Inliners International Forums General Discussion Hi Performance 848 cylinder head questions

Guys,
I've run into a problem trying to assemble the top end of my 261.
The 848 cylinder head I purchased has been extensively milled, and I'm having trouble getting the valve train geometry correct. The previous owner of the head had switched to racing 292's when they became available, and no longer remembers how much he had this head milled!
I have cc'd the combustion chambers, and recorded the results. If someone can tell me the combustion chamber volume of the 848 head when new, and, how much (in cc's or so) the volume is reduced per some amount of milling, I can figure out how much to raise the rocker shaft, pushrods, etc.
I thank you in advance for your help. Toolman

Milling the head doesn't affect any part of the geometry directly, except the pushrod length.
However: the intake valve seat is frequently recessed back into the bowl for clearance if the seat is removed by the cut, or to prevent the valve from striking the piston at TDC.
This makes the stem taller above the head, and would normally require a shim added to the stands by almost exactly the change in valve height (actually slightly less since the valve is at a slight angle). Don't assume the valve height changed by exactly the milled amount, you have to measure it. If the amount is small it can be removed from the stem tip by grinding, although it should be done carefully and re-hardened afterward.
The exhaust valve height isn't affected, so any adjustment made to stand height will make the exhaust stands too tall.
If the cam is stock, no compensation is needed for this.
You don't need to know how much the intake valve moved up, you can just do the geometry with a few trial shims.
If this allows no adjustment on the pushrod they must be replaced (at least the intakes) to give 1-2 adjuster threads exposed when lashed.
The usual geometry means that the rocker's pad contact point begins with the valve closed at the closest position to the shaft, walks across the stem tip as the valve opens, reaches the center at 25% lift, and the farthest point at 50%. It then reverses, and walks back toward the shaft and reaches 75% lift at the stem center, and the same closest point at full lift.
I suggest you make a trial, and give us the results and we can go from there.

Panic,
Thanks for your prompt reply. All that you said is true. However, I believe I over-simplified my problem. My head has been extensively milled. Unfortunately, it was milled incorrectly. Apparently, the deck surface was not level when milled, resulting in combustion chambers that get progressively smaller as you go from no.1 to no.6! The intake valve seats do not appear to be recessed, indeed, the edge of the no.6 valve rises above the deck of the head! Only the thickness of the head gasket (.053 compressed) and the deck height of the flat top pistons (.080 down in the bore) prevent piston/valve contact.
As it is now, my pushrods are too long, and I don't seem to be able to find a set of shims for the rocker stands that will work. I'm hoping that I can progressively shim the rocker stands to get the rocker shaft parallel to the block deck again, and work from there. I think I can arrive at the correct amount the head was milled if I can find out the original volume and the amount of decrease per amount milled.

If it's just for measurement, a U-shape cut out of sheet brass or aluminum with shears will be fine (the U is the slot so it can pass under the stand without removing the bolt).
AFAIK, the deck and the upper surface were made exactly parallel, which means that inverting it to level on a mill bed (unless the guides are out, you'll need blocks to span these areas) and passing a quill and dial indicator across the length will give the error in thousandths.

It seems to me like your first problem is the different combustion chamber volumes. If the cut was at an angle, lengthwise, then first, you will have to re-machine the deck square to equalize them.You really don't want a different compression ratios in each cylinder. Your rocker shaft pedestal height will all be the same and can be shimmed equally. There is quit a bit of adjustment, are you saying it's all used up?! There should be enough material in the head to cut the intake valve seat to the original position. Don't forget lash caps are on the table too. In the old days folks routinely whacked 1/8th inch off these heads and I personally have came close to that, although, for racing I zero decked the block along with notching the pistons for valve clearance. If you can not re-machine this head, you will need to find another or have a very Micky Mouse set up.

I don’t know about cc’s as yet but the head is 4.000” thick +- .005 and parallel when untouched and I find a granite table/height gauge quickest for detecting thickness/parallelism. The casting thickness from deck to jacket is 3/8”so removing .090” puts it at 3.91”which is probably safe or if you can live with ¼” to the jacket then 3.875” is about minimum (1/8” total removed). If it cleans up parallel at that value or maybe a little less it should be usable but plan on some combustion chamber reshaping which becomes polluted as radiuses are replaced by sharp corners affecting exhaust flow. Custom pushrods may be a cost effective solution too.

I haven't been able to pin down the nominal CC of an 848 head because of different static CR listed for different years, etc. With low ratio, even a few 1/10ths make a big difference.
Using nominal engine size: chamber volume in CC of 1 cylinder =
((displacement ÷ 6) ÷ (CR - 1)) × 16.387
Example:
261 ÷ 6 = 43.5"
If the static ratio were 8.25, divide by 7.25 = 6", multiple by 16.387 = 98.3cc
To get 9.0:1 in a 261 you need 89.1, etc.

