The Stovebolt Site Forums The Shop Area Engine Shop re: 261 versus 235 head specs

Someone who understands this, please take the time to reply.

I posted this question the other day but hit a dead end and am still seeking knowledge: http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=000008

I'm very puzzled to understand why GM went through the trouble to beef up the 261 valve springs and change guide heights if it makes no difference. I'm convinced they didn't do it just for the fun of designing, buy, stocking and using no-value-added part numbers parts in the factory.

Mike, you might think about taking this tech question to the guys with all the info at:
www.inliners.org
They will most likely give you an answer that will fill in all the gaps.

Well let me try. I have personally swapped a number of these with no problems. All I did was the steam holes drilling on all but a couple. Those were to be used like a hot rod motor so I used heavier valve springs, not 261 factory but heavy duty from a cam grinder.

My take on this is that there is a theoretical need for heavier springs to follow the slightly steeper ramps on the cam. But, at the rpm we normally use a 50 year old motor, it is just not needed. Same for the guide specs, in heavy duty truck usage the added support is needed, but not for our use.

My dad and I used a 261 with an 848 head in a 51
Chev 1 ton for a wrecker daily for 5-6 years with no issues.

I think this was just a design for worst case use, trucks at fairly high rpm under heavy load for long periods. That is not the description of most Stovebolters usage.

I would have guessed that it would have been for durability also instead of a performance advantage. Considering that the 261 was intended for the bigger trucks and their operation.

Long story short - I was told the head is the difference (with the smaller combustion chamber), not the valve train, and if there is a difference in the two valve trains, the reasons are not enough to dump the 848 head for the 261 head. My machine shop didn't even acknowledge that much difference in the two valve trains and used parts from both to assemble the valve train. Maybe there is something to learn here, but I have never heard anyone mention any significant difference. I remember that the 54-55 Corvettes used a 261 cam, but I'm not sure just what head was used or valve train. Another differnce was the use of solid lifters with all 261 engines - perhaps that is sigificant.

Heck, I just followed the crowd on this, but would be interested to know if any significant difference is really notable. Let's keep talking about this - I think it is a very meaningful subject to allot of us for our engines.

Joe

You guys already eliminated my fears with ignoring the spec differences. I am a curious george on this subject. Thanks for all replies and please do keep this subject alive by jumping in.

Mike

I've noticed that with the larger trucks (i.e. 1 1/2 ton and up) that they are typically driven different than the 1/2 - 1 tons. When higher velocity is needed, the foot goes all the way to the floor, the carb opens all the way, the engine is providing max torque (for that rpm). When the desired speed is reached, the gas pedal is let up on to maintain the desired speed. In these cases the weight of the truck is so large with respect to the engine capability, that acceleration is always done at full engine torque.

Now think about a 1/2 ton short bed hotrodded with a 350, Holley 4, etc... If you put your foot to the floor every time you want to accelerate, you'll be buying new rear tires often from all the burnouts you do. With smaller trucks people tend to push down on the gas pedal proportional to the desired acceleration they want. Therefore in a smaller truck the engine is seldom producing full torque for that rpm.

An engine that is subjected to a harder life, i.e. 261 in a 2 ton, should be made stronger to last the same amount of time as a 235 in a 1/2 ton. I think that is why the 261 has some stronger components, but specs out similar to the 235 in torque output.

Now what do you want from your engine? Put the rebuilt 235 head on the rebuilt 261 engine, dump it in a 1/2 ton and it'll be a hot engine in comparison to the stock versions, and the head will probably be sufficient for non racing driving. Dump in a offset grind crank, over bore the cylinders, hot cam, increase rocker ratio, port and polish head, headers to a dual exhaust, triple rochestors or 500 cfm Holley, etc... In this case beefing up the valve train sounds like a good idea to me.

All this being said, I've got my eyes out for a junk yard 261 to rebuild (with the 235 head) and drop in my 58 1 ton.

My $0.02.

Much of the 261 design was for maximum cooling. The cylinders are siamesed, and the valves tend to get pretty hot under hard load. The 261 used a bypass thermostat and steam holes to alleviate the problems associated with this, along with heavier springs which tend to keep the valves in contact with the seats better, to avoid burning. The 261 used Stellite exhaust valves. The funny thing is that when I pulled the head of the 6 in my BMW 325i it has almost exactly the same configuration and design of cooling passages and a bypass thermostat. The valves are much bigger though.

A lightly worked 235 does not require this stuff, although the presence of it may prolong engine life by keeping the heat evenly distributed in the block.