The Stovebolt Site Forums The Shop Area Engine Shop Engine Compression (and ID) Specifications for 261 engines

The topic of 235 vs 261 compression ratios was recently raised in another post.

Here is a compilation of 1954-1962+ 261 and 235 engine compression ratios, and ID information for 261 engines (USA Chevrolet truck 261).

The 1955-2nd series information is incomplete/confusing, because the 235 and 261 engines changed at some time in 1955. The significant change was the location and style of water pump. It is not clear if the head changed when the block changed (there was no need to change the head, but, we can only speculate, unless someone has GM documentation).

Chevrolet Truck Engine Information
please note: the casting codes listed below came from various web sources: they often conflict, and no source indicates that their casting codes come from GM documents.

If you use this information in another post/publication, it would be polite to indicate where you got it (as I indicate at the bottom of this post).

1954/1955st compression: 7.17:1 (7.5:1 in 235)
Head 3703414, 3702436, 3703414
Block 3635499 ???, 3703570, 3836850
1954 Engine Serial Number:
F54Q, T54Q (5000 series)
F54N, T54N (6000 series)
1955st Engine Serial Number:
F55Q, F54Q55 (5000 series)
F55N, F54N55 (6000 series)

1955nd compression: 7.17:1 (7.5:1 in 235)
Head ?? (earlier head ??)
Block ?? (later, 1956-style block ??)
Engine Serial Number:
1955-2nd series: F255N. T255N (6000 series)

1956 compression: 7.8:1 (8.00:1 for 235)
Head 3703570, 3836850
Block 3733950, 3836340, 3837012 (, 3738813 in 1956)
Engine Serial Number:
1956: F56TA, F56TD (P.T.) (6000 series)

1957 compression: 7.8:1 (8.00:1 for 235)
Head 3703570, 3836850
Block 3733950, 3836340, 3837012
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1957-t-ser.pdf

NOTES:
- the 1954-1957 261 might have had a by-pass oil filter system.
- the 1958 and later 261 supposedly came with a factory installed (standard equipment) full-flow oil filtration system (the first GM/Chevrolet/GMC truck full-flow oil-filter system).
- apparently, factory replacement 261 blocks (cast after 1958) came set-up for no oil filter (or, for a by-pass filter), but a plug on the lower driver's side of the block could be "punched-in" for the full-flow filter system.
- a 261 set-up for the full-flow filter had to be run with the filter installed (or with a hose/line between the two filter-line/ports on the block.

1958 compression: 8.00:1 (8.25:1 for 235)
Head 3836340, 3739365 (Jim Carter), 3759365, 3769717
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717
Engine Serial Number: http://chevy.oldcarmanualproject.com/chevyresto/58t074.htm

1959 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925
Engine Serial Number: http://chevy.oldcarmanualproject.com/chevyresto/59tsn03.htm

1960 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925, 3769923
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1960-t-ser.pdf

1961 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925, 3769923
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1961-t-ser.pdf

1962 compression: 8.00:1 (8.25:1 for 235)
Head 3836850
Block 3836340, 3739365 (Jim Carter), 3759365, 3769717, 3769925, 3769923
Engine Serial Number: http://www.1954advance-design.com/chev-specs/1962-t-ser-ID/

1963 and later (replacement blocks) compression: probably same as 1962: 8.00:1)
Head probably same as 1962
Block probably same as 1962
Engine Serial Number: any code (or even no code) could/might be used/found here (usually/probably the code from the engine being replaced)

please note: the casting codes listed above came from various web sources: they often conflict, and no source indicates that their casting codes come from GM documents.

If you use this information in another post/publication, it would be polite to indicate where you got it.
Much of this information is posted on a page on my website:
Casting codes/etc: http://www.1954advance-design.com/Stovebolt-engine/casting-numbers.html
Serial numbers/etc: http://www.1954advance-design.com/chev-specs/

I obtained most of the above information from GM documents (Shop Manuals & Specifications Manuals), and from:
- Block codes and series/sequence codes from Beck's Machine & Tool on Inliners International (and from Jim Carter) - some of the information from each source is probably incorrect
- Keith Hardy (The Old Car Manual Project - OLD ONLINE CHEVY MANUALS) kindly hosts GM engine serial number prefixes and suffixes and some GM Specifications Manuals
- Truck sequence/series codes/pre-fixes are nicely presented by John Milliman on The Stovebolt Page

One observation on compression ratios: people do a macho growl/grunt about using a 1956-1962 235 "848" head on a 1958-1962 261.

It looks like that change gets you a 3% increase in compression ratio?

Maybe a member who knows engine specs/performance will comment on the value of having drilled and installed a sought-after, hard-to-find without cracks, holy grail (and, relatively expensive), "848" head in order to obtain 3% higher compression (besides, of course, the macho/bragging grunt/growl value).