As stated above, the rocker arm to valve stem head contact point will not change no matter how much you mill the head if the correct push rods are used. If you do nothing but bolt the head on using stock push rods, then yes, you have changed the rocker contact point and will wear the valve guides out much quicker. You can not shim the whole rocker stand higher to restore this. The rockers must hit the valve stem head at the correct points all across the whole opening and closing values.

There is only one good way to fix this, get shorter adjustable push rods.

Recessing the valve in the head will change the contact points a little, but I would think you would would really have to sink it deep to really need some rocker shaft shimming. If this is the case, you might think about new valve seats to restore the factory valve stem hight measurements. If they are not to bad, the adjustable push rods should compensate for the machining.

On a lot heads, its not a one to one process for milling the heads and raising the compression. If the chamber is tapered, more compression will be added the deeper you cut. So the first pass may only raise the compression a little if any, the second pass will be more and it will multiply as you go. So its pretty hard to come up a number for how much cutting will raise the compression. You need to find a stock head and measure each to get an idea, and then its not 100% since each head is slightly different and with there age, the history becomes sketchy.

Mock up the head on the block and get an idea of the push rod length needed to place the rocker arm in the center of the valve stem at about 2/3 lift on the cam. With this measurement you have a starting place to work from. Milling 1/8" off the head should mean a 1/8" shorter push rod if all was correct before the machining.

Joe

As an amendment to the above, raising the rocker shaft will not correct the marching because of the racker arm contact to the valve stem. You would need longer valve stems that equal the shims you would have to add under the shaft. You cannot change the way the rocker hits the stem with out adding length, or removing length from the valves or push rods. Any change from the factory design will cause extreme stress on the guides and stems.

Its hard to explain, but I think if you mock up the head and block, you see it yourself after studying it a bit.

The number one concern is for the rocker to hit the stem in the center just before total lift or opening. For a full opening and closing the wear on a valve stem head should be equal distance from the center point of the stem. If its off to one side, the geometry is wrong.

Joe

Not following this.
The stock pushrods can be only 1 of 3 things:
1. pushrod is correct (able to get an adjustment): geo is correct.
2. pushrod is too short, adjuster has too many threads showing: geo is correct but slight loss of lift due to slight change in adjuster angle.
3. pushrod is too long: the geo doesn't matter, because if you tighten down the stands the valve is open already.

Gentlemen;

Someone tried to "reinvent the wheel" here.

A stock 261 CID head is 40cc and a slight
bit of milling doesn't change much, that is
done with custom pistons at around $100 each.

You could have pushrods that are designed for
"juice" lifters, using the old style (milk
bottle) solid lifters etc.

Check with Patrick's for the lenghts of each.

Good luck.

PS: The intake vlve can't hit anything as It's
closed at TDC for the piston(s).

261" + 40cc chamber in the head, allowing .030" gasket thickness and .030" deck clearance = 15:1 compression.

Panic you are right about the pushrods. I would delete my posts if I knew how!

What I was trying to say was, you can not correct a push rod problem by messing with the rocker shaft or rocker arms. The geometry will not work out.

The rocker must stay where it is or the contact point on the valve stem head will change.

One other point on the compression figures,did you multiply the final answer by about 70 percent to account for the efficiency of the head and intake system ?

Joe

Panic, re-figure that compression ratio with a .053 gasket and the piston .080 down in the hole. I believe those were the numbers given a little earlier.

261 (43.488" per cylinder) = 712.6 cc, + 40cc in the head, + .053" gasket (estimating the gasket at 3.875" both for oversize clearance and the exhaust pocket) = 10.2cc, + .080" below deck = 14.5cc.
Total chamber volume: 64.7cc.
Static CR: 12.01:1.

The intake vlve can't hit anything as It's
closed at TDC for the piston(s)

Only true if overlap is less than zero (which happens: the intake valve opens ATDC, but not true in the stovebolt even with the 216 cam).
There's enough room to clear a stock or small cam because of deck clearance and gasket thickness. The high-lift 261/Corvette cam only clears because the overlap is so short - the valve has hardly moved by TDC. Cams with less lift but more overlap may have problems.

Dear Panic;

I think I just posted that??

The "hi lift-Corvette cam" only
holds the valve open longer, as
the piston moves downward, Sir.

About every cam grind possible,
has been tried (over the years) &
the intake valve is still closed,
when the piston reaches TDC.

Even "maxed out" 5,000 RPM + they
still don't hit.

Good luck.

...this is where I came in.

Can someone else try to explain it?

I can't say much about stock cams even Corvette but I still have the timing sheet from the engine Ray and I raced at Bonneville. This is at .020 Tappet height and is grind 100-GTL276.
Intake opens 31.2 degrees BEFORE top dead center and closes 64.8 degrees ABDC.

