The Stovebolt Site Forums The Shop Area Engine Shop Boring a 261

Gentlemen,

What would You all consider the maximum possible overbore in a 261 Chevy engine?
Do You consider possible going to 3 7/8"?
A very good friend of mine is considering a high performance 261, so I was thinking of boring and stroking it. Stroking is the easiest part, since we can offset grind the crank, but overboring is a little suspect, since here the first bigger piston available is the 3 7/8", I'm into messing with an used block to check it out, but would like to hear from You first. The same question for decking the engine, how much is considered safe?

Thanks.

What year? The '54 has more to work with, but all can vary depending on core shift. Checking one may not give a definitive answer for the other.

Might be best to limit the boring to .080 inches for street use.Some drag racer used to go .110 oversize, but like Grant says ,the little bit you gain may not be worth it if the cylinder walls are weakened.

Alexander;
Unless you plan on pulling tree stumps, why would you want to stroke it? Boring for displacement and possibly increased rpms I can see, but these things already have too long a stroke to my way of thinking! It limits rpm. If it was possible, (it's not) I would bore it .250" and destroke it! Suggest you bore it .080" and use a good cam and carburetion.

Alexandre: Grant is right about the 54. The 54 and 55 1st 261's are the best of the 261 blocks as far as casting quality, (nickel/tin alloy) and thickness in wall and main web areas. But, there is a trade off, the later 58 and up blocks have the improved oil system, and better cooling. Also the later crank and rods are superior forgings to the earlier 54/55 1st. Life is full of trade-offs.

I also agree with Tony, in that I think for street use, stay with .080 over max. I think thats the biggest oversize 261 piston you can get anyway. And it's a cast piston at that. I would try to stay as close to stock bore as possible for wall thickness/integrity.

You run into all sorts of problems with a large overbore. And as mentioned, the core shift issue is ever present. I have bored several 261's out to .020/.040 and .060 oversize. I have not taken one to .080. I would not even try to take one to .125 over, at least with conventional boring bar set-up. Your out there in no-man's land.

The only way to safely bore to that size, would be to sonic test each bore at 12,3,6,9 o'clock positions and at about 3 inch interval levels throughout bore height. Then you could establish exact core shift, and wall thickness position on all bores. The bore centerlines would almost certainly be off center to crank axis. Would have to bore block on a horizontal boring mill. Then you could reposition for each cylinder offset. All the rods/pistons would have slight angularity to crankshaft centerline.And by the time you get to this point you will have marginal wall thickness and less than desirable cylinder wall integrity. As you can tell it would not take that much to make this set up fail, and after all that expensive machining. You could have casting cracks, or early heat failure from such a thin wall, once you got up to operating temps, or high compression and under heavy load.

After all that, your only out to what a 270 GMC is. Which is the piston that you would have to use. Then you have the problem of the pin size. As you can see the list of difficulties increases with bore size. You could just get a 302! It's a 4 by 4.

You can weld, and then offset grind the crank to adjust the stroke, but be prepared to spend lots of time and bucks there too. You have to weld, offset, rough grind, send crank out for heat treatment, undersize the mains, then finish grind the crankpins. Believe me, I have done this, and it's a big chore. All the welding just seems to pull the weights around and before you know it, the crank is all spazzed out!

I have safely taken .040 off the deck, with no troubles. Like I said, if you want power, keep that bore small and wall thick, and jack that compression up. Forged pistons, headers, porting and lots of head prep/flow and valve work are the secrets to more HP! Let me know if you find any forged pistons for a 261. Good Luck, and repost your ideas!

Think Chevy! :rolleyes: :p

Thank You all Fellows,

Tony And Grant:

The point is simple: the 261 is a big and lazy engine. They worth little or no money here. We had them prodiced in Brazil from late 50's to mid 70's and I can get no sonic checker here. But as I told, blocks are for free, machine job is also inexpensive, and some other fellows stovebolters here always talked into increasing displacement of their 261. And I'm talking exclusivelly of dealing with block made in brazil, not U.S.
This point it turns a li'l shaddy, we cannot cross it out. Just guess.

Chevyman:

The whole ideia is: offset grinding the crank .105" off to use 292 rods, then boring it ou to 3.875, using 230 pistons, also for free here. So it would end up 293 CID, and I have no GMC around here to make a bigger stovebolt without all this hassles.
In fact it is a sort of a gather togheter of some inexpensive stuff to see what happens. It's fairly common to sleeve used blocks here and they take it nicely never hitting waterjackets, so I'd bore one to 3 7/8 and then some more, let's say to 3 15/16 and finnal to 4 and watch the holes appearing slowly to see if it'd be acceptable.
Then camshafts and other stuffs would be another story, if the assembly proves itself worth, we would go deeper into it.
Carb and headders are easy to overcome, either buying new or having them fabbed here.
This set up, if feasable, would require no forged pistons for a 261, (283 V8 are much easier) and and also would give a good compression with flat tops.

Smitty62
Destroking it and going to a full 4" and less stroke would give me it even more lazy, and I'd need to grind a cam with an wider LDA, not easy task using a stock cam. Grinding a cam is easy, getting a new one not so! This would really be the ticket to fly high, but the overall idea is to produce even more torque, even using a worst rod/stroke ratio that would make even more low end grunt, but also would make it a li'l more peppy.

