How far can one safely deck a 225 block?

-
SpriceyStuff

SpriceyStuff

Well-Known Member
Joined
Mar 12, 2022
Messages
309
Reaction score
67
Location
Richland, WA
Hey guys. I just got my nice flatties in today. I went with flatties to boost compression, and while in a compression boosting mindset, i was wondering just how much one can take off the deck safely in a 225. I imagine that a 77 motor is gonna have some irregularities in the deck surface that will need to be corrected anyway.

So I have these questions:

1.) How much deck height can be safely removed on a 225?
2.) How much does the compression ratio increase putting flat top pistons in a 1977 225?
3.) How much compression can a slant six actually take?

B6655E02-3CA0-44D3-BBBD-DCBF5595F8B7.jpg
 
Easy .100 Long time Slant racer Mike Jeffery's saying was 100 off the block,100 off the head and 100 overbore.
One of my Slant engines had 13.8 to 1. How much of a increase with flat top I can't say.
 
I decked my block .050 which gave me pistons .137 in the hole. I then shaved the head .060 which made chambers of 54cc. This gave me a 8.75 compression ratio. This is with a '78 block. The thing to watch out for, is you start to get close to the studs that hold the alternator bracket on.
 
So, I have this question. Why Std bore? The cylinders probably have wear. and be out of round. You are thinking about increasing the CR by decking the block, and maybe milling the head. You might have a problem with ring seal. Without making sure the cyls are round and straight, keep your CR reasonable.

EDIT: Question #2. Zero increase. All slant six engine (USA Automotive) had flat top pistons.
 
On my one block, the block was decked 90 thou and head cut 10 with a 30 thou overbore. That’s as far as it proceeded
 
I've never seen anything BUT flat tops for the slant, so you're not gaining anything "there". I know from direct experience the head can stand over .150" because I've done it. As for the block, all I have is what others have said. Maybe @Charrlie_S will chime in and give some experienced figures on the block. The .100" figure given for the block wouldn't surprise me though.
 
The only reason to cut the block really is to try to achieve quench. Most 225s have a deck clearance on the order of .150 - .175", so trying to get zero or close to zero deck height might not be the smartest thing. If all you're trying to do is get a little more compression, then the .100 give or take off the block will work good.
 
I took 0.060 off the block and 0.010 off the head with oversized valves. Ended up with about 8.8:1.
 
I took .060 off the head, pistons were about .175 in the hole. 55CC chambers. That came out to about 8.4 static compression.
 
halfafish said:
I took .060 off the head, pistons were about .175 in the hole. 55CC chambers. That came out to about 8.4 static compression.
Click to expand...
Mine was milled down to 34CCs. lol
 
.125

There is an article about building a 300hp slant 6 somewhere that was in H..R magazine. They cut .125 off the head and .125 off the deck for .250 total.
 
Last edited:
Going with 198 rods or aftermarket long rods and 2.2/2.5l or other appropriate aftermarket pistons gets you to a zero deck piston height.
That and a light clean up cut on the block and head got my slant to a 10.5 static compression ratio.
 
Darter6 said:
Easy .100 Long time Slant racer Mike Jeffery's saying was 100 off the block,100 off the head and 100 overbore.
One of my Slant engines had 13.8 to 1. How much of a increase with flat top I can't say.
Click to expand...
I wonder if new custom push rods were needed ? 300 thousandths is well over 1/4 inch
 
Usually, any milling over .100 total is VERY close on needing custom pushrods. I have found on 225 engines you can get away with a little more than on the 170 engines.
 
There is alot of leeway on them because of the adjustable valve clearance. If it is a hydraulic lifter engine, "not as much" so you're much more likely to need custom pushrods
 
The head on my 225 was milled "somewhere" north of .150" and the stock pushrods fit fine. Been runnin great for dang near two years. I'm sure if I wanted it "dead right" shorter pushrods would have been the correct thing, but everything fit together good and it runs fine.
 
my68barracuda said:
Going with 198 rods or aftermarket long rods and 2.2/2.5l or other appropriate aftermarket pistons gets you to a zero deck piston height.
That and a light clean up cut on the block and head got my slant to a 10.5 static compression ratio.
Click to expand...
I considered that... the problem with that idea for me particularly is that my engine is a late model cast crank engine. The 198 stopped production in 1974, and thus never saw production with a cast crankshaft (began in 1975). Since the cast crankshaft has narrower journals, there was never a 198 connecting rod made that fit a cast crank.
 
RustyRatRod said:
The head on my 225 was milled "somewhere" north of .150" and the stock pushrods fit fine. Been runnin great for dang near two years. I'm sure if I wanted it "dead right" shorter pushrods would have been the correct thing, but everything fit together good and it runs fine.
Click to expand...
That's the thing I like about solid lifter motors. Those manually adjustable rockers make life so easy when it comes to changing clearances
 
SpriceyStuff said:
I considered that... the problem with that idea for me particularly is that my engine is a late model cast crank engine. The 198 stopped production in 1974, and thus never saw production with a cast crankshaft (began in 1975). Since the cast crankshaft has narrower journals, there was never a 198 connecting rod made that fit a cast crank.
Click to expand...
When K1 / Molinar released their ‘long rod’ slant six rods they had a forged and a cast crank rod. The cast crank rod was not available for long, but I jumped on it and I got a set and that is what I am running in the 87 cast crank slant that is in the 68 Barracuda.
Unfortunately and I don’t know why but K1 / Molinar quickly canceled the cast crank rod option. But for those that want to go in that direction, get a set of K1 / Molinar or 198 rods and have the crank end of the rods milled down to the thickness of the cast crank rods. You will need to also have the bearing lock slots cut in, but like the width reduction, not a big deal.
 
my68barracuda said:
When K1 / Molinar released their ‘long rod’ slant six rods they had a forged and a cast crank rod. The cast crank rod was not available for long, but I jumped on it and I got a set and that is what I am running in the 87 cast crank slant that is in the 68 Barracuda.
Unfortunately and I don’t know why but K1 / Molinar quickly canceled the cast crank rod option. But for those that want to go in that direction, get a set of K1 / Molinar or 198 rods and have the crank end of the rods milled down to the thickness of the cast crank rods. You will need to also have the bearing lock slots cut in, but like the width reduction, not a big deal.
Click to expand...
Yeah I wanted to get the molnar H-Beam rods for my build but they’ve thus far been impossible to find. I’m not too worried about it though. I’m not planning on zero decking the 225. I was just curious how far it was safe to cut it down cause if I end up needing to due to warpage then I wanna know how much I can take off without having to junk it, but it sounds like I don’t need to worry to much about that
 
You must log in or register to reply here.
-

Back
Top