The Stovebolt Site Forums The Shop Area Engine Shop 261 chevy six

What were the years, and models that the "261s" were available in? I`m guessing that most of the external parts are interchangeable with the "235". Thanks Guys, JD

261's were built from 54 to 62.


Virtually all the parts of a 261 interchange with 235's except the rods and the pistons.

Look for them in 2-ton trucks and school busses.

Thanks guys, What about the "848" head? Whats the deal with that? Bigger valves? Higher Compression? What are they found on? It sounds like thats the setup everyone wants to run. I`d like to put one of these things together, and I`d like to do it right the first time, with the right pieces. Any helpful suggestions, and info. would be great. Thanks, JD

The 848 refers to the last three numbers of the serial # found on the top of the head next to the valve cover on the manifold side. I think the last year made was 1959. Its a 8.25 compression head stock and is a good platform to upgrade for the 261. You'll have to drill a steam hole that matches up with the 261.

The 848 head is a 235 head, made from mid '56 to the end of the production run in '62. It is cast with a higher nickel content, to help reduce cracking. It also has the highest stock compression ratio offered for a 235. It will bolt right up to a '261, but like 52pu said the two steam holes will have to be drilled.

As far as ports and alloy go, the 848 head and the corresponding "850" 261 head are exactly the same. Only the deck height is differant (and the extra water passages are drilled) on the 850 head, as far as I can tell, to give 7.5:1 compression on a 261. Also, the 850 head has Stellite exhaust valves. The 848 may or may not. The valves and ports are the same size between the two, as are the rocker arms and the oil passages.

The 261 needs extra water passages to the head to prevent steam pockets, since it has siamesed cylinders. It also uses a bypass type thermostat on some applications, which helps prevent heat pockets while it is warming up.

The 848 head is put on 261's in an effort to increase compression slightly. If you are building a 261 from scratch it is not completely necessary since you can deck the block or mess with the pistons to increase compression. If you are putting a blower on the engine a lower compression head might be better.

Please bear with me while I try to understand, I`m new to the 6cyl. scene. So the reason most guys are seeing close to 9.0 compression with the "848" head is because they`re using it on the 235 eng? (smaller bore than the"261") How about this. Do you think the higher compression "235" is a fair trade off for the "261", or is torque still King? Buy the way, the intended purpose is nothing fancy or wild. Just a good dependable, bullet proof "6" that is respectible. JD

Two additional passages (small water holes - for coolant) between the block and the head are needed for each pair of "siamesed" cylinder bores - six additional holes in total (in comparison to the 235 head/block).

The two small holes are shown below for one (of three) pairs of cylinders. The holes in question are the two smallest diameter holes in-between the cylinder openings in the head gasket.

Tim

[img]http://ares.cs.siena.edu/~lederman/truck/Photos/261headgasket.jpg[/img]

JD54

If I recall correctly, the factory horsepower rating for the 235 was 140, and for the 261 was 148. Not much difference. Either one will make a good, strong running engine. Parts are available for both. You will probably pay for the 261 initially, but after that it will be the same as a 235. In my opinion, its 6 of one, half a dozen the other.

The huge difference between the 235 and 261 is TORQUE ! I don't have the figures here but I know there is a big difference. Jim

My 2 cents... The Canadian 261 used in Pontiacs came from the Ottawa factory with the 848 head. I've checked several of them at shows,auctions,junkyards,etc. and know this for sure. I have a spare one in my garage I scored for peanuts. Look at Jim Carters site for more info. Here's another one going cheap if anyone's interested.
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&category=6386&item=4548727307&rd=1

Curt B.

You get more compression using an 848 head on a 261, but not 9:1. It's more like 8.3:1 unless you also deck the block.

You can also use a much more radical cam on a 261. 200 hp is pretty easy to get.

Also, you can build a Cadzilla 261 by using Cadillac pistons and rods and offset-grinding the crank to increse the stroke. Inliner's 12port news has an article on this.