By the way, I would use the "848" head for conversation-value (If I had a spare "848" head).

Tim, you have noted in your post above starting in 1958, "(8.25:1 for 235)".
For 1957 it is, "(8.00:1 for 235)".
Any idea what boosted the compression by 0.25?
Don't they both have the same 848 head?

Carl

Look again- - - -these are TRUCK engines- - - -none of them are listed with 848 heads, which were used on passenger car 235's. Anything that raises 235 compression will make noticeably more difference on a 261 which has 26 cubic inches more displacement, or a little over 4 cubic inches per cylinder. That's 66 more CC's of cylinder volume being stuffed into the smaller combustion chamber. If a thin steel or copper head gasket is also used, the compression gets another bump.

848's are the highest compression(smallest combustion chamber) stovebolt heads available, equivalent to shaving .100" off the earlier head. It makes a noticeable difference in performance, especially if it's combined with some valve work and a big overbore. Drilling the "steam" holes makes it compatible with the Siamesed cylinder walls of the 261 engine.
Jerry

I figured thinner gasket, about .025 thinner. Wasted effort after reading Jerry's real answer. Thought I really had something.

To really see a compression bump do the "Cadillac" stroker modification on a 261 which takes the displacement up to about 292. That makes an 848 equipped engine a premium fuel only application, even with a relatively radical cam profile. I'm considering doing that to the 261 I've got stashed away. One interesting bit of trivia is that the 261 uses the same diameter wrist pin as a small block V8- - - -.927". That might open up all sorts of interesting piston combinations, including some of the domed pistons made for the 265's, with the same bore size as a 261. Some of the custom piston makers like Jahns, Keith Black, or ForgedTrue used to be willing to make a one-off set of pistons with special compression heights, dome shapes, etc. The stroker uses a 292 rod with a smaller-diameter big end bearing. which also has the .927 wrist pin, IIRC.

Jerry

Carl,

I do not know what changed the compression in 1958.

I would hope there might be speculation on that, and speculation on what the 848 head on the 58-62 261 might result in (which already has 8.0 vs 8.25 on the 235).

There is nothing wrong with discussion on exotic 261 upgrades, they just do not interest me.

Once the temperature in the shop gets a little more friendly, I'll drag out the CC tester and do a comparison of combustion chamber volume on an 848 head compared to some of the other casting numbers. From there, it's pretty simple to calculate static compression ratios using such info as cylinder volume, piston deck height, head gasket thickness, etc.
Jerry

Thanks, Jerry

Hey Jerry, I for one would really be interested in seeing that
set up when you CC the head. Please post some pictures
they're always worth "a thousand words".
Thanks ahead of time,
Denny Graham
Sandwich, IL

Jerry, I have read somewhere, my notes don't indicate where,
that a stock "848" head is 77 cc. Hope your testing will verify that.

It would surprise me if it's that small. The combustion chamber on a low-compression 350 V8 is 76 CC, and just an eyeball comparison of the stovebolt chamber seems to indicate it's considerably larger. That big hole up close to the exhaust valve takes up a lot of space!

Denny, it's pretty simple, just a 1/4" thick piece of clear Plexiglass with two holes in it, and a 100CC burette. The Burette is inserted into one hole, and the other one is an air vent. The plastic is sealed down to the head with Vaseline, and dyed alcohol is the test medium. I use Methanol from the hardware store (shellac thinner) with a few drops of blue food coloring in it. Run the chamber full, let the air escape, and measure the volume used when the chamber gets full. Easy! Of course, a clean combustion chamber with good-sealing valves is a must! Remember to put the spark plug in! I'll post some pics the next time I've got the system set up.
Jerry

Understand the principle just like to see your setup. Always enjoyed watching other people when they work. Keeps me from making expensive mistakes. I almost always pick up a few pointers for the shop from what's in the back ground.
dg

Denny, As a man who craves knowledge, you can buy a relatively inexpensive set up to "play" with along with all your other interests.
Actually, I'm surprised with your back ground that you don't have a burette stored somewhere in your cache.

Thanks for the description, Jerry
(thanks for the question, Denny)

I grew up watching Dad brass-fill and CC cylinder heads on flathead Ford V8's. He ran bored and stroked "239" engines displacing close to 300 CID, with 11.25:1 compression, on an alcohol/hydrogen peroxide fuel mix, and turned a 4 1/8" stroke crankshaft a little over 6,000 RPM! All on 3 main bearings! When one decided to come apart it looked like a hand grenade went off in the oil pan!
Jerry

What are Stellite Valves? There is one 261 block from 1958 only that has listed stellite valves. Why the block? The head says its an 850. Ideas?

One of my sources has it listed as:
3738813 Chevrolet 55-63 261

the other says its a 1958 model with stellite valves

Tims has it as a 56 model.