It's the reason I HAD to notch the piston tops at zero deck height and would have even if at the stock height. Modern cam grinds open way before TDC.

To All:
Thanks for your inputs. I haven't been able to look at these replies untill this evening. They are very interesting, and it's clear that there is a large pool of knowledge about these engines on this BB!
I have built several engines in the past, but all have been 'modern' V-8's, such as SBC's and BBC's, along with some Chrysler V-8's. This 261 has been a different kettle of fish!, and much more difficult (read expensive!) than I would have believed!
I went out to the garage to try to find the notes I've made while building this engine, so I could share them with you. I could only find some of the preliminary measurements. I will try to look more tomorrow evening for my build notes.
For what it's worth, here are some of what I've found.
The engine is a 1961 261 'Jobmaster' with the full flow oil filter. It was removed from a '61 dumptruck.
After dismanteling (sp?)the engine, I had the block hot shot cleaned, magnafluxed and minimally 'decked' to present a flat and true deck. The main bearing bores were checked, no align boring or honing required! A deck plate was attached, using the stock head bolts, and the block was bored and honed with it in place. The pistons I purchased are from J&E, .080 oversized, forged flat tops, full floating pins, with moly filled, zero gap rings. The rods are N.O.S. GMC's, magnafluxed, polished, shot peened, bushed for floating pins, and all are 7.000 inches in length. Since the pistons are light weight forgings, I felt (feel) comfortable retaining the stock N.O.S. rod bolts.
The bore: 3.830, stroke: 3.937 (stock), Rod length: 7.000 Piston compression height: 1.840. Deck height of block is 10.870, while deck height of pistons are 10.809. This leaves the deck of the pistons at -.061, below the block deck at TDC.
The crank was magnafluxed, ground and indexed. Mains are .010 undersized, while the rod throws are ground .030, leaving the largest radius possible between throws and cheeks(?). Rod and main bearing clearances are right at .0025-.003.
The camshaft is a Howard's F-298 grind, single pattern, with advertised duration of 268 degrees, and overlap of 48 degrees. Duration at .050 (lifter)lift is 231 degrees as measured. The cam is installed "straight up", and the lifters are new mechanical, from Patrick's. The pushrods are N.O.S. of stock length tubulars from Isky Cams.
I checked all of these measurements several times during assembly.
There's one other thing that may be of interest. I purchased four N.O.S. head gaskets from Tom Langdon. One of these was used during the boring and honing operation. At my request, the speedshop placed a lead shot fishing weight between the torque plate and block deck, so he could measure the gasket's compressed thickness. It was (is).053. I then drew the outline of the fire ring on a sheet of 1/4 inch graph paper, and spent some time filing in the squares, four at a time, and counting them. I did each cylinder, and each one three times! I then averaged the results, coming up with 13.8125 square inches. Multiplying by the ,053 thickness gave me 0.7321 cubic inches, or, 11.999 cc's, gasket volume. (12 cc's, for short :))
This post has been longer than I expected, and somewhat off the subject. I hope, however, that some of you may find it interesting. When I finish this engine, I'm going to write the results in an article for the 12 port news, (if anyone cares):)

Its all interesting. Sounds like you have done your homework,

Joe

That's .114" quench - or, none to speak of.
IMHO, the motor is going to be knock sensitive (the cam will help some).
Unless you're already set on this, I certainly wouldn't run those gaskets. I can't recommend anything specifically, and I'm not sure that going down to .030" will be enough to make a big difference (still have .084"). Unfortunately, the complete cure is either new (taller) pistons or re-deck the block to get the total quench down to .040-.060".
For those light on cam data, that's:
intake opens 24° before TDC
intake closes 64° after BDC
intake lobe CL 110°
exhaust opens 64° before BDC
exhaust closes 24° after TDC
exhaust lobe CL 110°
LSA 110°

Panic,
Like you, I believe you are correct about this engine's knock sensitivity. Unfortunately, I'm now unemployed and am burried in this combination pretty deep! My original aim was to 'zero deck' this engine and mill the piston decks for the intake valve clearance! However, the person (a well known adviser to this club) I ordered the pistons from, either forgot my intentions or just made a mistake. I have tools to measure, but, no means to machine or fabricate. As they say, "It is what it is."
The long block is now assembled, and I'm ready to start assembling the induction system. I'd really like to finish the valve train first, though.
I have initially installed the rebuilt rockershaft and rockers, coating the tops of the valve stems with Dyechem so I could see the contact pattern. This showed that my pushrods were too long by a bunch, and got worse as one progressed front to rear. Hence, my original question.
I really don't want to go to custom length pushrods, as, ideally, I would have to have 16 different lengths! I also don't know who makes adjustable pushrods! Can anybody offer some suggestions about them? Toolman

I'm going to fall back on my original statement. You must make sure the head is equal front to rear. Do us a favor, you and me, put a lash cap on one of the valves and see where it marks. Remember, the adjusting portion of your rocker arm is an extension of the push rod length. To raise the valve stem with a lash cap effectively shortens the push rod. Again, the head has to be equal front to rear, it must be done! Put the head on the block and mark it where the cylinder and combustion chamber mate. Radius off all sharp edges around the chamber and maybe a little on the edge of the cylinder. Polish the chamber and piston tops. Grit your teeth and go for it!

