The Stovebolt Site Forums The Shop Area The HiPo Shop Remember the "Cadillac 261"?

Some time ago, there was a lot of discussion about increasing the stroke length on a 261 engine, using connecting rods for a 292 Chevy six, and a very scarce set of Cadillac pistons to end up with an engine very close to 300 cubic inches that looks like a 235 at first glance. Here's anther sneaky modification that might make a little more sense, and won't involve quite as much work and expense. By some sneaky parts swapping and a little machine work, I believe I can assemble a "216" engine with 231 cubic inches, aluminum pistons, 292 connecting rods with replaceable rod bearings, and eliminate the spray-oiled connecting rods in the process. The engine would be impossible to tell from an original 216 from the outside, since it would use the same engine block, cylinder head, and sheet metal as the original, with the exception of the oil pan. The pan would come from an early-production 235 with the same bolt pattern as a 216, minus all the spray-oiler hardware inside. It would use an oil pump from a 230 or 250 inline six, one that flows nearly twice the volume of a 216 pump. Other modifications would include a Melling camshaft that's slightly hotter than the "Corvette 235" cam, and a leakproof rear crankshaft oil seal. Even with these modifications, the engine would probably cost less than a stock 216 spray-oiler rebuild. I wonder if anybody would be interested in having a "modern" engine that looks like a bone-stock 216 and has 15 more cubic inches and 30-40 extra horsepower or more? It could probably cruise all day at 3500-4K RPM with no problem.
Jerry

Wow. That sounds like a whole lot of work..BUT it will be a conversation starter and big fun to drive. Go for it.

Its very clever and I can not wait to see how its put together and how it runs. Way more interesting then a V8 chevy!

The backbone of the conversion is a 235 crankshaft, with the thrust bearing flanges widened to accommodate the 216 main bearing. That is a simpler conversion that machining the block web thinner to take a 235 main bearing on the #3 position, and it's something I can do on my lathe with a carbide cuter bit and a tool post grinder. The 292 connecting rod is somewhat narrower than the 235 rod, so it gets a spacer washer pinned and silver soldered to both sides of the rod cap to center the rod in the journal. The small end gets a bushing to reduce the diameter to the size of the 235 wrist pin and make the pin full-floating with Teflon thrust plugs to keep the pin centered. The 216 block gets bored 60 over to accept standard bore aluminum 235 pistons. Now, here's the sneaky part- - - - -since the 216 block deck height is shorter than a 235, the pistons would stick out the top of the block. The crankshaft will be destroked from 3 15/16" to 3 7/8", and the tops of the pistons will be machined down just slightly go give a deck clearance of zero, even with the top of the block. The final displacement with standard bore 235 pistons will be 231 cubic inches. I just finished fitting a 250 oil pump that will almost double the flow of a 216 pump. A "216" with 40+ pounds of oil pressure at idle????? Of course it's going to get the leakproof rear main bearing seal while I'm doing the machine work. An early 235 oil pan without the 216 spray-oiler plumbing will replace the original pan, or the spray hardware could just be removed from the original pan and the oil supply port to the pan will be plugged.

I've also got a 258 cubic inch version of the 216 in the works, using NASCAR H-beam connecting rods and a lot of modification to a 235 crankshaft!
Jerry

Jerry, have you thought of doing all that to a GMC 302?

Ed

302's are scarce as hens' teeth, and priced accordingly. People are using 216's for doorstops and boat anchors because they don't know how to work on Babbit rods, and I'm figuring out ways to make "original-looking" engines in 1940's and 50's trucks keep up with traffic without swapping to a 235 or a 261. The idea of modern internal parts in an original-looking package interests me, and I believe if I show up at car shows and swap meets with one of my engines on a trailer with a dyno attached, I could rake in orders for them like leaves in the fall.

