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drkiwi

Dr 650 je high comp piston issue

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Hi all, Just joined the forum after years lurking, as I have an issue that has never been discussed as far as I know.

My dr650 09 is getting a rebuild with 3rd replaced and bottom end inspected and using a je high comp piston. The piston removed is the original Suzuki item. Original top ring gap is still 20 thou with less than 1 thou variation anywhere in the bore as best I can tell, and the cross hatch still faintly visible in the entire bore.

As the bore looks great I went ahead and ordered the je replacement piston, but this is where things get interesting. At 1/2" up the skirt the piston has clearance, unsure as to measurements as I do not have the measuring gear, but seems slightly less than the required 15 thou. The bottom of the skirt is wider and has no clearance, but fits in the bore touching both sides, and will not slide in the bore by gravity alone but rocks freely (no rings of course). Seems the piston manufacturing may be inconsistent, or an anomaly in cylinder manufacturing.

Has anyone experienced this in the past? Is there enough thickness in the plating to hone to the new piston? Can the piston skirt be resized sightly to the bore? I will have the piston and bore measured this week, but would appreciate the knowledge of the old hands in the forum, and this may be of use for others doing the same mod.

Thanks in advance!

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Have you completed your build?  I don't have specific information on this build so it's probably of little value but since no one else has chimed in...  The piston is probably ok, the head of the piston will experience much more heat and will expand more than the skirt will, hence the additional clearance when cold.

Regards,

Ed

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Cheers Ed, I have had the bore and piston measured, turns out it is 3 thou tight from specified clearance, the mechanic said just run with it and it 'should be fine' so that's what I'll do. Still seems odd that a new piston can have tight clearance in an old bore, its borderline but i am a gambler. Got the bottom end back today so will be running late this week, then follow up with a tm40 next month. Keen to see if I feel a difference!

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Grats!  I just got a new DR650 and being a tinkerer I'm already looking at ways to raise the compression a bit, that always adds power across the entire band and mandatory if you increase cam duration/overlap (static vs effective compression ratios).

After I get some miles on it I may get the kit from ProCycle but I'm a grey beard and it's probably got all the power I need...NEVER ENOUGH POWA!!!

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I'd talk to Procycle and see if they will swap pistons with you. Three thou tight is a bunch, I wouldn't run that. Also if your mechanic doesn't know, you should check and adjust the squish clearance and valve pocket clearance on those pistons. They're not just plug and play.

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6 hours ago, nobade said:

I'd talk to Procycle and see if they will swap pistons with you. Three thou tight is a bunch, I wouldn't run that. Also if your mechanic doesn't know, you should check and adjust the squish clearance and valve pocket clearance on those pistons. They're not just plug and play.

I would expect that all of their pistons would be the same, and the anomaly would probably be in my cylinder.  My piston is not bought from procycle anyhow, but good thought! I am going to run the stock cam for now, so interference is a non issue, but will certainly check if I switch to a 223 cam. I have heard of squish clearance but not looked into it, can you explain the process of checking it?

Anyone else have opinions on the bore clearance? Procycle or MXrob may have some insight here too?

Thanks

Edited by drkiwi
Grammar

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7 hours ago, Ezerin said:

Grats!  I just got a new DR650 and being a tinkerer I'm already looking at ways to raise the compression a bit, that always adds power across the entire band and mandatory if you increase cam duration/overlap (static vs effective compression ratios).

After I get some miles on it I may get the kit from ProCycle but I'm a grey beard and it's probably got all the power I need...NEVER ENOUGH POWA!!!

Power is a hard one, what do we really need? In 2 years ive gone from 170k monos with rsv tuono, to wr450 (loved it, crap adventure bike, and 2 pistons in one year 200hrs). Getting too old for hard riding and like my license so the dr is actually not a bad compromise for most situations with bit of work!

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To check squish you assemble the barrel and piston, lay strips of rosin core solder across the piston in the direction of the wrist pin, bolt on the head, and gently turn the engine over past tdc. Measure the solder and you will see how much clearance you have. You want to shoot for a little less than a millimeter, around .035". You face off the bottom of the cylinder to close it. Of course Procycle can tell you the details and precautions...

