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2000 xr70 smokes...new piston or just rings?

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I have a 2000 xr70 that has started smoking. Is it okay to just replace the rings, or will I probably need piston/rings? If I get the piston, is the cylinder plated and will I have to bore and go oversize? I am acustomed to working on vintage and I always bore to match the new piston but I hear this new stuff is tighter.

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It's difficult to say. The only way to know for sure what to replace (and if you have to bore the cylinder) is to measure the piston, rings and bore and compare to the specs for the motor. Doing anything else is just guessing. A machine shop should be willing to measure the piston and bore for you.

You might also consider replacing the valve stem seals if you have the head off.

You'll definitely need documentation. Get yourself the Honda shop manual. I don't think it's more than $30 from the dealer.

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The 70 cylinder is not plated, it is boreable. Personally, I would replace both the piston and the rings, that way, you know what you have. As stated above, while it's apart, replace the valve guide seals, they are usually included in the top end gasket set.

Check with your local bike shop to see if they bore cylinders, they can get you the piston you need and bore the cylinder accordingly.

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I appreciate it fellas, normally I would not have asked the question and just gone .25 over, but I kpt finding standard pistons claiming to fit and it got me doubting myself. I have a good machinist that does cylinders for $45, I'll just get the piston/rings,course new valve stem seals and a cam chain.

Thanks.

Edited by surveywaters
spelling

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Okay, new question. I am seeing piston, rings and cylinder for less than the cost of piston/rings and machining, is this a valid option or is this a cheap chinese job that's not sized right? Anyone with experience here? Also, anyone know of a place to get good prices, maybe even a full topend kit with gaskets/piston/rings/cylinder/valve seals?

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Okay, new answer...

Go with a topend kit from a ThumperTalk Vendor/Sponsor who will answer the phone if you have any questions. The TrailBikes Kit is one of the most popular, reliable, and dependable kits you can use. A much better grade of product, without a doubt. You wouldn't be disappointed.

Click on the logo below...

newestt-boltlogo.jpg

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That looks like a great deal, what needs done for the carb? Can I run the stock 70 carb, maybe with a rejet?

Edited by surveywaters
punctuation

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Anyone have a suggestion on a complete kit? Meaning one with a carb too. Why do they recomend an oil pump and not include one!? I think it may be cheaper to go ahead and get the 88 kit with the oil pump, carb, and maybe a new profiled cam. I found some for around $200-$250.

How does this look? http://www.ebay.com/itm/Honda-CRF-50-XR-88cc-Big-Bore-Kit-70-XR50-CRF50-CRF70-/130608414926?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item1e68de40ce I know it's not TB stuff but what's the difference beside customer service?

Edited by surveywaters

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Okay, I bought the kit from tbolt with the cylinder/cam/carb/etc. and it all looks real nice. I started assembly today. I currently have 2 engines on the stand, the xr70 and a 70cc chinese. I noticed that the chinese head has a much larger intake port and exhaust port. The valves are the same as my 70. The clone head has "E22" stamped on it. My Question is obvious, which head is better? I will be using the TB cam.

DSCN5124.jpg chinese serial number

DSCN5118-1.jpg Chinese port , notice the chamfer already ground in.

DSCN5110-1.jpg XR70 port (sorry the pic sucks, there's a huge hump of aluminum there, can't see the valve seat at all, unlike the clone head.

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The XR70 head and the Chinese E22 head both use 23/20 for valve sizes, but, the TB cam probably won't work in the Chinese head. I'm pretty sure it is made to fit the OEM Honda or TB race heads only.

Performance-wise, it's a matter of personal choice. I've used both heads in various engine builds, using the TB 20mm Shengway carb, with no issues or problems whatsoever. Keeping the displacement at 88cc, you would be okay to use either head. If you were using a stroker crank to up the displacement, then the E22 would be the better choice, simply because you could use a 24mm carb effectively.

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I appreciate the advice. I am wanting to use the cam that was provided, so I guess I'll stick with the ET-2 oem head. I still have a Z50 build to take on, I'll save the E22 for that one.

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I discovered why I am having to replace the topend on this bike. I noticed excessive cam chain noise when I bought it a month or two ago. Apparently someone had removed the cam chain tensioner accidentally and then after reinstalling, failed to prime the hydraulic cylinder causing an inefficient tension, resulting in a worn out oil pump drive sprocket. I have heard of this on many other 70's and would say that anyone doing a topend would be wise to go ahead and replace this gear as well as the tensioner spring which seems to fail with out the assistance of the hydraulics.

DSCN5140.jpg

Edited by surveywaters
added image

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Thanks for all the help guys. Big bore is installed and running strong. When I first ran it, she smoked like crazy. Turned out the head needed to be retorqued after a few minutes. It was running lean and was hot enough to burn the carbon built up in my exhaust!

Now I have a new problem, I was running a break in session on the home track and spun the rear rim! Time for a new tube and a bead lock.

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On 11/19/2011 at 7:03 PM, dirtbkr188 said:

Okay, new answer...

Go with a topend kit from a ThumperTalk Vendor/Sponsor who will answer the phone if you have any questions. The TrailBikes Kit is one of the most popular, reliable, and dependable kits you can use. A much better grade of product, without a doubt. You wouldn't be disappointed.

Click on the logo below...

newestt-boltlogo.jpg

Do you have a link to this kit?  I tried looking it up unsuccessfully. 

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