Back when GMC 270's and 302's were plentiful, they got hotrodded frequently. but those days are long gone. My father said the 270's gave his flathead Fords a little competiton on the dirt tracks, but they were so torquey the Fords were halfway down the straightaways before the Jimmys quit spinning their wheels coming off the turns. On a long, high banked track, they had a lot better performance because they could stay above the torque peak RPM.
Jerry

Jerry,
I'm intrigued! I have a '46 Aerosedan that your 216 mod would be perfect for. What would the final price be, provided I delivered a serviceable 216 to your door?

Tyler - that topic should be discussed in a Private Message.

Well, I'm glad I have a few spare 302's for backup.

Ed

Tyler Watts, Jerry (HRL) answers his PM’s. Buying and selling is not allowed in the garage Forums.

Drop me a PM. I'm still in the R&D phase of a few similar engine projects, but it's like a lot of other internal engine modifications- - - -your checkbook is liable to need some CPR before the job is done. Back when I was building race engines, the old saying was "Speed is expensive- - - -how fast is your pocketbook?"
LOL!
Jerry

OK- - - -progress! I've got a set of 292 connecting rods in the process of being modified. I made some spacers to increase the width of the rod caps to match the width of a 235 connecting rod (1.240"). Two rod caps were bolted together, and I made a holding jig to keep the thrust washers in place while a local welder made several short passes with a TIG machine to attach the thrust washers to both rod caps. Then a milling attachment and a slitting saw on my lathe cuts them apart, resulting in six rod caps that will stay centered on the 235 connecting rod journals. No thrust face will be needed on the upper part of the rod, and I'll recondition and weight-match the rods before reassembling the engine.

A set of standard-bore 235 pistons in the 216 block, and a 235 crankshaft with a stroke change to 3 7/8" to keep the pistons from sticking out of the top of the block will result in a displacement of 231 cubic inches, a connecting rod journal size of 2.100" (same as a 350 V8), and oil pressure to the connecting rods from a drilled crankshaft. This engine should cruise all day at 4K+ RPM and allow guys with a stock-appearing "216" engine to keep up with traffic!
Jerry

Jerry, could you post some pictures of your modified rods to help us understand your modification.

I'm still in the process of finishing up the conversion, but I'll be glad to post some pics of the finished product. The original "Cadillac Stroker" article required the center of the connecting rod journal to be ground down to the width of the 292 connecting rod, and most machine shops run from that kind of modification. That's the reason I bought my own crankshaft grinder. My method allows the journal to be ground full width to the 292 bearing size, and the widened rod cap self-centers. I'm also in the process of doing the same kind of modification to a set of NASCAR H-beam rods for a LONG stroke 235 crankshaft that will turn a 216 into a 258 cubic inch "sleeper"!
Jerry

Have you plotted out which combination of stroke length, rod beam width and length that just clears the cam?
Longer stroke and/or beam width = closer, shorter rod C-C adds clearance but shortens the rod ratio.
The 235 cam is directly below the rocker arms and close to the crank; the Gen-3 250 etc. moved the cam away from the crank (pushrods angle back toward the rockers) which adds clearance.

Yep, I already have a mockup engine assembled from junk parts, and everything clears. It's not my first rodeo, building custom engines from parts that aren't supposed to work together, since my family was doing it for two generations before I came along. Building an engine by assembling parts somebody else did all the engineering on should be child's play for anyone with the unmitigated gall to call himself a "mechanic". Making mismatched parts accomplish an improvement in performance is a different degree of difficulty.
Jerry

Excellent!

Here's a few pics of the modified rod caps More to come.
Jerry

That's a 292 rod in the photo, mated to a 235 piston. The rod will get a bronze bushing installed to reduce the small end diameter to match the 235 wrist pin diameter. I'll use aluminum or Teflon thrust buttons to keep the wrist pin centered, since the 235 piston has no locking clips in the pin bosses. The rod cap has two thrust washers attached to it to keep the rod centered on the crankshaft journal. The rod in the picture was an early prototype that I attached the spacers with small flat heads screws, and then silver soldered them to the cap. I found that it was simpler, and probably stronger, to just TIG weld the shims to the cap, using an alignment fixture to make the shim concentric with the cap. Since the only function of the spacers is to keep the rod centered, only the cap needs to be modified. Here's a few more pics, showing a 235 rod cap alongside the 292 cap. The 292 cap is ground to the same width as the 235 rod, 1.240".

