John Deere 720 Pistons

58 Deere · Feb 10, 2014

I am building a John Deere 720 GAS tractor with a 720 LP parts tractor. Both block have been bored .045 over at one time. I am hoping to use the better of the two blocks without boring again. My question is about using the LP pistons in the Gas tractor. I know this will raise the compression. But how much is too much? I run premium gas in my 620 so that would not be a problem. I just do not want to worry about spark knock. I made a similar mistake with an automotive engine. Or should I just sell the LP pistons? Thanks for the help.

BushogPapa · Feb 10, 2014

LP pistons give 8:1 Comp and will knock on 87 octane gas when under a good load.
Premium gas (and careful timing settings) should cure that.
I really like to hear a Gas JD Idling with LP pistons..they sound very "Ready to Go"..!
Put an Improved Cam in while it is apart and you will like it even better..

Ron.

730-LP · Feb 10, 2014

I agree with Ron

buickanddeere · Feb 11, 2014

(quoted from post at 01:09:47 02/11/14) I agree with Ron

I agree with 730-LP

The cam and followers will be worn. May as well grind them to match the pistons and 91 octane gasoline.
There is power in a good multi angle valve job . Blending the rough edges out of the valve bowls is good too.

5Deere · Feb 11, 2014

I plan on having the cam ground. The followers are worn. Who
do you recommend. I live in NC.

58Deere · Feb 11, 2014

I plan on having the cam ground. The followers are worn. Who do you recommend? I live in NC.

forty40 · Feb 11, 2014

(quoted from post at 10:33:46 02/11/14) I plan on having the cam ground. The followers are worn. Who do you recommend? I live in NC.

My experience with a JD A w/power block, stock head with moderate port work I finished up last fall, 8:1 comp ratio with 91 octane knocks like a diesel at idle. Tried retarding the timing as much as 5 deg, but still the same. Haven't taken the time yet to try 93 octane, that may help, but VP Late Model Plus 113 octane cured the knocking. Can't say if the A relates to a 720 or not.

Mike Aylward · Feb 11, 2014

I have a 60 with a Power Block and propane pistons in it. I have plowed with it on hard pulls with 87 octane gas and it does not knock. When I had it apart I was careful to smooth all sharp edges in the combustion chamber. The compression ratio (after a head milling) is somewhere around 9:1. It runs fine with no detonation whatsoever with timing slightly advanced from stock. It has had the cam reground. Mike

730-LP · Feb 11, 2014

Sonny at Bullet cam in Olive Branch Miss does a good job ,sorry I don't have a #

John 37A · Feb 11, 2014

(quoted from post at 14:41:38 02/11/14)

(quoted from post at 01:09:47 02/11/14) I agree with Ron

Click to expand...

I agree with 730-LP

Lol

gtx1967jewison · Feb 12, 2014

That does not sound right to me at all- have you tried a colder
spark plug? If the ring in the block is sharp that can cause it
too. Have you checked pressure with a gauge? Perhaps
you've got more compression than you think?

John Deere 720 Pistons

58 Deere

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BushogPapa

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730-LP

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buickanddeere

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5Deere

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58Deere

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forty40

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Mike Aylward

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