The Stovebolt Site Forums The Shop Area Engine Shop Looking for 261 .080 over Pistons

Anyone know where I can get 261 .080 over pistons? I have a block in the machine shop bored .080 over waiting pistons to finish hone. Patrick has pulled out all the stops looking for me and they just don't seem to be available anymore.

Thanks

Grant,
You might give Egge Machine a call. I think they can make just about any size and type piston you would want. When I called about a set of pistons for a GMC 302 they said a custom set would cost around $350. I didn't think that was too bad for a custom job, considering a new set of Venolias were $500. Their web site: http://www.egge.com/

Here is another list of piston manufactures. Some will be able to make custom sets also. http://www.flatlanderracing.com/pistons.html

Grant
Try Kanter Auto Products
www.kanter.com
I know they carry .060 over
but you would have to call on the .080 over
1-800-526-1096-1
I bought my 302 parts from them .060 over
and very satisfied.
Mike

Just incase you don't know-- You are reaching the limit of boring on this engine and are reaching the area where you "will" have overheating problems to contend with. You may want to consider boring it over a bit more and sleeving it back to near stock. Easier to find parts for and you won't have the overheating problems that come with thin firewalls. I have had one bored 90 over but it wasn't good. Boring 60 over is about the most I would ever do again. Just be careful here because- Just because you "can" do something doesn't mean it is "good". Talk to your machinist about sleeving it back to 40 over.

I know someone who used to race these engines. 080 was standard race engine. After racing it he took one and put it in his car hauler and ran it for years. Sometimes bored to .125 over.

Ross Pistons, an advertiser in Inliners International's 12 Port NEWS can make you anything you want.

Ross Pistons
625 S. Douglas
El Segundo, CA 90245

(310) 536-0100

Ronnie

Grant, you better drive with a door propped open, you can bail out if she blows."Scotty here Captain, she can't take much morrre of this abuse"
I have no experience with an 80 over 261, but Joy has a stock 60 over 261 in the 51 truck.The truck pulls a 1500 lb high frontal area travel trailer on summer trips.The truck never runs over 210 degrees even when pulling the trailer up hills steep enough to require down shifting .And the radiator is a standard cheapo reproduction not a heavy duty "desert cooler"
Grant again,although piston weight doesn't affect the balance of an inline 6, I do know inline engines running larger oversized pistons tend to run a little rougher than smaller bores.Perhaps you might want to compare the weight of .080 pistons to standard pistons.Hurry up and get that engine finished so I can blow by ya in the 37 lol.

Another place to try is Vintage Auto Parts, www.vapinc.com

Bolters:
I've got a totally reworked 261 in my 52 3600.
I haven't fired her up yet but the engine was speced out by Pat Smith (Inliners International) who really knows his Shi.engines! The engine is .110 over and uses Ross Pistons. (I've got a complete build sheet that I can e-mail to anyone who'se interested.) Inliners tech advisors say you can take the 261 out to a max of .125, incredible as that sounds. We went .110 to leave margine to open it up in the future if necessary. Now all I need is to find the formula which will allow me to calculate the actual displacement of a .110 over 261 with a stock stroke. In my experience Tony really knows what will and wont work on 261 from practical hands on experience (Actually he just turns the wrenches Joy is the brains of the team).

Grant--

This is the first Ive heard about boring out cylinders to 0.08 as being problematical but what do I know. Patrick's has pistons to 0.08 over for 235/261.

Now I'm worried about my 292 being bored .060 over. Hope I don't pierce a cylinder wall or have overheating problems. We shall see in about another month or so.

-ftyler

Formula to determine cubic inches -

bore x bore x stroke x 2 x .786 = eng displacement

Example of 235 engine -
3.5625 x 3.5625 x 3.94 x 6 x .786 = (235.81951 ci)

Example of a .060 over 235ci engine -
3.6225 x 3.6225 x 3.94 x6x .786 = 243.82979

You can confirm the bore from the oversized piston box. Plug that into the formula above and finish.