Of course, the rods will be balanced, and the big ends reconditioned to get them round and the right diameter again, and keep the "crush" on the bearings correct. The 292 rod uses the same bearing as a small block 350 V8.
Jerry

On the LONG stroke conversion of the 216 that uses the recycled NASCAR rods from Carrillo, I'm doing the same sort of widening to fit the crankshaft journal, but those rods will get a spacer on the top and bottom of the big end. That conversion will be using a rod bearing for a 2.4L Nissan/Datsun engine and a 4 5/16" stroke for a total displacement of 258 cubic inches in a 216 block. I've found a supplier for Nissan H beam rods that will cost about $450.00 a set, not the $3500.00 that Carrillo wants for their custom-length rods.

Here's a few pics of the NASCAR rod conversion. I'm using recycled rods from retired engines, bought from various salvage sites on Ebay. A set of 8 Carrillo rods that originally cost $3K+ is available from dismantlers of retired engines for $100-200. Of course, they have to be reconditioned and adapted to custom applications, but that's no problem for a competent engine machinist. The NASCAR rods are shorter and lighter than the 292 rods, and they use a smaller bearing, so there can be a longer stroke increase to the 235/261 crankshaft when using these rods. The downside is that the crankshaft counterweights must be machined down and the piston skirts need to be trimmed to prevent counterweight/piston interference. I'm also machining both ends of the rods for more readily available rod bearings and wrist pins than the outrageously expensive racing parts and adding a bushing at the top instead of using a coated wrist pin. The attached pics show an unmodified rod, and the spacer rings, a rod modified for the Nissan rod bearings, and a small end that's been reamed for a bushing. There's also a pic of the welding jig I use to attach the spacers. Once the spacers are installed, I use a narrow slitting saw in a milling attachment for my lathe to separate the rod and cap again. It's a fairly labor-intensive process, but the machine work is pretty simple for someone with the right equipment to do it.

More pics:

As always, parts don't care what they were originally designed to fit. Once accurate machining, welding, balancing, and clearancing is done, parts from different vehicle lines or custom-made pieces made from scratch should work well together- - - - -it's just a matter of making the project work on paper before you start whittling on metal!
Jerry

This is what made America the center of innovation: clever people creating their own solutions.

Jerry ... Naturally, you have this brainstorm *after* we complete my 261 upgrade project ... do you have a platform in mind for the final install and roadtest?

The prototypes of both engines are going to get a dyno workout- - - -probably on my 16 foot trailer so I can haul it around to reunions, car shows, and homecomings. I believe a 150+ HP "216" might generate enough interest to keep my grandson busy building engines for a few years after I get too old and feeble to do it. The same uber-long stroke modification could be applied to a 261, resulting in a 300+ cubic inch engine that will out-torque anything else remotely resembling a stock stovebolt!
Jerry

John,

I volunteered years ago! Too bad he can’t do this to my 261 for me!

Chris

Jerry, I wonder if you might share a little more detail on the oil pump, maybe a couple pics? Is the main mod the pump or the block?
Thanks

A 300 cube stroked 261 would really be something.

I'm putting the finishing touches on the pump drive- - - -it's basically a shortened version of the drive shaft used on a small block V8 or a V6. I made a stub housing and a flange to bolt the 250 oil pump into the same bracket the stovebolt pump uses, with the single setscrew and lock nut like the original pump uses. The oil gallery that connects to the pan is plugged, and either a 54 or early 55 235 oil pan can be used, or the spray-oiler plumbing can be removed from a 216 pan. There will be a slight rerouting of the inlet and outlet tubing for the original pump, but it looks like the original pickup screen will work with a little modification. I believe the spray-oiler engine could probably be adapted to run the same pump. Now, that would really be radical- - - -a 216 with oil pressure!
Jerry

Mike, I lied a little- - - - -with the 4 5/16" stroke I'm planning and a 60 overbore, running the numbers on the "261" comes out to 295.46 cubic inches. Boring 80 over would break the "300 barrier"!
Jerry

Then I will rephrase, a 295 ci stroked 261 would really be something!

