John deere A rod length

[Bill Marcusky]

I bought 5.625 dia M&W style high compression pistons for my 1950 JD A. I'm told those pistons with everything else stock my compression will be around 6.34:1. I don't want to deck the head so it stays strong and doesn't blow head gaskets. I'm taking the eyebrows out of the head and cleaning up the intake and exhaust passages. My question is with all other stuff left stock how much will I have to lengthen my rods to get my compression to 7.45:1? and with my setup what cam work if any will be necessary to make it run good on 93 octane?

 

[FactoryFarmer]

You will get alot of answers for this but the only way to truly figure it is to find your deck height and figure out the how many cc your combustion chamber is.

 

[Bill Marcusky]

What do you mean by deck height? and I found out a stock
combustion chamber with the eyebrows blended back is 184cc

 

[FactoryFarmer]

Deck is the top of the piston to the bottom of the combustion chamber. If I'm figuring this correctly with that CR your deck would be .81 assuming your pistons are flat. If they are not then that has to be figured out and added to the combustion chamber.

In order to end up at your target CR you would need a deck of .61 so your rods need to be .20 longer.

 

[buickanddeere]

Time and money ahead to leave the rods alone. Sell the pistons and purchase a set of factory style model 60 propane pistons.

 

[Anonymous-0]

You can tape the sides of the pistons and fill the center with water from a CC"d horse syringe. Area of the head gasket and the cc"s of the head combustion chamber. Get out your calculator

 

[forty40]

(quoted from post at 15:46:12 01/01/14) I bought 5.625 dia M&W style high compression pistons for my 1950 JD A. I'm told those pistons with everything else stock my compression will be around 6.34:1. I don't want to deck the head so it stays strong and doesn't blow head gaskets. I'm taking the eyebrows out of the head and cleaning up the intake and exhaust passages. My question is with all other stuff left stock how much will I have to lengthen my rods to get my compression to 7.45:1? and with my setup what cam work if any will be necessary to make it run good on 93 octane?

Piston to deck dimension on a recently disassembled 1951 JD A was .75". Using .75", a 5.625" bore dia, 5.750 dia head gasket w/.060 compressed T, and a 184 cc combustion, results in a 6.34:1 compression ratio. Using those numbers, to achieve a 7.45:1 ratio, the required piston to deck dimension is .532". If the piston is a flat top piston (a very important feature in the formula), it has to move .218" further up in the bore. If the piston TDC changes that much, the spark plug or fire slot clearance needs to be checked. Mix matching features and dimensions is not usually an as-is bolt in assembly. If the piston has a dome or dish, naturally the numbers change. The above numbers assume zero piston cc volume in the combustion chamber. Flat top pistons in automotive applications w/valve reliefs usually add 3-6 cc volume, domes subtract, whatever dim the builder needs for a specific cylinder head and resulting comp ratio.