|
|
Joined: Nov 2006
Posts: 7
Active BB Member
|
OP
Active BB Member
Joined: Nov 2006
Posts: 7 |
How much do you usually have to take off of a 250 to get zero deck with 307 pistons? The reason I am asking is I have my block back form the machine shop so I could mock up the bottom end to measure the deck heigth, and all of the pistons are .040 down in the hole.
The pistons I have are some new old stock Ohio brand .020 over flat tops with no champher and four valve reliefs.
I am going to deck it .040 anyway but I just thought this seems like a bunch! Any input from those who have done it would be nice.
Thanks!!
Jeff
|
|
|
|
|
Joined: Sep 2008
Posts: 3,673 Likes: 42
1000 Post Club
|
|
1000 Post Club
Joined: Sep 2008
Posts: 3,673 Likes: 42 |
Jeff, comparatively speaking, .040 deck clearance isn't that unusual for production engines. If you have ever built any 440 Chryslers, many of their mid seventies smog engines in motorhomes had .080-.100 deck clearance. Most 292 L6's have .070+, so in a way the 250 is more blessed with only .040 or so....
Class III CNC Machinist/Programmer
|
|
|
|
|
Joined: Oct 2002
Posts: 787
Major Contributor
|
|
Major Contributor
Joined: Oct 2002
Posts: 787 |
I can't find Leo's book at the moment, but there is a max that should be removed, and I believe it is less than .040. IIRC it is .020. His recommendation is for the 292, I assume it also applies to the 250. Can someone chime in here.
Larry
Ignorance can be fixed Stupidity is forever
|
|
|
|
|
Joined: Apr 2006
Posts: 534
Major Contributor
|
|
Major Contributor
Joined: Apr 2006
Posts: 534 |
Leo's book pg30 recommends max .030" decking.
The deck on my stock engines I have handy are, with the engine codes:
292 stock bore THR .032 deck
292 30-over THH .023 deck
250 stock bore TAX .027 deck
AFAIK these blocks have not been decked but I couldnt guarantee that.
So at least for me .040" deck clearance would be a lot.
I wonder, is there a thinner head gasket to use? Do they seal?
Some of the steel shims are only about .020" thick and it is the total quench clearance that matters, so 0-deck with a .040" gasket would be the same as .020" deck and a .020" gasket -
|
|
|
|
|
Joined: Sep 2004
Posts: 5,840 Likes: 1
1000 Post Club
|
|
1000 Post Club
Joined: Sep 2004
Posts: 5,840 Likes: 1 |
It sounds like those pistons he has sit down too low in the block,if they are copies of 307 pistons.
IIRC, the TRW forged flat top pistons I have,I only had to deck the block about.010 & now it has a positive deck from.001 -.004
Or the block has a little taller deck?
I would just deck it .040 if it is not an all out race engine,it should be fine.
Or buy pistons that sit @ the proper height.
MBHD
12 port SDS EFI 
|
|
|
|
|
Joined: Nov 2008
Posts: 43
Active BB Member
|
|
Active BB Member
Joined: Nov 2008
Posts: 43 |
There where 2 different piston heights, 1.640 & 1.655, due to the water pump bolt as tight as it is to the top of the deck and a known crack issue here I wouldn't cut more than .025 off of the deck on a 250, also as stated befor maybe the block may have been a slight bit taller, in this case I would check the piston to see what the compression height is first, if its the 1.655 you might then be able to deck the block even more, I would also check another block and measure the bolt hole to the deck on both blocks.
|
|
|
|
|
Joined: May 2007
Posts: 79
Active BB Member
|
|
Active BB Member
Joined: May 2007
Posts: 79 |
If those are older pistons, Ohio,Badger and others compensated for the increase in compression brought about by the overbore by reducing the pin height....normally I find them to be .015 to .020 less, given the variances in the block deck height this is probably what you have encountered.....interestingly the last set of 307 (TRW) I used with the chamfered head (couldn't find true flats) came out at zero with no decking...the variation in parts can be different from one production run to another I suppose.....Fats
fats
|
|
|
|
|
Joined: Nov 2004
Posts: 420
Contributor
|
|
Contributor
Joined: Nov 2004
Posts: 420 |
I also used TRW forged flat top pistons and like Half Dozen I decked the block .015 for a zero deck.
Jerry Davis II#4711
ol Smokey said "one test is worth a thousand expert opinions."
|
|
|
|
|
Joined: Nov 2006
Posts: 7
Active BB Member
|
|
OP
Active BB Member
Joined: Nov 2006
Posts: 7 |
Well, I checked the pistons and they are the taller than the original ones so maybe I have a little more material on the block?
Anyway, I going to deck it .040 and run the tar out of it. If it cracks or something I have six other 250 blocks and a set of domed forged trw pistons for the next go round.
I know some of you are thinking that I should just check one of the other blocks and see if it is shorter but I already have the money in boaring and cleaning this one so I am going to just use it instead of spending another $400 in machine work on another block. Yes I an just that cheap!!
Thank all of you who did respond! If you have any more ideas please let me know!
Thanks!
Jeff
|
|
|
|
|
Joined: Sep 2004
Posts: 5,840 Likes: 1
1000 Post Club
|
|
1000 Post Club
Joined: Sep 2004
Posts: 5,840 Likes: 1 |
Personally , I do not think there wil be a problem decking
it .040.
Just put a good chamfer/radius in the holes/passages after decking it.
MBHD
12 port SDS EFI
|
|
|
|
0 members (),
125
guests, and
27
robots. |
|
Key:
Admin,
Global Mod,
Mod
|
|
|
|