Yep, 3/8" more stroke should make it a torquey beast- - - - -an old adage on the dirt tracks was "Horsepower is to brag about- - - - -torque wins races!" Some of the best shows I've seen on a quarter mile dirt oval were between a guy with a 361 MOPAR and another one with a 302 Chevy V8- - - -the Chevy would turn 8K RPM at the end of the straightaway, but he couldn't out drag the Plymouth off the turns. Those guys would run lap after lap side by side all the way around the track.
Jerry

OK, more progress- - - - -I machined some bronze bushings the right size for a press fit into the 292 connecting rods, to reduce the small end diameter to the size of the 235 wrist pins, then I pressed them in and honed them for a correct fit on the 235 pins. That Sunnen hone machine is earning its keep! Next I'll assemble the widened rod caps onto the 292 rods and recondition the big ends back to the right diameter and compensate for any distortion the TIG welding might have created. That's done by grinding a few thousandths off the rod and cap mating surfaces to reduce the size of the hole, then using the proper size mandrel and honing stone to make it round again. That procedure shortens the rod a tiny amount, but on a non-racing engine the difference isn't critical. On a round track engine, I'd go a couple of steps further- - - -measure the center-to-center distance on all the rods, find the shortest one, and recondition all the others until they were all exactly the same length. Then I'd check the deck clearance between the piston tops and the top of the block, and mill the block to get whatever deck height I wanted. I'll also use my triple-beam scale and a holding fixture to weight match both ends of each rod (with the bearing installed) to a tolerance of +/- 1/10 of a gram. (A dollar bill weighs almost exactly one gram). Ditto for the reciprocating assembly of pistons, wrist pins, rings , wrist pin thrust buttons, etc. That's done by weighing all the parts together, finding the lightest assembly, and machining the balancing boss on all the other pistons to get that 1/10 gram weight match. Since an inline six crankshaft is inherently balanced, all I'll need to do with the crank is spin it up to as fast as my lathe will turn to make sure there's no vibrations there. I can get the lathe cranked up to over 2K RPM, so getting a 100 LB. crankshaft spinning that fast sort of increases the "pucker factor" a little! It's surprising how smooth an engine with weight-matched parts will run, even without sophisticated spin balancing.

The "long stroke" engine with the NASCAR rods will get a similar procedure, with bushings being installed in the 235 pistons to accommodate the 21 MM diameter wrist pins.
Jerry

Thank you for the oil pump info. When I read something that I think may apply to my long term project I get interested. Heck, this whole project your on is really interesting.
I drove a 1960 chevy pickup with a Corvair turbo on the 235 all through high school, and I want to sort of recreate that power plant in my 49 pick up. I have no notes from back then (50 years ago), but the more I think about it I think I had plenty of oil with the stock pump, even supplying the turbo.
Don't mean to hijack, just wanted to say thanks.

Originally, I had an idea about using a small block V8 oil pump on a 216 to get past the "no oil pressure at idle" problem the spray-oiler engines have, but building an adapter for the offset drive system and the angle the pump mounts in relation to the drive presented some problems. Then I noticed that the 250 pump has approximately the same size gears as the V8, but adapting the mounting and the drive shaft is much simpler. The same pump on an unmodified spray-oiler should come close to doubling the oil pressure. That pump is common to several engines- - - -the Chevy II 4 cylinder, and the 194, 230, 250, and 292 engines. Fun stuff!
Jerry

So I'll throw out one of my first thoughts when I started reading this, what about machining the housing and putting larger gears in the existing housing? or maybe machining the entire gear cavity off and installing a longer housing and gears (the 250 oil pump as doner)?
I'll also try to answer my own question, Crazy time and money??

