Slant Six Head Mill

I am in the process of getting a remanufactured cylinder head for my 225 slant six. When it gets to me I plan to take it to a machine shop to get the head shaved down. From what I've seen on other forums and sites is that the general amount to take off is either .090 or .060 inches. What is the max amount of material I can take off without having to worry about other things such as valves and such?

What compression ratio do you currently have (exactly)?

What compression ratio are you trying to get/obtain?

Has the replacement cylinder head been resurfaced, and if so, by how much?
Note: any machine shop will remove a few thousandths of an inch as a quick way to check for flatness, as part of their operation.
BudW

I don't know my current compression ratio due to not taking the original head off to find out the exact depth the piston sits inside the cylinder at TDC. All I know for sure is the engine(1980) has all stock parts and has never been machined.

I have a lot of Chrysler publications from pre ’60 though present – but do not have ’80 or ’81 parts books nor do I have each year FSM (Factory Service Manual).
The two closest FSM’s to '80 is my ’77 and ’82 FSM. Both years, the /6 (225) had 8.4:1 compression ratio.
I would suspect your original engine would have 8.4:1, as well.

Now with that said, a majority of cylinder head machine shops will resurface the head when performing a valve job – as well as with any remanufactured cylinder heads.

The question that is unknown - is how much has been taken off of your replacement cylinder head, already.

Sometimes the reman cylinder heads will come with a card saying how much was shaved off – but typically that is for high end operations (ie: Diesel engines, which compression is already tight, and in racing engines). 98%+ of the rest – you have no idea.

The next question is what compression ratio are you trying to achieve?

Most cars running pump gas (87 octane) – you don’t want to go over 8.7:1 compression before pinging develops. Unless you are already planning on paying for higher octane (ie: every fuel fill-up will be more expensive). The engine will run more efficient with higher compression – but for a daily driver, I wouldn’t want to do away with the cheaper fuel.

If the starting point is 8.4. Max is 8.7 (if planning on using pump gas) and also to consider your replacement head has had some amount shaved off of it already, you might be closer to 8.7:1 than you might think.

If you are planning on a compression ratio higher than that, then let us know.
BudW

I haven't really thought about what compression ratio I look to get, what's a good ratio to have, this 8.4 just isn't cutting it for me. Also I've been using 92 octane fuel ever since I got the car and started working on it. Would 92 octane be acceptable for compression ratios higher than 8.7? If so how high of a ratio can I go with 92 octane fuel

I should back up here a bit.
There are two different ways to calculate compression ratio. SCR (Static Compression Ratio) which is counting the of combustion chamber at BDC (Bottom Dead Center) and counting the same volume at TDC (Top Dead Center), and DCR (Dynamic Compression Ratio).

Again SCR takes the volume at BDC – which for sake of discussion, is 500cc. Volume at TDC is 50cc. 50 divided into 500 (500/50) = 10/1 or 10:1 to 1 compression.
This is how most everyone looks at measuring engines – but is not really a good tool.

DCR, on the other hand is more of a “real life application” of compression ratio.
Compression does not start to build, on the compression stroke, until the intake valve closes (which happens on the upwards stroke). I’m not sure what specifications are for an ’80 /6 is, but my ’77 FSM shows the intake valve closes at 48’ after BDC (or ABDC).

Note: 180 degrees from BDC to TDC and intake valve closes after 48 degrees has taken place, so 26% of the piston movement upwards is wasted.

Based off of the FSM of Intake Valve Close angle of 48’ ABDC, that would give you roughly 7.5:1 DCR (in stock condition).

I did some research on best compression ratio for you and one thing I found was consistent . . . it depends.
Factors like vehicle weight, gear ratio, Altitude, normal driving ambient temperatures, cylinder head design, length the intake valve is open, intake/exhaust valve overlap, and about 100 other factors – so I just gave up trying to get everything factored in.

MY BEST GUESS, to get to the compression ratio you desire, is to reduce the cylinder head combustion chamber volume about 6 cc (from stock measurements).

If cylinder head combustion chamber is also at 3.40” in diameter (same as cylinder bore), then you would need to shave off approximate 0.040” from an un-shaved cylinder head.

Before I was to do anything to your reman cylinder head, and after your old cylinder head was removed, I would get both cylinder heads CC’ed (combustion chamber volume checked). Then compare volumes between both cylinder heads. You want the “newer” head to have about 6 cc’s less volume than old/original head.

Then, if combustion chamber is 3.40” diameter and is round (at head gasket surface) then 0.040” milling would reduce new head down about 6 cc’s. If combustion chamber is not round or if CC difference is not 6 cc’s exactly, then the 0.040” figure is tossed out the window.

I am 99.2% certain your new cylinder head has have some material shaved off of it. The question is how much. If I was to “guess”, I would say 0.020” – but that is only a guess.

The problem with machining, is if you took off too much – you now have a boat anchor, for you can’t put the metal back onto a cast iron head. Kinda like cutting a piece of wood. Measure twice and cut once. A board cut too short . . . is useless.

Attached is a picture taken from my ’77 FSM and combustion chamber appears to be round. I couldn’t find anything to indicate what combustion chamber diameter is, for if it is wider or narrower than 3.40” – then the shave thickness will also be different.

BudW

What is cc's and what is its effect on compression ratio, also how would I go about finding out how many cc's each head has. Is there also any way to measure the "new" head and fund out if there is, if any, material taken off prior to my ownership of it?

CC is Cubic Centimeter.

Perform a Google search on “CC Cylinder Head”.
Many shops can do it but many kits are inexpensive and a person can do it him/herself fairly easily.

Knowing what your old head CC measurement is vs. new cylinder head CC measurement is can determine how much was taken off and what you want additional to remove (if wanted).

