The Stovebolt Site Forums The Shop Area Engine Shop 261 head vs. 848 head

Currently running a 235 with a 261 head. Recently found a 3836848 head, literally a barn find. It's dirty but hopefully rebuildable. How much of an increase in compression and noticeable power would the 848 head have over the 261 head I'm currently using?

The 261 head number is 3703570.

This is off the top of my head but a 261 head could be found in a 86 cc version or a 91 or 92 cc version (I forget whether it is 91 or 92). The 54-55 5913 head is 86 cc. For the 1956 model year, the 848 head was introduced with a chamber of 79.1 cc. This is why everyone heads for the 848 head.

I’ve long wondered about this. I understand the smaller combustion chamber of the 848 increases compression, and in theory, should net a performance gain.

HOWEVER, I’d love to hear from someone that noted a “seat of the pants” performance increase by going from a larger chamber 235 head to an 848 head. In my simple brain, it’s enough CCs that the difference ought to be to be material.

On the other hand, we are dealing with a pretty low performing engine that has many other limitations.

I only owned one with the early head. I remember noticing more power when I got a 1956 year model, but there were other changes along the way. I figured they all helped in degrees. More than anything else, it seems to me the right camshaft and the right gearing helps with power management/optimization. The best performing inline six I ever owned was the Pontiac OHC. I hoped the Datsun inline six would be good (first version of the Maxima). It ran very smoothly but performance wise never lived up to the advertisements.

Don't automatically reject the heads with larger combustion chambers. The 1954/55 "low compression" head can be made to equal the compression ratio of the 848 by milling .090" off the head gasket surface, and recessing the intake valves back into the head the same amount to protect the edge of the valve from the flame front as it travels across the face of the head. That modification was pretty common with hotrodders in the late 1950s after the 848 head was produced. BTW- - - -stainless steel exhaust valves for a Chevy small block V8 are a drop-in fit on all the stovebolt cylinder heads. There's no need to drop a truckload of dollars installing hardened valve seats!
Jerry

I added an 848 head in place of the 55.2 head (don't know the casting #). I noticed two things: better fuel economy, more torque. It was noticeable on the road. I will admit that the intake valves on the original head had recessed into their seats to the point where I'd run out of adjustment on the rockers. Hence the change of heads. I'd calculated the CR to be about 6.8:1. So bumping to 8 was a significant upgrade.

Jerry, could you explain a little more? Why we don’t need hardened seats on stovebolts? I know that’s been your position for a long time, but I don’t recall the reasoning.

I'm not Jerry, and I certainly don't have his knowledge or experience but virtually no cars need hardened seats except maybe high revving race engines that get beat on constantly.

When the US mandated the removal of lead from fuel, every car on the road didn't suddenly burn its exhaust valves nor did they die premature deaths. Contrary to popular belief, unleaded fuel wasn't introduced in the 1970s. AMOCO introduced its famous super unleaded in 1923.

Every vehicle I own and drive uses unleaded fuel with no additives. None of them have hardened valve seats. I've put 150,000+ miles on my '56 Cadillac in the last 20+ years and it burns unleaded fuel without any problems. Ditto for all my other vehicles from the pre unleaded mandate era.

I think the issue of valve recession depends on the service the engine is put to. I think hard seats would be unnecessary in a passenger car or pickup application even with no-lead fuel. I have found that performance motorcycle engines suffer the valve recession fate and hard seats are a good solution.

In truck applications many if not most Chevrolet engines used valve roatators on the exhaust valves. These reduced the tendency of hot valves to temporarily weld themselves to the seat at the same point repeatedly, creating pits and eventually enough to begin leaking. Other truck engines have used sodium-filled stems along with rotators, amongst them heavy duty Ford V8s and International V8s that I have worked on. These engines run full throttle most of the time.

I had a head rebuilt using stainless valves and bronze guides but no rotators. It's run about 50k miles and is still OK but next time I pull the head for any reason, I'll install a set of NOS rotators I bought off Ebay.

Ditto to all the above comments. The only engines I've found that absolutely need hard seats are the ones using "dry" fuels- - - -Propane or compressed natural gas. For some reason, those fuels can cause the exhaust valve seats to erode and the valves to sink deeper into the head. If someone who is rebuilding a stovebolt head wants a little extra protection, replacing the exhaust valves with stainless steel ones for a small block Chevy V8 is a lot less hassle and expense compared to replacing the valve seats. They're a drop-in fit- - - -no modifications but a good 3-angle valve seat job needed, with an.090" valve to seat contact patch.
Jerry

I believe the lore of the "848" head is due in part to the increased compression without the pain of milling and recessing the valves. As 1Ton tommy said you would eventually run out of adjustment on the rocker arms. Just using a Compression Calculator on the web it's easy to see the difference the head swap would make.

Do 235 heads use the same valves as the 261?

Maybe that's why a stovebolt intake valve is a half inch tall above the keepers? Every valve grinding machine I've ever used in the past 60+ years has had a fixture for shortening the valve stems accurately. On hydraulic lifter engines with non-adjustable rocker arms, adjusting the stem length to get the proper preload on the lifters was always part of a good valve job. Doing stuff like that is what separates the pros from the shade tree wrench jockeys.

Another option to correct the valve geometry when milling a 54-55 head to get 848 compression would be to put a 3/32" (.093) shim between the rocker shaft stands and the head, and use a hardened steel lash cap on top of the exhaust valves.

Most 235s and 261s use the same valves. The only exception I know of was the 1950-53 spray-oiler 235s used with Powerglide transmission applications- - - -they used a bigger intake valve diameter- - - -1.94 inches. The later model 235 and 261 engines can be fitted with the bigger intakes.
Jerry