It's possible to make a spacer that fits between the pump body and the cover, and installing V8 gears from a Melling M-55 oil pump in the 216 or 235 housing, but there are also other modifications to be made. The pivot shaft for the driven gear is longer, and larger diameter, so a new one needs to be made and pressed into the original housing. Then a different shaft needs to be made for the gear the distributor shaft drives, and pinned into place so it doesn't break loose from the gear under load. Doing it that way was my first attempt, and when I saw how much work was involved, it was simpler and less risky to adapt an oil pump that somebody else had engineered and tested instead of performing heart surgery on a 70 year old original pump.

The spacer plate idea isn't new- - - -a kit to do the V8 gear swap to a Corvair pump used to be available.
Jerry

Yes, while I was typing I could see the hours piling up and the reliability going down.
Funny how things come back to memory from discussing it, I now remember when I removed the pan to weld in an oil drain for the turbo I also took out the pressure spring and stretched it a little and reinstalled it. High tech mod eh?

Only if you follow the proper stretching procedure. I'm quoting my Dad on this one- - - - -he had a classroom lecture for his high school auto mechanics students about the right way to stretch a spring. Here's the condensed version, as he could take up 15 minutes of classroom time on the procedure:

1. Using a yardstick, measure the original length of the spring.

2. Grab the spring firmly by both ends with Vise Grips

3. Stretch the spring to exactly 3 times its original length, using the yardstick as a precision measuring tool.

4. With a pair of diagonal wire cutters, cut the stretched spring into pieces no more than 1/2 inch long.

5. Throw the pieces of the cut spring into the nearest trash can.

6. Buy a new spring

7. Do not stretch the new spring!

After he taught the auto mechanics course at that school for 15 years, he retired. Another instructor took over, and ran the shop for the next 7 years. When he retired, I took over the same shop and ran it for another 15 years until the school board in their infinite wisdom closed the program. I spent one year at another school, and also retired. Now neither of the schools have any technical trade education courses of any kind!
Jerry

OK- - - - -more progress- - - - -The modified 292 rods are finished- - - -bronze bushings in the top end, and the big ends are reconditioned for an exact fit for the bearings, with no distortion or damage from the welding procedure to install the spacers. BTW, since the guy with the TIG welding rig really knows his job, I was not able to measure anything other than routine "used rod" wear even before I reworked them.

Here's a few specifications, comparing 216 and 235 engine blocks. The main bearing diameters are the same on both engines, but the 216 thrust bearing (#3) is about 1/8" wider than the 235 for some reason. Widening the thrust flanges of a 235 crankshaft makes it a "drop-in" fit into a 216 block. Here's the main difference in the blocks. To compensate for the use of the same length connecting rods and pistons with the same compression height on both engines, something must be done to prevent the pistons from sticking out of the top of the block with the longer stroke of the 235. ( 3 15/16" instead of the 3 3/4" stroke of the 216)- - - -so Chevy made the 235 block 1/8" taller from the center of the main bearings to the top of the block. When a 292 connecting rod is used in a bored-out 216 with a 235 crankshaft, the shorter 292 rod almost compensates for the added stroke. The piston tops only poke out the top about .015"- - - - -it's easy to shave that much off the piston crown without weakening it, and the deck clearance goes to zero- - - - -the pistons end up level with the head gasket surface.

End result- - - -a "sleeper 235" in a 216 block- - - -and one that also runs a higher volume oil pump, replaceable rod bearings, a leakproof rear main bearing seal, and a camshaft just a little more radical than the Corvette/261 item. The lightweight aluminum pistons and much stronger rods with smaller-diameter bearings should allow the engine to run 4K+ RPM with no reliability concerns so you can outrun your Stovebolter friends with a "216" engine that looks bone-stock!
Jerry