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I've read a bit about it and typically you can take about 10 thousandth's off the cylinder to get your squish right if you don't measure it as described above, it also raises your compression about 2 tenths of a point.  I've looked around for thin head gaskets to accomplish the same thing as I've done on cars but haven't found any for the DR.  Cometic has some gaskets for the DR but I think their the same as stock since I could find no listing for thickness's.

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Just read a couple of threads on this process, sounds like a good measurement to check. Is there much detonation risk with a looser squish with this mod? With my wr I would just check the bore clearance and gap the rings, and pop it in (wossner). No issues at all with detonation and that was at standard 12.5:1 compression. Maybe I was just lucky!

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I did see a thread where bergdonk was able to adjust his by taking a layer off the metal sandwich base gasket, I wonder if that is an option here as 10-11 thou seems to be a common amount to machine from the barrel. Food for thought 

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There's a lot of factors that go into detonation issues, higher octane gas always helps reduce detonation.  The effective compression ratio is controlled by cam timing so a mild cam with a 12.5 compression ration piston might detonate and maybe even hard to crank over where a cam with more overlap timing might actually reduce the effective ratio to 10.5.  Squish helps reduce it by forcing the mixture to burn faster and controlled and helps reduce detonation.  Lots of factors...

 

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Cheers Ezerin for the new knowledge. Done the squish test with solder and got 39 on the exhaust side and 37 on the intake side, for a 38 thou average (squish areas are fore and aft rather than over the wrist pin).

Not a perfect result as 32-36 is optimum but milling 2 thou seems a little pedantic? This was at 25 ft/lb instead of 28 specified so that may make a small difference when fully torqued down. I will clay the valve clearance tomorrow just to be sure as well.

Now to quiz procycle on the bore clearance issue, to be sure.

 

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Pedantic... good word!  Plus one for you!

Lots of folks don't realize the running comp. ratio is not the same as the static ratio and wonder why they don't see the performance increase they hoped for from a cam change.  Since your effective ratio generally drops from a performance cam change, you need to go to a higher compression piston to recover it and fully realize the benefit of the cam.

*Anyone here see an issue with buffing the piston skirt with some fine sand paper (if it's non coated of course) to gain a little clearance? I don't have any experience with air cooled cylinder behavior and we certainly don't want any piston slap.

My DR only has 1100 miles on it so I hesitate to tear into it but my wrenches are taunting me!!!

Good luck with your project!

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4 hours ago, Ezerin said:

Pedantic... good word!  Plus one for you!

Lots of folks don't realize the running comp. ratio is not the same as the static ratio and wonder why they don't see the performance increase they hoped for from a cam change.  Since your effective ratio generally drops from a performance cam change, you need to go to a higher compression piston to recover it and fully realize the benefit of the cam.

*Anyone here see an issue with buffing the piston skirt with some fine sand paper (if it's non coated of course) to gain a little clearance? I don't have any experience with air cooled cylinder behavior and we certainly don't want any piston slap.

My DR only has 1100 miles on it so I hesitate to tear into it but my wrenches are taunting me!!!

Good luck with your project!

Not to be too bold, but I do think luck is what i got. Got a great reply back from ProCycle. ...

Screenshot_2018-10-17-05-42-48.png.c6d0d7bd3bd51431f8de71a8ca298041.png

This is gold for others doing this mod, im sure others have or will encounter this 'problem'. Now just the pockets to look at. Which is lucky as the piston is in the bore, with rings, wrist pin circlips,  lubed up ready to go. I will run with the squish on the high side, sounds like 40 thou is too much, so I'm on the borderline but will see how it goes.

Thanks guys, Happy riding!

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Another one from ProCycle. ...Screenshot_2018-10-17-11-12-28.png.55d85cbfa494423efb0fbaed89d2df36.png

Wish we had a shop like this in nz, but our country has the population of a decent town in the other hemisphere. Big ups for procycle, the guy is a gem for us diy blokes. 

I guess I'll just button it up and go riding!

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