The Stovebolt Site Forums The Shop Area Engine Shop 848 heads

Did GM make a lot of these? What percentage do you feel are cracked? Like to get one if I need to rebuild my 235--Thanks Bill

If you did the Coke/Pepsi test: 848 head vs. 913 head in no way would you be able to tell the difference in performance. You would have to have Hotrod Lincoln's experience to tell the visual difference (outside of the obvious number designation).

The 848's are getting scarce enough that it might be necessary to go back to the old-school way to increase compression- - - -mill the older head .090" and recess the intake valves the same distance to keep them from hitting the pistons. 848's were made standard equipment on passenger cars around 1957 or so, I think. I'm sure one of the part number gurus will be around shortly with more accurate information. I don't think one particular stovebolt head is worse about cracking than another- - - -they all have a habit of doing it.
Jerry

I am not ready to put the blame of cracked Stovebolt heads on the manufacturer. These trucks were farm implements which were rode hard and put away wet.
Brass radiators were not cheap. The radiators would spring a leak and the farmer would pour in stop leak until it fixed the leak or filled the water jacket, whichever came first. Ever wonder what all of that hard black concrete-like junk is which we find in the bottom of the water jacket on many of these old engines?
Repetitive over-heating is what likely led to the cracked heads, not design flaws.

Yes, these engines received a lot of abuse, but what about all the pampered passenger cars that developed head cracks? Pouring a hundred pounds of molten iron into a fairly primitive sand mold had to be a pretty good challenge with the foundry methods in use 70-something years ago, and the basic design of the stovebolt six engine originated in the 1920s. The head design has to be a factor, just like the design of the cooling system of flathead Ford V8's made them prone to cracking blocks- - - -right across the exhaust valve seats. Finding a 59-A series flathead without cracks to modify for the dirt track got to be quite a challenge, also.
Jerry

Yes, GM made a lot of the "848" heads. All high oil-pressure car/truck 235 engines from 1956 through 1962 (and all post-1962 GM replacement heads) were "848" heads. Only the 1954/55 235s did not have the "848" heads.

Nonetheless, many "848" heads seem to have cracks, and good ones will become rarer every year. Good 235s are often kept in service. Bad-headed 235s are taken out of service. Spare heads come from out-of-service engines - which may well be out of service due to a cracked head.

If you are buying a head from someone, pay to have them test/check the head for cracks, before paying for the head (or, pay more for a pre-tested head).

Depending on where the crack is, and how deep a particular stovebolter's pocket happens to be, some head cracks can be repaired. There are a number of methods of fixing a crack, some more expensive and/or effective than others. One of my favorites involves "sewing' a crack with a series of overlapping threaded tapered plugs made of similar cast iron material as the head. The fix involves finding both ends of a crack, "stop-drilling" it to prevent it from spreading, and drilling, reaming and tapping a series of tapered holes which are then plugged one at a time with cast iron inserts, The insert is cut off flush with the head surface, the next hole is drilled and tapped to overlap the previous one- - - -etc. until the crack is eliminated. The downside is the setup cost for a machine shop to equip itself to do the work- - - -several hundred dollars' worth of tooling, and plugs sold in minimum quantities of 100.

Welding is another option, with its own set of difficulties- - - -mostly preheating, post-heating, peening, slow cool-down and expensive special nickel-alloy welding rods.

Wait a few minutes, and some Google Guru will be along with a page full of research to "prove" I don't know what I'm talking about!
Jerry

So Jerry what is the difference between a 848 head and
a 913 head?

Is the 913 head good as the 848 to improve performance?

Jeffrey

As I said, the main difference is a .090" mill job, resulting in a smaller combustion chamber and higher compression. I don't believe there's a noticeable difference in the port size or shape, so breathing should be the same regardless of the casting number. The access hole for the temperature gauge is different on the older head, so fitting some types of aftermarket temp gauges can present a problem.