To clarify my previous post- The engine I was refering to was a Corvette 235 and it was almost 40 years ago, so things are a little foggy but not "that bad"-LOL

When you bore an engine oversize you decrease the ability of the cylinder wall to dissipate the heat. You also, when boring past .060 over, get the chance of compromising bore strength and getting bore distortion. If you (for what ever reason) figure you need to bore your engine past .030 over I would suggest that you have it "Sonic" tested. Of 235's bored past .060 over about 95% will fail this test. A prefered thickness of cylinder wall is 0.200" thick and I like 0.2500" better. Talk to your machinist and then talk to some other machinsts. Let "logic" dictate which way and what you will do. If still in doubt- get ahold of me, what I don't know- I know a couple of good machinists that do know.

As to the over heating issue- I will lay this out as I see it. The cooling systems on these old trucks are barely adequate, at best, and when they aren't maintained properly (as Tony does his) then they are prone to failure. When you increase the size of the firing chamber (generating more heat) and simultaneously decrease the ability of the cylinder wall to dissipate heat, you need to increase the ability of your cooling system. But, here is the draw back- When you increase your cooling system you will need to use power from your engine to operate the increased cooling and thereby reducing your ultimate horsepower output. Sometimes "bigger doesn't equal better".

There are better ways of increasing the output of the I-6 then overboring it to a large extent.

FTyler, I wouldn't be extremely concerned about a 292 bored 0.060" over unless you have a poor cooling system. Or, unless you are making it into an experimental racing engine. Now if you were talking a 235 (261?) bored 0.060" and were going to be turning it at 4-5,000+rpm then there would be other concerns to think about.

Oly's point about the large overbores reducing cylinder wall strength is sound advice.The small gain in displacement from a large overbore on a steet engine is offset by the structural compromise.However it's becoming incresingly difficult to find standard bore 261 engines.So many of us have to live with the bore being maxed out.The 261 in my 37 had nice bores, only requiring boring to minimum oversize. The other 261 had a 5 beautiful std cylinders ,but one was ugly and need .060 to clean up properly.Very few 261 engines are placed under the strains of constant heavy duty truck use so a big bore is probably no problem to live with if you have a decent cooling system.
Maybe one of you can explain this to me. To my way of thinking(dah),the engine heat is transferred to the coolant. Thinner metal tranfers heat faster than thick metal. So why may a thinner cylinder run hotter if the cooling system is up to the task?

I had the same thought as Tony on this. It is my understanding that the block stores heat requiring cooling passages to remove the heat. Less metal less heat. Moreover as you increase the size of the piston the distance to a water passage is diminished thus bored cylinders should cool more efficiently. However; the combustion volume is increasing apparently about 5% if the calculations on this thread are accurate so......Agree with Oly and Tony that structural integrity is a problem.

Tony, If you think a little about what you just asked "To my way of thinking(dah),the engine heat is transferred to the coolant. Thinner metal tranfers heat faster than thick metal. So why may a thinner cylinder run hotter if the cooling system is up to the task?", you will see that the answer lies within the question. Being an Electrician, think of a heatsink.

I don't believe that Oly's comments have any basis in reality. The block isn't a heat sink, the water is. The cylinder wall is just a thermal interface to conduct the heat but contain the combustion and provide for structural support. To put this into simple terms, the cylinder wall is an insulator (it really isn't that great of a thermal conductor compared to Cu, Al, or Ag). Think of your house. Are you going to keep warmer with thicker insulation or thinner insulation. I think that this really has more to do with cylinder ballooning.
Also, think about this with sleeving: There is a sleeve/block thermal interface which could include an air gap. Remember that the machining isn't perfectly smooth or round. You could be worse than what you started out with.

