I was wondering what is recommended to mill the block and head on a 250. What needs to come off to make up for the added gasket thickness? I was going to take enough off the head to clean it up, and the rest off the block. I know I've seen this somewhere, but I can't find it.
How much to take off block/head?
- Thread starter Duane
- Start date
The block is very deap, and the pistons park about 100 thou below the deck. Rods are 5.88, throw is 3.91/2, and piston is 1.53. Deck is 9.469". This means the blue-print piston site 104 thou below the deck.
I'd personally check this with a feeler gauge and straight edge, and take of 130 thou. The replacement +30 pistons will be about 1.500" .
The Aussie 250 runs a shorter 9.38" deck. If you are stuck for pushrods, get some from the Aussie 72 to 76 XA-XB Falcon200/250 log head. This should allow you to run the stock head shaved a little, with the 130 shaved block compensated by the shallower Oz pushrods. There are even 50 thou shorter rods, so you could shave the head 50 thou.
Beware though, the CR will skyrocket.
Get some alternative advice from Jack.
One other option is to forget about decking the block, and take the 6" 2.5HSC con-rods with over size 1.500 2.3HSC pistons.
Get some advice from Jack or Rickwrench.
I'd personally check this with a feeler gauge and straight edge, and take of 130 thou. The replacement +30 pistons will be about 1.500" .
The Aussie 250 runs a shorter 9.38" deck. If you are stuck for pushrods, get some from the Aussie 72 to 76 XA-XB Falcon200/250 log head. This should allow you to run the stock head shaved a little, with the 130 shaved block compensated by the shallower Oz pushrods. There are even 50 thou shorter rods, so you could shave the head 50 thou.
Beware though, the CR will skyrocket.
Get some alternative advice from Jack.
One other option is to forget about decking the block, and take the 6" 2.5HSC con-rods with over size 1.500 2.3HSC pistons.
Get some advice from Jack or Rickwrench.
: Dumb question I know, But when you Deck the block do you have to remove the Pistons or can you machine the block with them still in the Block?
Unless you have a rising mill table, and a lifetime supply of petrolatum or vaseline to fill the piston bores, or some plumbers pipe blocks, then its unlikely you can mill it without a full strip down.
The milling fluid must cover the mill to take the metal off.
I've worked with a bloke who has used a rising table before, with the block mounted on top.
Any machinists here?.
The milling fluid must cover the mill to take the metal off.
I've worked with a bloke who has used a rising table before, with the block mounted on top.
Any machinists here?.
Howdy Duane and all:
Assuming your doing a complete engine rebuild, a poor mans partial solution to the huge deck clearance on a 250 is to: measure your engines deck height with new pistons and bearings in place. Use the 255 V8 pistons which are .085" taller than stock replacement pistons. Deck the block only enough to achieve a zero deck height. For example, if you're fortunate enough to have a block with only a .100" deck height then the 255 V8 pistons will take up .085" of the difference and you'll only have to deck the top of the block .015". Decking will be a variable depending on your engines dimensions. Do not deck the top of the block anymore then necessary as removing material here compromizes structural rigidity.
Do not mill the head past a minimal trueing cut to get the mating surface dead level. You may need to reshape and open the chambers to get to your CR goal.
Be sure to use washers under the head bolts when assembling.
I have not heard of anyone using the 2.5 HSC 6" connecting rods, and the corresponding flat-topped pistons yet. It sounds like a perfect solution. It should work but will likely take some debugging to get it right. I've found the 6" 2.5 Tempo rods rather scarce. I sure wish anyone whose done this will share the details. The improvements in reduced deck height and improved rod to stroke ratio sound rewarding.
Offset crank grinding is another option to be explored. It will give a slight increase in stroke. It will only gain .010" to .020" at the most so would have to be used in conjunction with other remedies.
I plan on milling a "D" shaped dish into the tops of the 255 V8 pistons to control compression to the 9.7:1 area, but I'm at 5,000 ft of elevations too.
If you are just milling to compensate for the thicker aftermarket composite head gasket, mill the difference in thickness between the OEM original and the replacement gasket you plan to use.
The key here is to measure and compare- build and then share!
Adios, David
Assuming your doing a complete engine rebuild, a poor mans partial solution to the huge deck clearance on a 250 is to: measure your engines deck height with new pistons and bearings in place. Use the 255 V8 pistons which are .085" taller than stock replacement pistons. Deck the block only enough to achieve a zero deck height. For example, if you're fortunate enough to have a block with only a .100" deck height then the 255 V8 pistons will take up .085" of the difference and you'll only have to deck the top of the block .015". Decking will be a variable depending on your engines dimensions. Do not deck the top of the block anymore then necessary as removing material here compromizes structural rigidity.
Do not mill the head past a minimal trueing cut to get the mating surface dead level. You may need to reshape and open the chambers to get to your CR goal.
Be sure to use washers under the head bolts when assembling.
I have not heard of anyone using the 2.5 HSC 6" connecting rods, and the corresponding flat-topped pistons yet. It sounds like a perfect solution. It should work but will likely take some debugging to get it right. I've found the 6" 2.5 Tempo rods rather scarce. I sure wish anyone whose done this will share the details. The improvements in reduced deck height and improved rod to stroke ratio sound rewarding.
Offset crank grinding is another option to be explored. It will give a slight increase in stroke. It will only gain .010" to .020" at the most so would have to be used in conjunction with other remedies.
I plan on milling a "D" shaped dish into the tops of the 255 V8 pistons to control compression to the 9.7:1 area, but I'm at 5,000 ft of elevations too.
If you are just milling to compensate for the thicker aftermarket composite head gasket, mill the difference in thickness between the OEM original and the replacement gasket you plan to use.
The key here is to measure and compare- build and then share!
Adios, David
rickwrench
Famous Member
FSPP is having custom forged pistons made, that's how I would go, no question. I think he said in his post about them he can get any cc dish you want. You would have to ask about a taller compression hieght, but since they are cut to order from oversized forged blanks, there is likely plenty of margin to play with dimensions. Shoot for a zero deck, use an .040 gasket and adjust you dish needs accordingly. I don't have my notes handy but if I remember right, a zero deck piston with a dish of 15cc will put you at 9- 9.1/1 using a late head on a 250. With a good quench surface you can run the crap they pass off as gas these days without any detonation at 9/1, no problem.
Zero deck (0cc's) + .040 gasket (just shy of 7cc's) + late head chamber (62ish cc's) + a 15cc piston dish = 84cc chamber volume + 1cc for "above the top ring volume" and it's 85cc total volume.
You don't have to fool with pushrod lengths or finding hse rods either.
Rick(wrench)
Zero deck (0cc's) + .040 gasket (just shy of 7cc's) + late head chamber (62ish cc's) + a 15cc piston dish = 84cc chamber volume + 1cc for "above the top ring volume" and it's 85cc total volume.
You don't have to fool with pushrod lengths or finding hse rods either.
Rick(wrench)
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