The Stovebolt Site Forums The Shop Area Engine Shop inline 6 engine specs

I have been searching for some info and been getting some conflicting information. This is for my own information. The 56 factory supplement to the 55 manual says the engine is a 235 however the 55 manual says it's a 235.5 the only difference I can see from the specs from the 55 to the 56 is the shims on the 4 main caps. What I have found is a 235 bored over .030 is supposed to be a 239.4 CI.

Here is where I get lost and math is not my best subject. I am trying to find out these questions to fit my application. I have listed my info below. Any help is appreciated.

#1 bore/stroke ratio
#2 rod/stroke ratio
#3 rod length
#4 stroke size
#5 bore
#6 bore and stroke

Here is what I have:

1956 chevy 235 bored .030 (block # 3837004 serial # 0365139 F56X) fresh rebuild

Reconditioned connecting rods

Head (3836848) hardened seats

Solid lifters W/ stock 261 cam

Manual transmission

Crankshaft -.010 X -.010

Dual Carter/Weber 2 bbl. Progressive carbs

Fenton intake manifolds 2X1

Fenton split exhaust manifolds

What I found using the net is
3.560 bore
3.838 stroke
6.813 rod length
1.7751 rod/stroke ratio
0.9276 bore/stroke ratio
3.5625 X 3.9375 bore and stroke
Torque 210@2400 rpm
HP 140@4200 rpm

Thanks guys

The 235 has been a 235.5 ci from the beginning.
http://chevy.oldcarmanualproject.com/shop/1941/41csm198.htm

And stayed at 235.5 ci when it went to high-pressure lubrication.
http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0647.html

They are commonly referred to as a 235, but some non-GM parts manuals/lists refer to them as 236.

http://chevy.oldcarmanualproject.com/chevyresto/56152.htm

not too clear what you're looking for Jamie, the "235" is the nominal size, many GM engines are a decimal amount over or under their nominal size, and any engine that's been bored will be slightly larger cubes because of that increased bore .... the 55.2 owners manual specifies "235.5", as do all the other year owners manuals .... hoping you've got a few more horses or more torque? probably, but 150hp rather than 140 isn't really chest puffing territory, that's just the stock 261 over the stock 235 .... also I believe the later 235's were rated at 135hp rather than 140, maybe GM fudged the numbers somewhere

Bill

Here's an easy formula that you can use to find the displacement of any engine, once you know the bore and stroke dimension.
Bore x Bore x .7854 x Stroke x # of Cylinders.

Hey hey no chest puffing here if I was going to do that i would have built a sbc or bought a hellcat. My real reason is that I always see the different online calculators and as I see the different numbers and different information they ask for. Me not really understanding the different terms or formulas I still am curious how many different was to get some information. It just helps me understand a little better. I just know it's better to post all the information I know and not try and leave anything out.

Longbox has the formula, and your understanding that the over bore makes it a 239.4 is correct

Bill

ok got it.

How do I calculate the bore/stroke ratio and the rod/stroke ratio?

What is the rod length of the 235 truck engine for 1956? I could not find that one.

Since I had the crank turned down -.010 x -.010 does that effect any of the calculations or is it the same as when it left the factory?

Having the crank turned undersize won't change anything. The bearings are thicker to compensate for the material taken off the crank.

Thanks Don, red58, and longbox.

One down and two to go.

How do I calculate the bore/stroke ratio and the rod/stroke ratio?

What is the rod length of the 235 truck engine for 1956?

Bore divided by stroke is the ratio your looking for:

BORE: ÷ STROKE: = RATIO:
3.560" ÷ 3.9375" = .904


ROD LENGTH: ÷ STROKE: = RATIO:
6.812" ÷ 3.9375" = 1.73

Copy that. Thanks a bunch you guys are great.

For some understanding and some basic math concerning rod length and stroke there is a member that has written and published some very good articles. The math is explained as well as the technical engineering stuff. These are copyrighted and his own product. But he has made a lot available for us to read. Look up Panic in the member list for more about him. This is a link to a paper on rod length and stroke. And why it matters.
http://victorylibrary.com/mopar/rod-tech-c.htm
Don't worry about the Mopar stuff. The paper applies to any recip engine.

