Stovebolt.com Forums The Shop Area The HiPo Shop "Baby Chicken" 261 stroker pistons- - - - -

As in "CHEEP- - - - -CHEEP- - - - -CHEEP!" A standard bore piston that fits a Buick 231 V6 is .050" bigger than a 261 piston, the wrist pin is just a few thousandths of an inch bigger, and the compression height works well with the 3/8" stroker crankshaft I'm contemplating running on 235 and 261 engines. It's also got a dish in the top that will allow a higher than standard lift on the intake valves, and/or a milled cylinder head. The 235's can run pistons for a Ford 223 inline six with minimal machining. 235's become 270's, and the 261's approach 300 cubic inches, depending on the oversize used. Ebay is loaded with brand new 231 pistons for under $100.00 a set!
Jerry

Jerry, just asking, but are you going to build a motor using one or the other or both of those pistions? The reason I ask is that for quite a few years I was told that with minimal machining you could install GMC 302 truck rods in a 261. It turns out while that statement was true, it was only sort of true. In fact, if you asked all the folks who were expousing this statement whether they had ever build a 261 with GMC rods, none had ever done so. I actually built one. Yes, you can machine the sides of rods down to 261 size, and rebuild the rods to fit a 261, but then comes the rest. No off the shelf pistons so to compensate for the longer GMC rod length and altered compression height so you need custom pistons, the GMC rod bearings have to be narrowed because they interfere with the235/261 rod journal filets which requires a lathe and a homemde tool, the cam has to be relived between the lobes on the flats to clear the rod bolts. I learned those things during the build. It was time consuming.

I have one more bare 261 block, a pair of spare 235 blocks, and a couple of spare 235 cranks. If you are building a stroker motor using these off the shelf pistons, I think I will wait to see how the build progresses, then use these blocks and then engage in outright flattery, adulation and then theft and follow on your coat tails. Truth be told, I would rather not reinvent the wheel if you are headed in that direction. My youngest son has been my right hand man for all things old cars and trucks for quite a few years now, since he was a teenager. A stroker 261/235 would be a good project for he and I to do together.

Yes, I have both 235 and 261 blocks available, and several crankshafts to work with. Both engines will run the same crank- - - -offset ground to have a 4 3/8" stroke, and using connecting rods for a Nissan 2.4 liter 4 cylinder. The rods are narrower than stovebolt rods, so spacers around the wrist pins will be needed to center the rod on the crankshaft journal. I'm also experimenting with putting a bushing in the big end of a stock stovebolt rod to adapt it to the Nissan bearing. Once I have a block and crankshaft set up, then I'll start tinkering with rod length and piston compression height to get the pistons close to the top of the block. The Buick piston tops have a circular dish almost 1/4" deep, so some tinkering with compression height can be done without compromising the strength of the piston crown in case the piston sticks out of the top of the block a little. Ford 223 pistons are 3 5/8" flat tops, and have a lower compression height than stovebolt pistons to compensate for the longer stroke.

You're welcome to copy all my designs- - - -just give a little credit to who dreamed up the whole Rube Goldberg idea- - - -whether ir works or not! I'm currently working on putting a pressure lubed crankshaft and rods into spray-oiler engines, 216's and 235's. That should work to salvage some of those spray-oiler "boat anchors". The big sticking point on those engines is main bearings- - - -they're as scarce as hens' teeth! I might need to work on a way to use my line bore machine to narrow the thrust face on the spray-oiler blocks to accept the narrower 235 thrust bearings.
Jerry

I just bought a set of hypereutectic cast pistons for the 231 Buick engine- - - -.020" oversize which will be .070 over in the 261 block. I have a 261 block that's 40 over right now, so with the Buick pistons and 3/8" extra stroke, it should come out very close to 300 cubic inches (296). I've also got a set of H beam Nissan rods that are rated for 600+ HP. Now- - - - -if I can get those roller valve lifters and rocker arms to work with an 848 cylinder head, things might get "interrrrrressssting!"

BTW, shipping included, the cost of the pistons came out to the princely sum of $14.00 each! Unsold inventory from a parts store that went belly-up! Ebay to the rescue!
Jerry

Jerry I’m looking forward to seeing and hearing the engine run once you get it all wrapped up and into a truck. I would assume a You Tube video will be in the works.