I do not know what your original cylinder head should CC at (other than you want new one at 6cc less).
BudW


Edit: I want to get another CC measurement set (my last set walked off . . .). If you get one, I might be willing to purchase it from you once you are done with it, possibly.

Here's some some basic information from the mopar engine book.

A cut of .0066 will reduce chamber volume by 1cc.

A .090 cut is needed for 45 cc chambers for 9.5 to 1 cr. Steel gasket.

It doesn't give stock cc.

I ended up getting myself a Felpro gasket, I assume that's a different thickness as compared to the metal one

Generally the “installed” head gasket thickness is published information – for that is something many engine builders need to know about.
That said – I looked and found nothing about head gasket thickness on that Fel-Pro gasket – which is odd.

Note: installed thickness is not the same as measuring it.

I prefer Fel-Pro head gaskets and have had good luck with them in the past.
BudW

Ok well I got my "new" cylinder head in the mail yesterday. What would be the effect of shaving off .09 inches off the head? Also how would I be able to tell if it was machined prior to me having it?

Unless it was recorded as to how much was shaved off of it (ie: enclosed tag) – there would NOT be any way to know. Without measuring the combustion chambers (ie: CC’ing them) and comparing results between both heads – you will not know.

There is nothing to prevent you from shaving off 0.090” (almost 1/10th of an inch) - but you will not know what the total thickness taken off (to date) will be – for there WILL be some already machined off of it, already (just part of a normal valve job and standard practice for a reman cylinder head).

Comparing the CC’s from an original cylinder head to your reman cylinder head will give you a base amount to know what more to machine down - to get to your desired amount.

You might want to sign up for and send this same post to slantsix.org/forums and see what they say. They work with /6's, a lot more than I do.
Note: I’m not a member there – but do access their data every so often.
BudW

Ok I'll be sure to make a post on that website thank you.

I know this is an older thread, but I believe I can add some useful information. The slant six cylinder head is one of few made where you can measure actual thickness of the head between the machined surface of the valve cover rail and the block mating surface. Generally speaking, an unmilled head measures "around" 3.625 give or take. I have several unmilled heads ranging from 3.610 to the thickest, a factory closed chamber head coming in at 3.625, with others I have coming in between those two figures.

It is perfectly safe to have an unmilled slant six head cut .100" on the 225. You cannot do that on the 170, as those engines are a zero deck height and compression will be too high. This will get compresion up into the low 9s on the 225, which is just right for a decent performance street pump gas build.

I have one that was accidentally milled down about .155". That reduced the chamber volume to 34cc. Factory is around 56cc. Plenty of thickness left. Structural integrity is still substantial.

Only thing is, now, even with the factory piston .175" in the hole at TDC, static compression is STILL over 10:1 with that head. I have a pretty rowdy camshaft (250* duration @ .050") to help keep cylinder pressure down so it can run on pump gas. So far, all my calculations on the milled head motor are just a tick below 8:1 at like 7.89 which is fine for pump gas.

So you can use a dial indicator to measure your heads as I described above. Should be somewhere close to the figures I posted, if the head is unmilled. Also, another little tidbit, "about" .0067 milled off equals 1cc, so with that, you can figure chamber volume closely using just the cylinder head thickness measurement, if you do not have a head CC kit. As long as the head has been milled properly and evenly, you should get fairly close.

Something I didn't see mentioned in this thread is pushrod length. Unless you're running an adjustable rocker arm setup custom length pushrods are probably something else to get.

All '79 and older /6's have solid lifters and adjustable rocker arms. The '80 and newer /6's have hydraulic lifters – which get pressurized via the rocker arms (and hollow push rods). The adjustable rocker arms (solid lifter engines) have a lot of adjustment in them – so push rod length is not normally a concern.

Push rod length is an issue with hydraulic lifter engines – and more so an issue, sense the lifters have the oil fed via the push rods. I'm not saying they don't exist, but I don't recall seeing adjustable push rods and/or adjustable rocker arms that also allow for oil flow to the lifters built in them.

Unrelated, It is possible to convert any RG (raised block /6) from solid lifters to hydraulic lifters or vice versa, by changing the lifters, push rods and rocker arm assy's (will work either direction). Not sure how practical it is to change all of that – but it is possible and has been done before.
BudW

BudW said:

All '79 and older /6's have solid lifters and adjustable rocker arms. The '80 and newer /6's have hydraulic lifters – which get pressurized via the rocker arms (and hollow push rods). The adjustable rocker arms (solid lifter engines) have a lot of adjustment in them – so push rod length is not normally a concern.

Push rod length is an issue with hydraulic lifter engines – and more so an issue, sense the lifters have the oil fed via the push rods. I'm not saying they don't exist, but I don't recall seeing adjustable push rods and/or adjustable rocker arms that also allow for oil flow to the lifters built in them.

Click to expand...

His motor is an untouched 1980 according to post #3. .090" off the deck of the head would certainly cause issues. So you either get custom length, oil through pushrods, or convert to an earlier mechanical cam setup.

I'm not familiar with the rocker setup on /6 motors. Are they shaft or pedestal mounted? I wonder if shimming could be done to make up for the decreased deck height.

Pre '80 /6 with adjustable rocker arms (I should say, also needs to be cleaned). There are a couple of different versions but differences are minor.


This is a '80 and up non-adjustable rocker arm (looks somewhat similar). You can see the internal oil passageway cast into the part.


Some part stores say this rocker arm will fit.

It might fit (never tried), but there is no oil passageway to the lifters (so DO NOT try to use).

BudW

Any time you remove. 090 from the deck or head combination you will need a custom length pushrods. With that much removed the pushrods will be too long and the rockers will not be pushing on the center of the valve stem.