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XR200 I need info on 11.1:1 high comp piston for 1984 xr200

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I recently got a rebuild/restoration project in the form of a 1984 xr200 twin shock. It had a atc motor in it so i bought a mid way rebuild xr200 motor (some assembly required) and was told to get a wiseco 11.1 : 1 high compression piston (stock bore) for the motor. Obviously I did, and now I have to figure out what else do I need to do to the motor to make it all work, and keep it reliable. Do i need to re jet the carb, get better head studs, hone the cylinder, modify the exhaust? Any and all help is appreciated.

Thankyou in advance.

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If you are unsure of the 36 years old bottom end's condition (not sure if that was part of your rebuild plans)

I wouldn't be too quick to add on extra compression and other power adding mods.

 

Air cooled engine often run hot so the fueling requirement (octane) will likely change going to higher compression to avoid detonation/pre-ignition.

Edited by mlatour
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The 200 was fairly high compression stock, 10.5 as I recall.  The standard Wiseco piston doesn't raise it very much.  

The death of a 200 engine is almost always valvetrain related.  Loose timing chains, worn gears leads to the chain jumping and then its bad news.  Likewise a 40 year old valve can finally wear out.  The camshaft rides in the aluminum head, so keep the oil clean and full.

I need to find a twin shock to add to my stable.

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14 minutes ago, chadzu said:

The 200 was fairly high compression stock, 10.5 as I recall.  The standard Wiseco piston doesn't raise it very much.  

The death of a 200 engine is almost always valvetrain related.  Loose timing chains, worn gears leads to the chain jumping and then its bad news.  Likewise a 40 year old valve can finally wear out.  The camshaft rides in the aluminum head, so keep the oil clean and full.

I need to find a twin shock to add to my stable.

I'm also finishing a 1986 xr200r dualsport build, and ya the engine did run and all but the cam chain was rattling in the channel for a long time before I rebuilt it. Thankfully it only needed a new chain, cam and cam bushing and a few dozen oil flushes to get rid of the aluminum shavings. Also changed the cam chain tensioners, and head gasket to an oem all metal one while it was apart. Now it's ready to be put back in the frame. 

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A new piston in a used bore usually doesn't work out well. Usually with a used bore you'll need to bore oversize for a good surface for new rings. Next oversize is 0.5mm larger so order the piston so the shop has it in hand when they bore the cylinder.

Stock XR185/200/R compression ratio is 10:1 for regular pump gas, a higher octane does provide some protection on hot days, I ran 10.5:1 for many years on pump premium and never had a problem on hot days in the mountains. Several years ago I built 2 Powroll 218 engines with 11:1 and they both had ping events at the same time and place, turned out to be a part throttle lean mixture was allowing over heat. One had stock cam and exhaust.   Pete at Powroll told me to limit total advance to 29 degrees on a 218, which I did on both engines.

No problem with ignition advance using stock compression; I had a very high hour stock 90 XR200R and it had lower than optimum compression, but the engine ran great and was one of my fav engines, but it was running 34 degrees total on pump premium. On the 10.5:1 engine I ran 30 total on pump premium.

The issues are the compression provided by good ring seal, inlet valve closing, proper ignition lead, squish band, gas octane, and proper jetting. Changes in any of those can upset things and cause problems. So for a  nearly stock engine I think 10.5:1 is safe, 11:1 is pushing it.  As a reference point the compression spec on a XR200 w/ 10:1 is 190psi. So maybe before tearing down an engine do a hot WOT compression test, than add a 1/2-1 teaspoon of oil to the cylinder and repeat.  Harbor freight has a Compression Test kit with an adapter for the small spark plug used on the XR200s.

on edit; install a new timing chain, check the chain sliders for wear and replace if the top layer is worn thru (common).

Edited by Chuck.
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I've always learned to make thing reliable as possible is to keep them all stock, once you start adding performance mods your on your own. High compression piston on an air cooled engine is questionable. But keep us posted, I'm interested in hearing how it goes.

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I put a 12:1 in my 83 xr200r had to back the timing down a little and keep premium in it but man it will walk off and leave my stock 82

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I would go bigger bore, stock compression. With stock airbox and exhaust you don't have to change the jetting much. 

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