Below is the technical data on the 1954 261 in 1954 Engineering Features book - it provides a comparison of hp and torque among 1953 Thriftmaster and 1954 Thriftmaster, Loadmaster, and Jobmaster engines.

1953 and 1954 235 Thriftmaster engines:
[img]http://ares.cs.siena.edu/~lederman/truck/Photos/2355354torquehp.JPG[/img]

1954 Loadmaster 235 & Jobmaster 261 engines:
[img]http://ares.cs.siena.edu/~lederman/truck/Photos/261235torquehp.JPG[/img]

The percentage improvement between 1954 and earlier engines is greater than the percentage difference between 1954 235 and 261 engines.

Nonetheless, my 261 was noticeably more powerful (low and high end) than the 235 it replaced (even before the 261 was "hopped" up). My 261 is a 1960 model from a large school bus and the torque/hp ratings most likely improved in comparison to the 1954 261.

JD54, You commented about 235 external parts fitting the 261 engine. Besides water pumps and thermostat housings being different but interchangeable (within same years - I think there was a change in 55/56 in both 235/261 engines), the only major external-part difference is the '58-'62 261 has a completely different oil filtering system (not interchangeable with the 235). I think the Canadian 261 has the same oil filtering system as the 235 (for all years).

The flywheel/starter changed with the switchover to 12v in both the 235/261.

Tim

I have noticed that my 261 is more agreeable to shifting into a higher gear before sufficient road speed. I loved my old 235 but the whole truck would shudder if I did that until I got the road speed and RPM's up. And I have a longer 4.10 rear-end now instead of the old 4.57.

I wouldn't go nuts in buying a 261 but GM and I think they are worth it.

Of course, I had the Carter W-1 on the 235 and have a Rochester Monojet on the 261.

Thrre is no reliable dyno tested power ratings for the 261 except for the article in the May 55 Hot Rod magazine.A stock 261 on a dyno devoloped about 112 net HP at 3600 rpm.The net torque was around 200.A fully modified one,considered borderline streetable then,made just under 200 hp.In the 50 years since that test,nothing has really changed with 235/261 speed equipment as far as maxiumum power is concerned.Even guys who grind cams for these engines,as well as the GMC's don't even know the correct rocker arm ratio.
The only person I know who's seen a modified street 261 or 235 on a dyno is Patrick Dykes.He claims he's never seen a streetable 235 with more than 175 hp or a 261 with more than 200.
Leo Sanducci,who wrote the 292 hop up book ,drag raced 261's before 292's .He told me an all out 261 drag engine with major head work and a 2000 rpm idle makes 280 hp on a good day.And it needs to be reved to a scarey 6500 rpm to get the power.
These are fun engines to mess with however.

Hey, Tim-

What about a 235 with a 261 cam. Reckon it will have an identity problem?

fab51,

When the time comes to rebuild a '54 235 for my '54 Suburban (that I am trying to keep as stock in appearance as possible), I plan to use the 261 cam in the 235 (as the Corvette did). Might even try to find a '54 261 carburetor (which the Corvette did not do).

Don't know if I would use an 848 head, though (I got one from eBay a few years ago, on a '54 Loadmaster engine). The 848 casting number is a dead give-away.

What do my students and NASCAR engine crews say? It ain't cheatin', unless they catch you.

Tim

Great info from everyone, thanks very much. I really don`t intend to Hot Rod this truck. I doubt if it will even see dual exhaust, however thats not to say that I`m opposed to good `ol stump pullin, raw dog TORQUE! The most important thing to me, that will make it the most fun is 100% dependability. Start every time, never run hot, and I`m happy. I think that with the stuff I`m learning on here from you guys, I can choose the right rear end, and trans, and be able to "putt" around town, and drive 70 down the hiway. That said, has anyone had any overheating problems with 261 siameased cyls? (provided steam holes are drilled in the head your using) JD


rod