So now the question is, are there any gains to be had from the weak link, poor breathing 216/235/261 heads by altering the rod length?
Carl

For the 216, there really isn't much gain to be had in the cylinder head department, though installing larger valves and porting can't hurt. Their ports are much smaller than in the 235 and 261 heads as are the valves. The later 235 and 261 heads can benefit greatly from using the available 21st century cylinder head machining and porting technology currently available. Also, the GMC rod swap is still an additional way to further improve the rod length to stroke ratio, plus add a better rod. A performance book explaining this procedure along with other Stovebolt performance books are fixing to become available.

12 Port, you mention the GMC rod swap improving rod length to stroke ratio. I am assuming that this swap increases the rod length-to-stroke ratio. By how much? With the stock 235 ratio being 1.73, what will the it be with GMC rod? Will this change enhance rapid evacuation of exhaust and/or improve the flow of incoming fuel/air through the inefficient intake ports? It is important for you to know that I do not 'know' anymore about this topic than what I have read in this thread. I am just trying to understand the information that has been presented.
Carl

Carl, there have been many schools of thought on this topic over the years. And whether increasing the rod length enhances the scavenging of those things is often debatable. I think initially back in the early days of the Stovebolt's popularity, enthusiasts did if for different reasons than they do today. First, aftermarket rods were still 25+ years away from being commonly available back in the 40's, and they no doubt saw the svelte GMC rod as a great upgrade in strength over the "pinchbolt" Stovebolt rods, plus it was an easy upgrade to do and inexpensive to find GMC rods. The GMC rods offered inserted bearings compared to the babbited Chevy rods. Second, they were longer than the Chevy rods. At 7.000" compared to the Chevy's 6.812", they provided the advantage of being able to use a lighter piston because of that fact. The GMC rods also had a full floating wrist pin in its S/E with a bushing, instead of the Chevy's "pinchbolt", and it was able to provide full oil pressure to the wrist pin via an oil hole that ran the entire length of the rod beam from the housing bore where the bearings are installed all the way up. And it even sprayed oil to the underside of the piston crown. Fast forward 60+ years, several aftermarket racing connecting rod companies are now today offering their rods with this feature as an upgrade for extreme racing conditions like NASCAR and other severe duty racing environments. And many high boost turbo and supercharger guys are benefiting from the ability to cool the underside of their pistons in these often high combustion chamber temps and ultra high chamber pressures that occur with these engine mods. Talk about being ahead of your time in innovation, GMC was on that one. So whether or not this rod swap gave them any true increases in performance or any other advantages, it definately gave them a peace of mind.

The GMC rod ratio difference is in the right direction, but:
1. very expensive (you also need special pistons)
2. below the threshold of detection (less than 3%)

There are many modern "innovations" marketed today which we find in these antiquated clunkers which we are working on. Solid brass battery connectors, for example. Many others as well. Listing all of them would be an entertaining thread all by itself.
Thank you and Panic and Steve for the education on the rod length deal.
Carl

I read somewhere that the gmc rods were stronger than the chevy, however I was not aware the rods were longer. So if the rods are longer does the pistons need to be shorter. I am just trying to understand the concept. Is there more torque or hp advantage? I have only built 1 235 but in the future I would like to do a cadillac 261. But only after I finish this project. I would like to know more about that preformace 235 book that coming soon. Data junkie that's me.

Google, Smokey Yunick long rod theory. You will have plenty to read.

Yes, the wrist pin location in the piston needs to be moved upward in the piston to compensate for the difference in rod length. As panic noted, this can become an expensive proposition since it will require a "custom" piston. Even back in the 40's though, speed parts were much in demand and performance piston companies of the day gladly accomodated hot rodders with their needs. Today, Ross Pistons gives generous discounts to Inliners members and has forging dies for many of the older vintage inline engines including the GMC's and Stovebolt's.

I got a real good chuckle at some of the NASCAR feuds he made. I especially like the one where Nascar had the rule of you could only have a certain size tank but they did not specify the size of the full line so he made a fuel line 11 feet long and 2" in diameter that held an extra 5 gallons of gas and where he inflated a basket ball and after the inspection he deflated the basketball in the tank. Now I guess I will search for the rod story. I got a little side tracked reading how people crossed the gray line in the world of black and white.