Ok, Jason said what I was gonna say,except he makes more sense than me. Balloning cylinders, friction, more heat, blowby?
Large industrial/institutional electrial equipment generally relies on overdesign to avoid excessive temperature.Sustained temperatures over 90C causes excessive vottage losses and increased resistance . Then the switch gear vaporizes,and out go the lights, lots of overtime.Large transformers are often oil cooled.After the power was shut down, we used stick our bare arms in the PCB oil to service the electrial connections.Don't worry, the managers said, those PCB's are safe.Then go out side to have a smoke with oily hands.Ah, the good old days.

I can't see where thinner cylinder walls transfer heat less efficiently, in fact it would be quite the opposite. This is why we have thin metal in a radiator. However, the stability of the thin cylinder wall is another question.

Tony, I used to get covered in that PCB filled oil when I helped service reclosers as a summer student. It seems that we survived a lot of things in the good old days ! I also remember working in garages where at the start of a brake job we would pull the drums and clean things off with compressed air creating great clouds of dust filled with asbestos lining remains !

Grant, Tony and truckernix, I "am" dealing with reality on this. I'm trying to speak in a language(?) that you can easily comprehend. I can see where this is going to be difficult for me to explain, but, by gumbys, I'm going to try.

I never implied that the block or cylinder wall (engine) was the heat sink. The true heat sink here is the radiator. The water is only the vehicle to transfer the heat from the engine (cylinder walls, head and block) to the radiator. You want heat in the cylinder- The more heat, the faster gases expand, which in a confined environment gives you more energy, in turn giving you more mechanical power. How well an engine uses the heat it generates is known as it's thermal efficiency. The average engine has only a 25-30% thermal efficiency, so 70-75% of the heat generated never gets used to make power. The cooling system takes heat from the engine, heat that ideally could have made power, so the cooling system actually takes power from the engine. It is a necessary evil, without a cooling system, the engine will overheat and the internal parts will have a very short life. I am no metallurgist, but I do understand the notion that a thick piece of steel can withstand and dissipate greater and more heat then a thin piece of steel.

When you overbore an engine you increase the firing chamber, thereby increasing the heat. The thinner cylinder walls can't withstand the increased amount of heat as well, nor dissipate it as well. The stock cooling system isn't designed for an engine that gives off more heat. So you need to increase the flow of water to cool the engine even more than before. The trade off is that you will need to run the engine at a lower temperature because of possible cylinder wall distortion and you will need to use more engine power to operate a faster water pump.

As far as sleeving goes- You can either "dry or wet" sleeve, and nowadays there isn't really to many problems with doing it if you have it done by someone that knows what they are doing.

An old hotrodder freind of mine had a 261 in an old 41 Chevy coupe. It always ran fine, but the coupe was rotted out. Well he found a Jeep Stationwagon and did a transplant. In the process he rebuilt the 261. Well he found that he had forgotten ( kinda like I do) that he had bored the 261 .100. BTW this was about 30 years ago. He looked at the Books and found that 292 pistons would work (4" bore). Had it bored and drove it like he stole it for as long as I remember. He also was in with the origional Drag Safari in the early 50's. Ran a dragster/altered at the time with a 261 and a Wayne head. Back to the question. There has to be a reason that all the old stuff overheated. Maybe there's a reason that the new stuff has thin wall castings. Just my $.02 worth.
Dennis

This is part of an email answer i received from a fellow I know. He is currently building Street rods and track racing machines. Even his "more modern" ideas are similar to my "old days" ideas and practices.

