1st of all.Thank you for letting me join.i have a few mopars but thay been mothballed for 10 years.wife had cancer and passed away in 2022.so i am getting back working on my cars.i have a 1975 duster had a slant 6 in it.i was going to use as parts car but its in better shap than expected so i am going to make a daily driver out of it.mostly stock but put some work in the engine.so my questions are.i have a complete 1966 225 to use in it.how much can you deck the block and head to increse compression and how much compression can the stock rods handle?

If your car is a automatic, you will run into issues installing the 1966 engine into the 1975 car. You can install a 75 engine into a 66 car with only one minor issue, but going the other way is problematic.
As to your original question, some people have gone as much as .125 off the head, and about the same off the deck. That is too much for a "daily" driver.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Welcome to the site. I'm fairly new here as well.

Try a search for mill head compression site:slantsix.org
Lots of past threads to read. It's almost information overload.

I just had my block milled .07 You have to cc the head and measure deck height to calculate compression.

Good luck and sorry to hear about your wife.

What problems would i run into putting older /6 into 75?i do have a 75 block can i use the 66 head on it.i did cc the head its 62cc +or-2 cc.

I believe the pilot hole for the torque converter changed sizes.

I still have the torkconverter from the 66 ill have to see if it fits into the 74 trans.

It will physically fit, but it won't work. The input shaft is different.

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Official Cookie and Mater Tormentor.

It's doable. My current engine is a '64. I had the machine shop enlarge the crank pocket to fit the torque converter for my '75 transmission.

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1975 Dodge D100 225 c.i., HEI, Parallel Offenhauser, Motorcraft 2150, 904 auto, 9.25" 3.55:1

Thank you.that was going to be my next question was if a shop can enlarge that or will a newer forge crank fit an old block?also how much compresson can stock rods take?thx

Also if i deck the head or block do i need to get diffrent push rods or is that a non issue with ajustible rocker arms?

Yup a newer forged crank will fit that block... I had to do just that when I found out about the converter hub size mismatch the hard way when I did an engine swap and they wouldn't go together. I ended up having the 73 crank in the bad engine reground and put it in a 63 block. This was about 35 years ago.

Do a search, most of these questions have been asked and answered a million times over the years. If you go back and read older posts you'll see the same questions cycle around over and over

Thank you.for your help

I have asked questions here then searched and found the answer. Have learned to search first. Though most results are old, most of the time the info is still good and relevant. I need to figure out how to search for recent results for paint recommendations. Going to search for that now.

Your rods can handle all the compression you are going to make. Only the most serious engine build would need aftermarket rods.

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Hyper_pak