From the mid 50's, Stellite faced valves were very common in medium sized trucks. Since these trucks were usually operated at full throttle, exhaust valves got hot and Stellite resisted burning. I'm not sure exactly what Stellite is, but it along with positive rotators greatly increased valve life. We always made sure we got Stellite faced valves when we had to buy valves.

I wonder why the engine block is listed that way. Its funny but that particular block, if you do a google search on 3738813 Chevy you get all three choices above about equally. Thanks Ken!

I'm still learning a lot on this site and I appreciate your teachings. My 261 has an 848 head on it. When I sent it to get rebored and hot tanked I was asked if I wanted to deck it. As I was unsure of the consequences I declined. The block could be smoothed out up top but what would that do for valve clearance on the head to deck surface with this head?

Stellite, was(is) used to make hard faced inserts and valves for a long time. If you have hard valve seats, and/or hardened valves they are most likely made of Stellite. Here is a Wikipedia link for more information:
http://en.wikipedia.org/wiki/Stellite
It may help answer questions about what it is made of.

Any head milling or block decking is going to move the valves and the pistons closer together. So will the installation of a "shim" head gasket instead of the sandwich-construction "no retorque" style of gasket. All those dimensions need to be considered when building a high performance engine with a radical cam profile- - - -not so much with a stock engine unless a lot of material is removed somewhere.

The 848 head has the equivalent of a .100" milling job on the earlier head, but the 848 compensates for the closer valve/piston relationship by sinking the intake valves further back into the head to preserve the valve to piston clearance. If the earlier head is milled to get the same compression as an 848, the intake valves must be recessed to prevent interference. Shrouding the valves hurts intake flow, so the compression increase comes with somewhat of a tradeoff in breathing ability. The stovebolt head has port shapes that suck like a Hoover upright vacuum cleaner, so nothing much short of supercharging is going to help the gas flow much!
Jerry

Anyone ever flow test an 848 against a standard head? Lincoln, you claim the stock ports are garbage, any idea what they actually flow? Also would it be a waste of time to try and clean them up? I understand power is made with the right combination of goodies, but 8:1 and a plugged up head seems like a lost cause.

I can give you a little anecdotal information, since you stubbornly refuse to build a stovebolt to see for yourself. Back in the early 1960's I was a pit crew member on a team where we ran two supposedly identical 46 Ford coupes in the "stock" division at the Nashville Tennessee fairgrounds race track. Our only competition was a local body shop that sponsored a 52 Chevy coupe with a 261 engine (They were cheating, but so were we.) The 261 was an illegal engine since it was a "truck only" application. They tried to skirt the rules by calling it a "Canadian Pontiac" engine. They had about a 20 cubic inch advantage over our engines, since the "239" flathead engine could only come up to about 250 cubic inches with an .080" overbore. (Max the track rules allowed) The .080 over 261 was noticeably bigger.

Despite the displacement difference, the flathead engines didn't have much of a problem outrunning the 261. "There's no replacement for displacement" doesn't work if the engine can't breathe! The flatheads had some restriction in the exhaust ports, but they responded much better to porting than the Chevy.
Jerry

I understand I haven't built a stovebolt. Nor do I plan on it, I was curious about flow numbers compared to the stock head. I'm not trying to start anything with you. I appreciate your expertise on the stovebolts. I thought sometime in the past you might have actually flowed a head was all. My 383 is only 8.5:1... Grab a drink and relax man.

What would be a standard/stock head?
The 848 was the standard/stock 235 head for a long time.

You might want to go the HiPo Shop to growl about esoteric oddities (esoteric to non engine builders).

In this thread, it would be interesting to know the difference(s) between the standard/stock 58-62 235 head and standard/stock 58-62 261 head.

Thanks,

This is about as close to anything useful I have found so far. When Jerry does his testing we will know for sure...

I haven't made an actual A > B swap to compare effects, but I think it's probably a case of "people tend to see what they want to see". Curt Bergeson CC'd a few common castings and found 79.1 cc (4.83") for the 848, which is the smallest chamber by a small margin. Other common heads: 850 = 86.2 (5.26"), 570 = 95.5 (5.83").
A worst to best comparison on a 235.5" stock motor: if the engine had 7.25:1 CR with a 570 head, its total chamber volume (including deck, gasket etc.) is 6.28". Swapping an 848 head subtracts 1" (5.83 - 4.83) for 5.28". The new static CR is 8.43:1, a very substantial increase, the largest gain for any chamber size-derived swap. A bigger engine or one with a higher CR will have a greater increase.
In round numbers, raising the CR from 7.25 to 8.43 picks up about 5.6% efficiency, and perhaps worth 7% in power.
For a cam with 50° ABDC closing, the DCR numbers:
the 570 head 6.40:1, 122 psi gauge pressure, about 32 hp @ 1,000 RPM
the 848 head 7.42:1, 148 psi gauge pressure, about 40 hp @ 1,000 RPM.
As you know, besides a head swap and (limited) head and block milling, the only remaining avenue to really high compression is a domed piston, $$$.