Can you elaborate or provide a link with details? Does it deal with the 235 engine?

Sure thing, it will be available soon, but here's the link. It covers installing the GMC rods in the later 235/261 engines.
Here

Looks like they have published one other book: Case File:#1948
(but, I cannot find it on Amazon - you can buy a tee-shirt for it, though)

You can buy the "Case File: #1948" book from them directly with the "Add to Cart" button it looks like. I'm sure the car book will be the same way once it's available.

Increasing the stroke (Stroking),
although a viable method on the Ford Flathead, is not recommended for the Chevy 235. Internal clearances between rod and cylinder base greatly restrict stroking. It has been done but is quite expensive and limits reliability. The small gain in CID just isn’t worth it. However, since the CID is proportional to the mathematical square of the bore, increasing the bore (Boring) yields a much higher gain in CID than stroking and is highly recommended. So how much bigger can we make the bore? The block can be over- bored 1/8" to a diameter of 3-11/16" increasing the displacement to 255 CID. However, the block must be closely inspected for bore alignment with the casting. Detroit gets a little lax when casting and machining the blocks so if you want to bore this big, start with a well aligned block. Otherwise you may make the cylinder walls too thin. Also keep in mind that you have to find pistons of this size to fit the bores. Good luck! A good over-bore is .060". This raises the displacement to 244 CID and makes the engine competitive with the Watertown and Waterloo 260 Flatheads of the era and still retains reliability. These .060-over pistons are still available as stock replacement parts and special racing equipment.

article by Larry Witter

Good info and read. Just recently had my 235 block bored out .060.
I used the above posted formula and came up w/ 243.489 ci.
I was going to call it a "243.5" but "244" works.

Dave

Tiny, good info for not stroking a 235. However the topic of longer rods is not synonymous with stroking. The crank dictates the stroke. The rod length dictates the geometry (angles) of the stroke as well as altering the time spent at top dead center resulting in changing stuff which I have no business pretending that I know anything about.
Carl

Stroking has been done effectively on a Stovebolt before. Walt over on Inliners had one that was almost 300 cubic inches in his recent Bonneville landspeed record holder. It had a homemade FI setup and offset ground crank using the 292 Chevy 6 rods.

12 port are you talking about the "cadillac 261" engine? So what I am really understanding one of the main reasons you would increase the rods size is because of the design of the GMC rod is built stronger than the chevy, however if the rod is longer, then the added length of the rod would have to be subtracted by a shorter piston so it does not hit the head. The angle of the rod as 52Carl suggested I am sure has engineering benefits that i can almost visualize it I understand now how to get a longer stroke by offsetting the crank or putting a different crank that would work if that's even possible.

Now my next question since I put a stock 261 camshaft in my 235 engine, I was reading soup that chevy article that was published in May 1955 issue by hod rod magazine. Would the valve timing now be altered or the same as in the article? The article states the intake opens 11 1/2 degrees before top center and closes 52 1/2 after bottom center, duration 244 degrees and a lift at the valve .405
Exhaust opens at 51 degrees before bottom center and closes at 13 degrees after top center and a duration .244 degrees with a lift at valve .414 of an inch. Are these numbers the same with the stock 261 cam in the 235? This is stovebolt 101 for me. As I have mentioned earlier I have know rebuilt one motor. As I am reading the different concepts and thoughts I really can visualize what to do on my next stovebolt. I am taking notes all the way.

I think the "Cadillac 261" is along the same lines, but it makes a 261 cube engine out of a 235 block by stroking the crank to gain the cubic inch. Whereas Walt started with a 261 block and stroked it in a similar manner but arrived at close to 300 cubic inch because the 261 block has a lot larger bore to begin with than the 235 does.

Yes, the timing events would be the same regardless of the engine your installing the camshaft in, as long as you are able to "tweak" the cam by either advancing or retarding it to degree it in the same as the article, and of course as long as its the same cam profile as in the article as well. This may require a multi-keyway timing gear to accomplish though.

cadillac 261

Here was the link to the article I was referring too.

Yes i've seen that article, but you can also do the same thing by offset grinding the crank and putting it in a 235 block and get 261 cubic inch out of it.

