab. The cylinder portion is pretty easy to calculate.

Good luck.

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Well, I am a retread newbie with motorcycles, but I have a lot of engine background. Add some Moroso octane booster, and if that cures your knock the compression is the issue. I don't think you will get there with just jetting, but I could be wrong. With todays gas, that shouldn't surprise anyone. Measure the volume at TDC and you can pretty easily figure out exactly how much you need to increase it to alleviate the knock. If you don't have a burette (graduated tube you fill with water and then see how much it takes to fill the combustion chamber with a piece of greased glass or plexiglass on the head) you can probably got to any community college and they will have one in a chemisty lab. The cylinder portion is pretty easy to calculate.

Good luck.

The XLs were notoriously underjetted from the factory. They're not extremely high compression either. All the RFVC motors have knocking issues, XL600s in particular. Every XL6 I've owned or worked on has had this problem. Many seized in less than 1000 miles. In the 80s the bike mags all ran rejet articles. My first 85 knocked like hell till I rejetted. The dual carb engines are actully much easier to eliminate knocking than the later single carb engines due to the better distribution the dual ports provide. Once you get over the pain in the ass the dual carbs are to work on it's not that hard to correct the factory errors in jetting. Bumping the slow jet up GREATLY helps starting. Shimming the needles at least .030" improves the midrange and upping the mains cools them down for freeway running and stops the seizures. I think the needles are the biggest culprit here, they only have 1 notch. At least replacing the needles with XR needles would be a good start. Depending on the year XL changing to XR carbs can stop the knock, improve hp, overall performance and reliability. Having the experience with XL600s I have I wouldn't cut the compression unless rejetting failed to givee the results intended. As I stated my XL600 motor with an 11-1 Wiseco bigbore piston and small chamber XR500 head does not knock at all. When I built it I feared I would have det problems but it turned out otherwise. And no octane booster or machined piston needed either. We all know turning the piston down will turn the power down too. Just saying I would rejet before even trying octane booster and certainly before playing with compression. Try search, there's plenty of threads in the 600/650 section dealing with RFVC knocking and jetting.

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I have a xl600r '86. The thing had a spark knock under load in upper gears so I started looking for the reason. I think I found it, the P.O. had put a weisco 11:1 piston in it.

I assume this was happening on at least 91 octane fuel? What jets? How is the plug coloring?

FYI, the Wiseco 11:1 doesn't make 11:1....more like 10:1.

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Thanks everyone for the help/ideas, yes 91 fuel, I don't want to have to run booster, had a six pack mopar I built and had to run it......pain in the ass. I have had stock 83,and 86 set up for jets and 62 pilot with 125's in the main and secondary carbs.Plug has light black on bottom of thread diameter, no heat band on ground above electrode, looks hot all the way to base, and insulator clean white after about 150/175 miles.

My thought about turning piston is to take the weisco to stock comp (or close) I figure it's just easier on the engine and doesn't make the heat higher comp. can.

I do remember when I first started riding it, that it ran without knocking, then I changed plug and it started, colder plug still knocked. I found the first plug that was in it and it was gapped at .018, I'm guessing thats how PO kept it from knocking, by keeping plug cold, and it ran fine surprisingly,

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I just put my bike together after machining the top of the piston to match that of stock. I want to thank valvesrule for the info on a stock piston. The bore and piston/rings only had a couple hours use so I didn't want to throw them. I knew taking a little off wasn't going to hurt anything, I machine engine parts all the time for model "T"s and outboards. It worked great, no more ping or knock. Thanks again valvesrule, you "got it" and seemed to understand a little rework is no big deal.

Runs great, and with 1.0mm over, it is a true 600, (601) to be exact.

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I've had to make more than a couple "chop top" pistons myself for the same reasons. With the old pentroof XR500s it got to the point that new pistons went straight from the box to the milling machine no questions asked.

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