Lowering Compression

[440sixpack]

What started as a light rebuild just keeps getting heavier. 

My 428 has low miles and a great bore but I find it has TRW pistons  ( that look like new )  with my heads should be in the 10.2 to 1 range.  with a little machine work it's probably a little higher than that.


Since it's in a 4x4 pickup I'm not after top performance I just want a street engine.  though it's been working fine on pump gas with the timing backed off a bit I'd really like to lower it .  so I'm looking at Cometic gaskets .  I know putting the pistons down in the hole isn't an ideal situation but it's sure the easy fix.
 
 So to get to say 9.5 to 1 what would I need ? I think stock is .40  and they list a .60 and a 80 which would I need ?.   or is this just a bad idea  ?  leaving it alone is an option.

[BattlestarGalactic]

I've been beating on my 428CJ for nearly 30 yrs with 10.1 compression in my '69 F100.  Yes, I run 93 in it, but I suppose if I turned the timing down a touch I could get away with something less if it got better mileage and wanted to drive it more often. 

[TomP]

Mill a shallow dish into the pistons.

[gt350hr]

   Your piston should already have a dish and four valve reliefs in it. To modify the piston 22 grams of material must be removed from the top of the piston. This gram weight relates to an 8 cc increase in the volume of the piston and is the amount needed to reduce your compression by .7 of a ratio. It's easier to do this by gram weight than by constantly cc ing the piston. Caution MUST be used to not let the material thickness in the dish get less that .220.  Less than that will result in the center "caving in" due to heat and combustion.
     Randy

[440sixpack]

Your description is correct.  I'm not sure how much a gram would take out of the center, but it isn't super thick.

Brent is working up a cam for me, if he thinks I can get by with my compression on pump gas I'll probably let it go.  if not I'll explore this option.  thanks

[Barry_R]

Depends which "TRW" piston you have too.

The L2245F is a passenger car piston with lower compression.
The L2303NF is a CJ replacement with a shallower dish & higher compression

[Ranch]

Maybe a thicker Head gasket
I take it you've measured and CCed everything  (guessing really doesn't work well)
I'd leave the pistons alone and try working the combustion chambers un shroud the valves
Do you really want to pull the engine?
Messin with the pistons would probably mean a re balance

[440sixpack]

the engine is already apart.

the L2245 is the lower compression piston that's correct.  but it's atill high  by todays standard.   somewhere between 9.5 and 10.5 with the larger cc heads depending on everything else. from what
I can find.   

 I think I'm just going to leave it alone.  66,000 miles and the pistons look new so it can't be too awful.

[My427stang]

the engine is already apart.

the L2245 is the lower compression piston that's correct.  but it's atill high  by todays standard.   somewhere between 9.5 and 10.5 with the larger cc heads depending on everything else. from what
I can find.   

 I think I'm just going to leave it alone.  66,000 miles and the pistons look new so it can't be too awful.

With a L2245, you are likely below slightly below 10:1 unless your heads and block have been significantly (ridiculously) cut.   If it were mine, I'd run a Felpro 1020 gasket, not the blue colored 8554, although it seems like it would add compression, worst case you are .016 below and .041 gasket thickness which gives you some decent quench although not perfect.

Cam correctly and it'll be a blast

BTW you may be a little gunshy with compression, it's OK to be that way, but it's not high compression by today's standards, it's in a good place for efficiency assuming you keep quench tight and the truck tuned properly