Milling the 54/55 head moves the intake valve closer to the piston, resulting in an interference problem unless the valve is recessed into the head the same distance as it's milled, and it's possible some shimming of the position of the rocker arm stands and possibly some shortening of the intake valve stems might be required to restore the proper valve system geometry. Any oldtime engine machinist would have been aware of these differences and the way to compensate for them, but it's getting necessary to hold a séance to ask most of them a question these days!
Jerry

I understand milling a head decreases combustion chamber
volume thus increasing compression ratio.

The thing is another poster said that putting on a stock 913
head will boost performance as would bolting on a stock
848 head.

So would those looking to hop up their engine
be just fine going with a 913 head being people charge a
premium for an 848 head?
Jeffrey

So what is the same or different between those two heads?

The 913 head has lower compression. The engine will produce marginally less HP and torque, compared to an 848, with it installed. Whether or not anyone can tell the difference in performance without the engine being on a dyno is debatable, as some people can be very imaginative when it comes to "feeling" a performance gain.

50-something years ago when I was working on Chris-Craft runabouts powered by the old Graymarine conversion of the flathead six Continental industrial engine, everybody wanted more RPM. Getting the tachometers calibrated to indicate 200 RPM faster than the actual engine speed at full throttle got me the reputation of being the best boat engine tuner on Old Hickory Lake. Everybody bragged about how much faster their boats ran after I worked on them!
Jerry

After milling the head and sinking the intake valve by the same distance, some adjustment must be made to recover the valve train geometry.
Shimming the rocker stand corrects the rocker-to-intake-valve relationship, but causes other problems:
1. the intake pushrod is now too short, and may be out of adjustment range.
2. the rocker-to-exhaust-valve relationship inherits the .090" error, in reverse.
Assuming the other dimensions are correct, the easiest fix is to remove .090" (or whatever mill-and-sink distance you used) from the intake stem tip + slightly more to allow a lash cap to be used to protect the softer tip; use whatever the mfg. gives as the cap thickness (not the height) here such as .060".

You could also angle-grind the head, removing most of the metal from the exhaust side of the head. The intakes won't need to be recessed, and the geometry should not be affected.

To add to the compression equation, I have been told that if you bore these engines out and use the commonly available oversize cast aluminum pistons, that those pistons are cast to reduce the compression back to approximately stock spec, so you have no increase in compression due to a larger bore size.

I wonder if this is true?

They would have to have a closer dimension from the wrist pin to the top of the piston, or a recess cast into the top. As long as the piston comes to the top of the block and the head gasket thickness is the same, the only way to lower compression pressure would be with a dish-top piston.
Jerry

Compression ratio can also be altered by adjusting the height of the top ring land. This method is not talked about often. In fact, I'm not even 100% schooled on it myself, but if my memory serves me correctly big block chryslers were one engine where you could get multiple top ring height pistons that affected compression ratio.

Chrysler owner since 1964, never heard of it for any purpose. Never seen or heard of a passenger car piston with an extra ring land.

If someone has an engine to use for test, I need the dome OD (not the bore), B&S, static CR, and how far the top land is moved (up limited by the minimum crown thickness, down limited by proximity to the 2nd ring) to do a calculation of the effect.

I've built round track engines with pistons that had an L-shaped top ring that put the top of the upper compression ring even with the top of the piston. The top ring groove had to be specially machined to accommodate them. The guy with more money than brains who commissioned the engines insisted on using them, and I was never able to get a measurable increase in HP or torque on the dyno compared to engines with conventional rings. I was glad to take his money and do it his way, however. He also had me build a few engines with "zero-gap" rings- - - - -same result, all they did was give him something else to brag about at the bar all of us went to after the races were over on Saturday night.

"A fool and his money are soon parted!"
Jerry

^^^ LOL

Sorry, didn't mean an extra land, what I meant was there were pistons made with the top land in different positions relative to the top of the piston. This affects static compression ratio.

The fun way to go racing is on the other guy's money! I've had a lot more enjoyable times racing as an engine builder and crew chief than I ever had running my own car!
Jerry