The Stovebolt Site Forums The Shop Area Engine Shop Compression Ratios of Blue Flame engines.

Gents, I have a '40 model 1/2 ton truck which is fitted with a '55 Blue Flame engine. I am wondering what the compression ratio is. I have seen figures ranging from 7.5 to 1, to 8.5 t0 1. I would like to bump it up a bit, fit valve seats for unleaded fuel, etc. Can someone tell what the CR should be, and what I could safely take it up to please? I don't intend to go overboard & make a real hot motor! Mant thanks, Riggles

Do not waist your time with hardened valve seats. That engine has run for years on unleaded fuel and hasn't need them yet. The only time to use a new valve seat is when the head is worn out and you are building a numbers correct engine that needs the original heads. If you plan to run this engine are top rpm with the bed fully loaded with sand, then add the seats, other wise save your money. Just remember, unleaded fuel became the main fuel in '73, but it was around way before that. Just for kicks, ask your local machine shop to show you a worn valve seat from the use of unleaded fuel.

Hello, Riggles,

The "Blue Flame" 1955 engines was the name of the car engine.

All 1955 235 engines were lower compression engine than the 1956 and later "848" head 235s (cars and trucks).

Here is a collection of Specifications Manuals for "Stove Bolt" years.
There is information for cars and for trucks.
In each part, there is an engine section with specifications that you are looking for.

Find and post the codes shown on this page.We can then help confirm the year/type 235.

My shade-tree opinion regarding installing hardened seats in a 216/235/261 head is the same as Joe's: they are not needed (for non-race-car driving).

Unless you're also installing a much more radical grind camshaft, why do you want more compression? The design of the stovebolt cylinder head makes raising the compression a difficult and expensive process. Simply milling the head creates serious problems with piston/valve clearance, making it necessary to sink the intake valves deeper into the head. That shrouds the valve, and makes a lousy-flow head worse. Whatever gain in HP you might get from the raised compression will be offset by compromised breathing due to the valve shrouding. Other than special-ordering some sort of custom piston with a weird-shaped dome, I can't think of a very effective way of raising the compression, or why it would be necessary in the first place. A stovebolt cylinder head is so poorly designed that any improvement in valve size, intake, exhaust, or camshaft is pretty much wasted effort. You can actually make a silk purse out of a sow's ear, but it makes the pig look funny, and it doesn't hear very well. LOL!
Jerry

The common head found on a 55 motor is usually the 5913 head. That head is about 86 cc. Stock cr is advertised at 7.5:1 but they rarely get there, usually a little less. The 848 head entered service in 56 for the passenger cars. Trucks may have continued using the 5913 head later then 55, I am just not certain of that. I have a pretty early 848 head that has a late 55 (I want to say September or October) casting date so this head was for the 56 passenger car model year. 56-57 cr was advertised at 8.0:1. That changed in either 58 or 59 when the cr was advertised as 8.5:1. The chamber in a 59 to 62 head is 79.1 cc. I have never measured a 56-58 848 head to see what difference, if any, there is in the cc of the chamber. I will have to add that to my list of things to do this coming year. Just as a matter of completeness, the 261 heads come in two flavors, 86 cc and 92 or 93 cc (I forget which one off the top of my head).

As for boosting cr, other then custom made pistons (read that as very very expensive), you are stuck with milling the head, block or both. But it gets complicated depending on how much you are milling off because of the shaft mounted rockers. You have sink the intake valves a corresponding amount, raise the rockers the same, and add lash caps or use small block chevy exhaust valves with longer stems to the exhaust valves to keep the rocker geometry the same, or flycut the pistons if you are brave and think there is enough meat on cast pistons. Years ago they also furnace brazed the chamber to fill it up a bit to build compression. Milling the block is also a possibility. If you were to just mill the block, you would not necessarily have to deal with sinking the valves or lash caps, but you might, and you would have to have custom push rods (shorter by the amount you milled from the block) and adjusters made up (Smith Brothers can make them but only in 3/8 thickness). The Smith Brothers parts are a work of art (I have them in one of my 261 motors) but really over kill for a street motor. Then you would have to sort out whether the intake would hit the piston, depending on the lift and duration of the camshaft (necessitating a flycut on the pistons to make room for the intake valve or sinking he valve and all of that). No issues with the exhaust because of how deep the exhaust valve sits in the funky shaped chamber. I have done all of that and can confirm that its a lot of work and frankly, not a lot of return. there are other ways, but all of them require a good bit of machine work. If you want really good compression without lot of fuss, and it does wake these motors up, save and get a set of good pop up pistons from Ross. Not cheap by any means but the easiest way to get compression.

So unless you want to get deep into this motor, I would say leave the stock pistons alone, mill the head and block the minimum needed to get them flat and square.

Hardened valve seats, I have done them on occasion. Here is what I will say. I had a 848 head, perfectly good to start with, end up a boat anchor after machining for valve seats. Turns out the casting was a little thin in an unexpected place and the machining hit the water chamber. That was the last one I did and that has to be 20 or better years ago now. Truth be told, I have a 57 235 with a 55 5913 head with stock pistons in the back of my garage that I beat on unmercifully for years. It has an old Chet Herbert cam, 480 lift and 280 duration and stock rods. That motor never missed a beat and sounded terrific. When I pulled the head off to freshen it up, the valves needed nothing more then an old fashioned lapping. I did not use hardened seats on the 261 I recently built either (very modified) just for that reason so I would say skip the hardened seats unless you expect to drive the vehicle every day and put 80k on the odometer.

How do you know if you have a 848 head? Is the number 848 cast in the head? Thanks for any replys. Bill

Yes

HTH.

An 848 Head Thread

Here are locations of casting codes.
With that information, you can usually ID the size/year of a stovebolt engine, and maybe a little/lot about the vehicle it was originally installed in/on.

I have had a brief look at the engine & found the following
Head 3835923 D 15 5
On the block 3836233
Engine number is 4R136909
I forgot to mention that I am in Australia, & I acquired the truck from an old gent who restored it 35 years ago and installed the later motor. I have recently imported a step side pickup bed from Jim Carter, and am in the process of painting & assembling it. I intend to fit new Coker Classic tyres, (old looking tread but radial construction).
Apart from the later engine, it is pretty well stock, which is the way I want to keep it. I was toying with the idea of just lifting the compression, cleaning up the ports & possibly fitting a twin outlet exhaust. I don't even know what the head looks like when off the block, but I was assuming that it has a combustion chamber which would let me get a skim off the head.
Any more advice will be appreciated, Riggles