"You are correct in saying that thinner walls cannot dissipate the heat as fast. Metal transfers heat much faster than water and anti-freeze, and the tops of the bores run the hottest. If the bores are thin, there is less metal to dissipate the heat. When too much heat is in one place, coolant can boil and form steam pockets, which is probably what caused problems in your 235. A nice thick cylinder wall acts as a better "heat sink" and pulls some of that heat down to the cooler lower portion of the engine. In other words, it's not as concentrated. People tell me all the time, " If the coolant flows too fast, it wont pick up the heat in the block and you'll have problems." That is not true at all. Faster flowing coolant is better, providing the pump does not cavitate and there is enough block pressure. Faster flowing coolant reduces steam pockets and helps even out engine temps. Coolant boils when it picks up too much heat. When it flows slower, it picks up more heat. Flow the coolant twice as fast and it can only pick up 1/2 the heat, then when it gets to the radiator, there is 1/2 the heat to get rid of, so it can do that twice as fast also. By doing this the coolant is only picking up 1/2 as much heat in the block, but doing it twice as fast. So where back to the same amount of total heat transferred, but we're reducing the chance of steam pockets by evening out the temps some.

There is also many other things that happen to the bores under heat and pressure that have a lot to do with bore thickness and consistency. Core shift can really mess up the works, it's bad enough that the bores flex (which they all do), it's even worse with the flex out of round due to walls being thicker on one side. That's another long story.

IMO, the old 6 Chevy's had pretty thick walls. 0.080" should still leave plenty of metal for good cooling, I wouldn't want to go any higher than that. I personally like to have at least 0.200" thick walls for any kind of performance engine, but the thicker the better. Joe"

I hope he has explained it better then I have.

So now we are back to were this started, an .080 261 should be fine. Did you know a 350 Chevy V-8 with a typical .030 overbore has a cylinder wall thickness of about .130 and even less with core shift? Pretty thin stuff for 400 hp street engines . I suppose all the built small blocks running around deal with it somehow.

Thanks for all the tips where to buy pistons. No one seems to have them, Patrick can't get them, But did talk to a fellow in NC, Troy Robertson who oredered me a set as well as himself. They came in yesterday, only two pistons instead of two sets. This just seems to be the way this 261 project is going. The only bright note here is people like Troy who are willing to help out and send me a set of pistons with just my promise I will send a check when we find out the total.

BTW, I bored this engine .080 not just for the heck of it, but because that's what it took to clean up. It was already .040 over and #1 bore was ugly. As far as the theories if water is a heat sink or metal is, this is all more than I want to understand even when simplified for me. I'm going to go whith .080 because people with more experience than I have done it with out problems.

On the 292, Leo Santucci has his 292 alcohol injected turbo race engine bored .070 over.

My point was that saying heat travels through thicker metal faster than thinner metal is incorrect. All the materials add resistance into the equation. However, the heat being dissipated further across the cylinder wall with the thicker wall is understandable and could result in more efficient transfer to the liquid.

All I have to say about all this topic is: Read my signature!

I must have misunderstood where you were coming from. Your later comments/quotes make more sense. Consider me enlightened. However, I don't agree that increasing the bore .080 is going to greatly increase the heat output. Percentage wise, you haven't increased the engine size very much.

True, but thinner metal heats up faster than thicker metal!

Grant,

Your biggest problem IMO (besides not being able to buy pistons) is sorting through all the BS in this thread. 0.080" overbore is the practical limit for an endurance engine, but some racers push to .125 and fill the bottom end of the water jacket with stabilizing epoxy and rely on water in head and upper cylinder jacket to do all the engine cooling. In the reading I have done between Leo, Bill Fisher and Roger Huntington, lots of folks push to 0.080 and beyond without problems as long as core shift isn't a problem.

Heat dissapation has not been mentioned once in the discussions of any of these authors, however, each of them have mentioned the reduction in wall stiffness. Make sure they use a boring (torque) plate to pre-stress the cylinder walls, your chances of having nice round bores under assembled conditions is much improved.

Grant,

If you cant find 261 pistons there is another option. You can use Buick 181 V6 pistons (3.800" standard) adapted to GMC rods. The downside is that there is a bit of machining involved and a lot of careful measuring. There seems to be several different compression heights and style of piston for the Buick so choose carefully. Once it's done it is a better setup than the original and you will have a better selection of rings, higher compression, longer and stronger rods, and maybe forged pistons.

Here's another link: www.kb-silvolite.com/