Not up to this yet, but you may have to drop your total spark (initial + mechanical) slightly to avoid WOT knock unless you use water injection etc.
Perhaps -5° was what I would start with, and since the static CR isn't high and the IVC is late vs. stock I would reduce the advance curve and leave the initial alone for now. Depending on how the cam feels, you may want even more initial up to perhaps 15-20° BTDC with the same total (std. -5°).
No carb details yet, but in general you need to have your initial advance pretty close before trying to set idle mixture etc. because it affects throttle disc angle vs. idle and transition holes.

Anything you remove from the total should be added to the vacuum advance travel to keep engine temp and cruise mileage under control.

DIY pushrod kits are available from most mfg. (Comp, etc.) consisting of both ends and a length range. Select the ends based on your tappet and adjuster types, length slightly longer than you'll need. Make an adjustable out of an old one by splicing in a piece of 1/4-28 female threaded tube, and measure each one to give 1-2 threads. Cut the tube, and press the ends on.

Re: "This showed that my pushrods were too long by a bunch"
More details on the pattern?

For testing only, Cut one of the stock pushrods at point where you can get to the cut once its in the engine. Remove 1/2" to 1" of material and weld a nut to each of the piece. Thread in some all thread or any type of threaded rod and you now have a adjustable pushrod. Once you have it in the engine, its just a matter of turning the two pieces till you get the length you need. Then when shopping for true pushrods, you have a good idea of the range you need.

Joe

A rough prediction for your scrub pattern: if you have mid-lift and correct geometry, a .400" lift cam will sweep only .010" across the stem tip. Anything more means something is wrong.

If the pushrod is too long, the rocker will always be angled too far down in all positions, the scrub path will begin at .00" lift too far from the rocker shaft, stop traveling across the stem tip and reverse before 50% lift is reached, and will return to a full-open position closer to the shaft.

Me again, guys,
Your inputs have convinced me that I need to remove the head and dis-assemble it. I'll then try to find a local machinist who will correctly mill the head to bring it back to the way it should have been milled in the first place. Once I achieve that (it may take some time ), I'll re-assemble and go from there. Thanks again for your advice. Toolman

Toolman, I think that is the best choice you have at this point. Good luck and keep us posted with your results.

I hate to cause you to doubt your future machinist, but in my experience more are good tool operators than good engineers. Meaning: you should be the general contractor if you want good results and give him very specific instruction:
1. the original error.
2. the exact angle or extra thickness at deep end to modify.
3. "please record what this was" i.e., "Took .036" off at 1.5°.
4. the intake seats at the deep end (largest mill) will probably need to be done again, and this means either:
a. the stem heights will vary, or
b. some seats will be sunk to match them
You won't know until you see it after the mill, so he should call you at this point.
5. the "hook channel" (gasket surface relief from the intake seat to the exhaust pocket) will be/has been removed by milling. IMHO this should be restored, but this is delicate hand work unless you want to pay him to make a bolt-on guide for a vertical mill cut.

A few years ago, Tom Langdon, before he retired from GM, was kind enough to provide me some combustion chamber cc's based on casting numbers:
3836848 79.1 cc's
3835913 86.2 cc's
3836850 86.2 cc's
3703570 95.5 cc's

I think i missed what you intend to use this motor for, but the cam specs look like a street motor. If your head is severely milled and milled incorrectly, give up on this head, particularly if this is a street motor. 1/8 cuts were common years ago, some milled more but 1/8 was more common. You had to play with those heads to get the geometry right. I did one and the performance gain was not worth the effort ( I was racing a 235). By the way, few of the block deck heights ever come up to spec and there is a good bit of material that can be removed from the deck. Big cuts end up with the same issues with geometry. Frankly, you will be much happier with an accurately machined head. The 848's are not that hard to come by and the aggravation of trying to save that head, shimming, custom push rods, etc., is not worth it in my humble opinion. As for the compressed volume for a 261 head gasket, a felpro engineer gave me the spec for their gasket (i was younger then and did not write the actual part number down, duh)so whatever part number they had in the early 90's was 10.82 cc's. Good building to you!