I'm daydreaming about setting up a demonstration engine with my dyno attached, put it on a small trailer, and making the rounds of a few car shows with it. A mockup engine turned by an electric motor, with a clear plastic valve cover and pushrod cover to show off the unique valve system would be an "extra added attraction"!
Jerry

Jerry, I know it would be a big hassle but, get a GoPro and record each step with narration of what you’re doing. Post the videos to YouTube. Only so many years left, buddy!

Good idea, Martin- - - -being the youngest and healthiest member of my household has me balancing caregiver duties with shop time. Right now, the shop is playing a pretty discordant second fiddle!
Jerry

Just throwing ideas out there. If you have a Technical School or even a High School close by that is offering a film/video class you might talk with the instructor about making it an ongoing class project. Could bring in a few $ for their program and you to. Might even lead to someone wanting to do an apprenticeship since you have a teaching background. I can see an Oscar in your future!

Those programs are just about ancient history. The state trade school program in the town 20 miles west of us used to have an auto mechanics program- - - - -I know, because I was the instructor. They closed it down in 1990, and laid me off. The construction trades class, welding, and machine shop programs also went away. Now the school doesn't do much of any kind of education that doesn't involve kids sitting around and punching a keyboard all day. After I retired from running an very successful high school shop in Nashville (which also got cancelled) I volunteered to help out (free) at the high school auto shop program my grandson was attending- - - - -"No, thanks!" They never got back to me!
Jerry

Amazingly my high school in Woodstock, Georgia still offers an auto mechanics program.

When I bought my car in high school my dad told me I'd better learn how to fix it if it broke down because I'd go broke if I had to pay somebody to do it. I immediately signed up for the auto mechanics class. I took it as elective for every semester after that.

Most useful class I ever had in high school or college

I could fix my own cars (or, at least diagnose them) into the 1970s, but even my oldest* is almost completely beyond my comprehension. The sad part: also beyond the dealer's service department's comprehension.

* 1992 Lexus SC300 with 2JZ-GTE turbo engine

Sad for all young people. Watch and listen to HRL!

Ed

I admire your attempt to get the most out of one of these "tractor engines".
Who, among us, have not dreamt of doing the same?
I realize that the only true arbitror of horsepower and torque gains is the dyno, but with your vast experience, what is your guesstimate for what you will realize with your modifications.
What compression ratio will this engine have, static and dynamic?

A couple of old sayings come to mind- - - -"There's no replacement for displacement" and "Horsepower is to brag about- - - -torque wins races!" I'm not going to venture a guess about HP gains, because the cylinder head flow is so terrible. An extra 3/8" of stroke should help the low end and midrange grunt by a noticeable amount, reducing the need to downshift on hills or when trailer towing, etc. About 5 extra cubic inches per cylinder from the increased bore and stroke should raise the compression about one full ratio or slightly more, which will probably put the engine on the ragged edge of needing mid-grade gasoline- - - -close to 9:1 CR, I believe. Dynamic compression is a function of the camshaft profile, since pressure in the cylinder doesn't start to build until the intake valve closes. A long duration cam that uses up a lot of the compression stroke to take advantage of high RPM cylinder filling due to inertia of the incoming fuel/air charge needs a high static CR just to bring cylinder pressure up to what a stock cam with shorter duration used to make. The 261/Corvette cam or the very slightly more radical Melling cam that I'm planning to use doesn't have a radical lobe profile, so there should be a pretty good gain in actual cylinder pressure with more displacement.
Jerry

I did some reading about the Buick Nail Head engine, so named because of the relatively small valves for a large displacement engine.
It was indicated that since the head design was similar to a hemi (it was called a semi-hemi by Buick enthusiasts), the valves are not shrouded like what happens on big valve engine designs, thus breathed as well as infamously shrouded big valve designed heads.
There was also talk of how the Nail Head designers addressed CFM through these engines. When designing all of the components responsible for air flow through the engine, their focus was on the aspect of the speed of the airflow, and not just the final CFM number.
Is CFM static, or is it dynamic, taking place over time?
What I do not know is what did they do to accomplish the increased speed of flow aspect.

