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August 23, 2019, 09:41:36 AM
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  340 Single Oversize
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   Author  Topic: 340 Single Oversize  (Read 151 times)
jerryswiss
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340 Single Oversize
« on: March 24, 2019, 07:23:42 PM »
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Ok, I am working on a '71 340 TNT single and I start scraping the carbon off the top of the piston and what doo I see? 080! So this cylinder has been WAY over sized. First question is WHY? The odometer only shows 950 miles, and I know the cable could be bad, but when I look at the track, it looks like new.

Second question, this motor does not have that great of compression (108 psi at 4800 ft). So is there any piston and ring set I can still fit in this engine? The piston and cylinder actually do not look that bad, no cylinder rim or gouges.

I was thinking of just putting a copper head gasket on it and seeing what happens. Thoughts?
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Bones
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Re:340 Single Oversize
« Reply #1 on: March 24, 2019, 08:51:29 PM »
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You ask " is there any piston and ring set I can fit in this engine" Yep, ones like you have are out there by the 1000s 
Your engine is NOT bored 80 over, it is in fact standard 
If you bore a 340 single cylinder to 80 over there is nothing left of the sleeve, you can`t doo it. 40 over really is max you should go. There is no question you have a aftermarket piston in there. Here is some FYI for you...….The piston for a 71 to 78 Rotax  type 302 (299cc) and the 69 - 320 cc are actually 100% the same piston and are standard of 76 mm. The piston for a 70 to 73 - 335 Olympic engine is also 100% identical in shape to the 300/320 piston but it is 78 mm as standard. so many, many of the aftermarket companies used the same tooling and made a standard piston of 76 mm then carried on with 76.5 at 20 over, 77 mm as 40 over, 77.5 mm as 60 over, 78 mm as 80 over, 78.5 as 100 over and ending at 79 mm as 120 over.  The piston marked as 80 is actually 78 mm and is std for the 345/340. You would use the 120 over as actually 40 over for the 340.
Have you figured out if you have a 70 or 71 head They are different. ALL aftermarket pistons are made to match the 70 head and are too low on the dome for a 71 head, compression is DOWN with the piston like you have and a 71 head. The correct piston to match the 71 head are near impossible to find. Aftermarket companies NEVER made a piston to match the 71 - 340 TnT head.
I set up a engine a few years ago with the low dome piston and a 71 head so the compression/squish was intentionally low to run the garbage fuel we have today. I doo not recall what the compression was but I am sure it was above 108 as you are getting. The engine will still run fine, if you are not racing, it is fine for a trail sled IMO.
I thin copper head gasket might work OK, I don`t really have a opinion on that.
Bones
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jerryswiss
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Re:340 Single Oversize
« Reply #2 on: March 25, 2019, 04:37:04 AM »
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Thanks Bones, it is definitely a 1971 machine, as by the serial number, so with that aftermarket piston in it, that would explain why the compression is low. I think I will try a copper head gasket and see what happens.

I am actually able to get some pretty good gas around here, 91 octane no ethanol. That is all I put in all small engines.
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