I got that from a friend on the internet. No idea myself. Looking forward to Jerrys tests.

The reason the 848 is so important to the 261 is not for the gruntvalue=max. Its because real 261 heads are very hard to come by, and the 848 was the most produced modify-able head.

A search of stovebolt.com and /or the "web" would lead one to believe the use of the "848" head on a 261 was to obtain a compression gain.

Yes, a few facts/figures would be nice : 58-62 261 head vs 58-62 235 head (with no special work done on either of them).

Great info Deve. Tc, sorry if I upset you and Lincoln. I'm just curious is all.

Hollow65 - you did not upset me (sorry for being "snippy").

I am hoping that someone does a comparison of the features/characteristics/differences of standard 58-62 235 and 261 heads.

We've beaten the "hotrod the stovebolt" subject to a bloody pulp repeatedly. There's nothing wrong with the engines just the way they are. They're never going to be tire-smoking torque monsters, no matter what's done to them. Somebody can always use a big hammer and a greasy shoehorn to stuff an outrageously inappropriate big block engine into most of these trucks, even if they lose all the originality (and streetability) in the process. At some point, it becomes an exercise in futility to get the idea across to those folks that most of us on this site have no interest in such a Rube Goldberg cobble job. Different strokes for different folks!
Jerry

Tired, smoking over-torque monster? I had one last year (which was not good). It was an original ’55 235. Anyways, Jerry was nice enough to provide me with a replacement '848' head for my rebuild last summer- pretty much just to help me out. Thanks again, Jerry.

I have since acquired a 1960 235 ‘848’ head to repay the kind gesture, but haven’t gotten it up there to him yet. This sounds like an interesting test, so as soon as the travel weather cooperates, schools stay open all day, and the stars align I'll deliver a 'good' cylinder head for the testing.

Brad

Thanks, Brad
Thanks, Jerry

Jerry, do you have available a 58-63 261 head, for comparison?

Thanks,

I've got several heads, not sure if there's an OEM one for a 261 among them. The 261 engine I traded for awhile back had an 848 head on it already, I believe, but I'm going to have to check the casting number to be sure. That engine is going to be the basis for a 292 stroker eventually, as soon as several other automotive and gunsmithing projects are caught up. Six months' recovery from the bike wreck has really put me behind the power curve, and traveling all over the southeast making some pocket change to finance my hobbies isn't helping.

I thought I was supposed to be "retired"- - - -whatever that means! At least now I'm not a slave to a bunch of incompetent public school administrators!
Jerry

We have the cc numbers for the 848 and 850 above. How that translates to power on the 261 is still in question. Then I would like to see a second set of numbers to confirm. I am not smart enough to do the computations, but isn't that computable?

79.1 cc (4.83") for the 848
86.2 cc (5.26") for the 850

Only in some engineer's pipe dream- - - -dyno readings, corrected for temperature, atmospheric pressure, and humidity, are the only figures worth considering. I'll be happy to provide those numbers, if and when I ever get my dyno back in operation.

There are far too many variables, such as fuel mixture, cam profile, exhaust system, carburetor, air cleaner, distributor advance curve, and about a dozen other considerations before something as basic as combustion chamber volume will make much difference.

Then there's Murphy's take on the whole thing- - - -"Constants aren't; variables won't!"
Jerry

I'll throw in my 2 cents on this topic. I flow bench tested an 848 head with various CNC machined alterations and sadly none of them made an appreciable improvement in flow. The reasons for this can explained in this picture. The short side radius is only 1/8" and as flow is increased the charge bounces off the back wall and becomes disorganized. Enlarging the runner from the intake (as documented by Frank McGurk)actually sacrificed velocity and was detrimental to flow numbers. The guy that did the testing for me had considerable racing engine experience and tried putting a clay "wedge" on the back wall of the port to help direct flow but it didn't really help due to the overall poor shape.

Exactly what most of us have been saying- - - -these engines are built for low-end torque, and efforts to create high speed HP are pretty much doomed to fail. The cutaway reinforces what we've been told about just how futile anything other than maybe a little pocket porting right under the valve will accomplish.

Thanks for the pics!
Jerry

The discussion about horsepower and torque might be interesting and might be fruitless, and it might be of more interest in the Hi-Po forum,

However, one question asked above is closer to being a matter of physics (I think) and it has to do with standard equipment/parts: what might be the difference in compression that would result from placing a stock "848" 235 head on a stock 58-62 261 block?

Thanks,

3835913, I think this is a 235 truck head but I lost the source. Anyone have compression #'s and accurate year for this head?