Does "do the same thing" include resulting in 292 ci?

As you guys who always move a discussion off-topic into other engines like to say, "there is no replacement for displacement".

Thanks, Tiny_Jaime for showing us the better-known and more muscular "Cadillac 261"

Not to stand on a soapbox however when I got started in this project a little over 2 years ago, if you asked me what is stovebolt, I would have thought it was bolt from a pot belly cast iron stove. I read these forums and your guys personal blogs and i still realize I know a little more than I did even with a complete rebuild under me and having hands on experience with both trial and errors. I still at the beginning stage of these engines. I am amazed and really appreciate your guys knowledge and experience that I never had. I had to put my OBD 2 in storage and buy a vacuum gauge.

The reason I started this thread was really to gather all the info possibly about my application of what I have done to my 56 3100. As my dad always sad a short pencil is better than a long memory. I want to pass it down the info and my truck to my daughter and hopefully she does not trade it for a Chevelle. Hopefully she keeps it and if there is ever a problem she has the info at hand to either fix it or have it fixed. All the car shows I mostly see have SBC, BBC or LS_ in them and rarely the inline six. I do not want these motors to be forgotten. As times have changed they have never seem 2-60 a/c, a floor dimmer, floor starter or even non power windows.

My next motor or project will be a cadillac 261 with all the tricks in another Chevy truck however not sure what year yet.

As I keep wading through all this info, I really have to thank everyone always for sharing their comments because it help me plan and learn especially when I'm broke.

I did not mean to rant however everything I do something new I do not want to know only how to do it I also want to know why.

Great site, great members and great threads.

Lastly, one of the reasons I liked the article listed above is because Tom Langdon opened his books and tried to find a simpler or easier solution in my humble honest opinion to use common parts when he wrote the article. Although I am having trouble finding a source for the pistons. It is much easier for me as an amateur builder (lol) with one build under my belt. Along with common things like Carter Weber from a Ford Pinto and a mini HEI distributor from an Chevy S10 etc. so if a part breaks on the road not close to home is easier to get it from the Flaps.

Once again thanks guys for helping me understand. TJ

1980-1985 Venolia Forged 368 Replacement Pistons with pins $895

80-84 368 pistons @$47

Thanks Tim

Every time I read or think about the "261 cadillac" motor I always do a search and come up with nothing. Now that excites me I will start doing a serious plan now. I will start a new thread have a few questions on the 261.

I've been looking at a similar project.

The 368 was so despised that the pistons are now Unobtainium. Flatlander says their listing is for "historical purposes only", and they can't get the pistons (but the listing does direct traffic to their website!).

$895 for a set of 8 Venolias is more than we'd likely pay for a set of 6 "custom" pistons from Ross, JE, Egge, or one of the other fine suppliers. So we may be better off finding out what bore our blocks need to clean up, what deck it needs to square up, what piston height will get the right quench, and what piston pin geometry will work with our rod choice. And then order *exactly* what our motor wants, not something close that was "off the shelf" at one time.

While designing pistons and deciding bores, also pay attention to available ring sizes. I'm looking at the Total Seal Gapless; they have a few options for the 3.80" bore and a couple of oversizes.

One other option I'm considering is the 3.0 Buick piston, with a longer (custom?) rod. Not sure if I'll stroke it yet, need to finish looking into the rod clearance question.

And 12 Port, the math on 261 = 235 stroked like the Cadillac build doesn't work out. 3.5625" (235 bore) with a 4.125" stroke (a la Cadillac 261) becomes 247 cubes. Overbore it 30 and you get 251. Overbore it 60 you get 255.

There is a huge number of engines out there with 3 3/4" or 96 MM bore sizes. Finding one with the correct compression distance and a wrist pin size somewhere close to the 292 connecting rod shouldn't be an insurmountable task for anyone who can read a piston catalog.

Folks, parts like pistons, connecting rods, valves, rod bearings, etc. don't care what kind of engines they're installed into. The machinist/engine builder with a few brain cells to rub together just makes everything fit with an idea and a few reams of scratch paper. I remember building up an air-cooled Volkswagen engine about 40 years ago with Flathead Ford V8 piston rings and Corvair wrist pins! It ran like a molested monkey!
Jerry