I heard a somewhat different story about the "nailhead" moniker- - - -The valve stems were almost vertical in the head, like someone had driven a bunch of nails in there. That made for a more direct flow path in and out of the head, although it made the pushrod and rocker arm geometry a bit more complex. Either way, the design produced an engine with plenty of low end torque. One of my competitors at the Merced California dirt track back in the late 1970's ran a nailhead engine, and trying to beat him off the turn with a tight-winding Chevy 327 was a real chore. He liked to run a little high in the turn and get a good slingshot down the straight. The only way to beat him was to hope he'd over torque it and spin wheels. He didn't do that very often!

Cubic feet per minute is simply a function of engine displacement and RPM. Port shape and valve size alters the actual volume, since it's impossible to get 100% volumetric efficiency most of the time. Here's a quick formula- - - -divide RPM by 2, and multiply it by the cubic inch displacement of the engine (cubic inches per minute)- - - -then divide that number by 1728- - - -(cubic inches in a cubic foot). For instance- - - -261/2= 130.5 X 4000 RPM=523,000 . Divide by 1728= 302 CFM. Since that's impossible, given the stovebolt port shape and gas flow, 90% of theoretical CFM is a more realistic number- - - -272 CFM.

Now- - - -how do some real world numbers compare to the "Use a 500 CFM carburetor" advice that some people give? The laws of physics are not "suggestions"!
Jerry

What are planning on using for a carburetor?

Most likely a Carter one barrel calibrated for a Ford 300 six. I might modify a stovebolt intake to accept a small bore Rochester 2 barrel for a 327 V8. Unlike the Rochester B, the 2G series carbs are very well designed and reliable. It's necessary to do some pretty extensive machining and welding to the manifold to extend the plenum under the 2 barrel throttle base all the way to the manifold runners. Using an adapter to mate the 2 BBL to the original manifold ends up getting less airflow than the original 1 barrel carb. I've also got an idea about using Keihin side draft diaphragm piston carbs for a Harley Davidson motorcycle on home-brewed individual manifolds, one for each intake port.
Jerry

The hemi comparison isn't accurate.
The Buick intake valve is vertical and inclined at 45 degrees to the bore axis (similar to some hemis), but so is its exhaust valve (parallel to the intake valve, which in a hemi has the reverse position).
The actual chamber shapes are completely different, the Buick is a very deep wedge with a closed chamber and quench.

W/r/t "261/2= 130.5 X 4000 RPM=523,000 . Divide by 1728= 302 CFM"
Easier, avoid 1 step:
261 X 4,000 RPM = 1,044,000. Divide by 3456= 302 CFM

If I read you right, you pretty much just described "semi hemi", didn't you?

A set of the pistons arrived today- - - - -lighter, stronger, and better alloy than stovebolt pistons, and with narrow, low tension rings. The hypereutectic alloy has almost zero heat expansion, so the skirt clearance can be fitted much tighter for better oil control and less noise. The drag on the rotating assembly should be much less than a regular setup, especially with a smaller diameter, narrower rod bearing. The connecting rods will also be considerably lighter and stronger than the original ones, particularly if I use the H-beam competition rods.
Jerry

Not in the slightest.

"(similar in some hemis)"?
I did not come up with the phrase semi hemi. Those who refer to it as such (way more knowledgeable than me) certainly know that the nail head is not a hemi, but were referring to it in terms that it was closer in design to a hemi than virtually anything else being produced at the time.

And speaking of hemi, the term today has become a marketing gimmick rather than an actual description of an engine's geometry.

These stupid Dodges with "hemi" written on the hood scoop have engines with geometry more akin to a big block Chevy than anything with hemispherical combustion chambers.

For a real hemi engine, check out the R-1820 Wright Cyclone on the WW II B-i7. It's an 1823 cubic inch 9 cylinder turbocharged radial that produces around 1200 HP.

"it was closer in design to a hemi"
Not at all, except for the comparison I made already.
A definitive factor common to all hemis is that the valve are opposite each other (not parallel) and open toward each other.

Much closer to other GM V8 engines at the time, including the 1949 Cadillac 331 and Oldsmobile 303.