Thanks again!

Alexandre: Now I get a much clearer picture of what you are trying to do with the 261 block. I've been thinking about the modifications you have mentioned, and been tossing thoughts around a little, and want to give you more imput.

I have sleeved many blocks myself. The standard sleeve set up involves boring block cylinder bore to within .250/.375 of bottom of bore, then stop! This "step", or ledge, is a positive register for the bottom of the sleeve to locate against. I like to keep the press fit on the easy side, say
0.0005/0.0007 inches, that way it will cause minimal wall distortion to block and adjacent cylinders. I always use locktite to bond the sleeve to block, then finish bore, then cut top down flush to deck. The head positively locates sleeve in place.

The 261 block does not have "much" of the bore or stock cylinder extending into the crankcase area. This means that for you to bore out that far (4.000), you will be well into the waterjacket
and will have no way to form a step or ledge for the sleeve. You will have to bore straight through (no material left) and may have seal problems in this area. Also without a register on the top of the sleeve, against the (counterbored)deck, you have no way to positively locate. How will you locate/lock in place? Also what does this "through bore" for the sleeve actually do to the blocks structural integrity? It will certainly not be as structurally sound or strong as an intergral bore block.

Offset grinding the crank to fit 292 rods.

Stock 261 crank pin is 2.311/2.312

Stock 292 crank pin is 2.099/2.100

The difference in pin diameter is .212, using 292 rods .106 offset from TDC stroke. No gain over stock 261 stroke of 3.940. The grinding will in effect make the pin diameter smaller, which will decrease bearing speed, but at the same time give up how much structural strength in the crank. The 261 crank already flexes with a larger pin diameter, and a larger, stronger, stock rod. The stock 261 rod lower end is larger in diameter than the 292, and is also wider than the 292 rod, so the impact/rod thrust load surface area is much greater with the 261 rod on the crankpin.

And what about this width issue?
The 261 rod width (OAL) is 1.242
The 292 rod width (OAL) is 1.030

And the bearings?
261 rod bearings are 1.033 (OAL)
292 rod bearings are 0.842 (OAL)

The narrow 292 bearings means that you will have to grind narrow offset on crank pins to fit, with smaller fillet radius of 292 bearings.

What about all the thrust on such a small bearing surface?

What about rod side flex and control, considering a portion of the radius, (TDC) will have no shoulder, or fillet?

What about oil control/throwoff?

I realize all these problems exist because of the differences, in the 4 or 7 main bearing cases, but this is the kind of problems you are going to run into.

You said you wanted to use 283 flattops. They will fit the rods, but I think you will have some other troubles here also. Need to check these exact block measurments, but I feel that the 261 is quite a bit taller block from crankshaft centerline to deck surface.

Stock 261 rod 6.8125 CtoC
Stock 261 crank has 3.940 stroke

Stock 292 rod 6.7660 CtoC
Stock 292 crank has 4.130 stroke

0.190 difference in stroke
0.0465 difference in rod CtoC

Compression height of these pistons (261 versus 283) are considerable. I think you are going to be a ways down the cylinder, and quite short.

Going to a smaller, lighter, piston certainly has weight saving advantages, but you may run into some control problems to.

With a 292 rod on a 261 crank and a short slipper skirt piston like the 283, I feel you will have rock/slap troubles, especially near BDC/TDC with compression and will have ring flutter or loss of ring seal. Stock type 261 piston is tall full skirted, and has less tendency to tilt/rock. More stable piston in a tall deck block. Also stock type 261 piston has wider top ring than 283 piston
and you already know the top ring does 90%+ of sealing of compression.

Anyway, I do not want to discourage you, or your experimenting. I think it's great! Thats how the neat stuff in modifications is figured out, with guy's like yourself pushing the envelope! I hope anything that I may have said, helps. Keep posting, and tell us of your trials!

Think Chevy! :rolleyes:

I believe some guys use a Buick piston from the later V-6 or V-8 engines.If you stroking the crank less than 1/8 inch you probably use the stock rods and hopefully the piston pin bore can be relocated higher up the same amount.The major limitation on any 235/261 engine is the restricted breathing of the stock head even when fully modified by porting.The only real way to find out is to do the work and see what the result is.Back around 1970 I was drag racing a 650 Triumph cycle in a class that was becoming dominated by better breathing ,higher reving, but much heavier Japenese 4 cylinder cycles.My only option to remain competitive was more power through higher RPM.I was told the absolute limit of mechanical reliability was 7800 rpm.The last season the bike was raced ,the engine was nearing 9000 rpm,Broke vavle springs and pulled the cylinder studs out of the crankcase but never broke a crank or rods.Alex, do it and tell us the results, might be interesting.

The Buick V6 pistons need GMC "419" rods with some modification to the small end for the smaller Buick pin, and narrowing on the big end to fit the Chevy crank. There are a lot of different compression heights and styles of Buick pistons, I am not sure which ones to use but they need machining for valve to piston or head to piston clearance. There is a better selection of off the shelf rings for Buick pistons. Standard Buick bore is 3.800".

Stroking with 292 rods has been done, but I think with custom pistons. There was an article in "The 12 Port News" from Inliners a while ago.

I looked into using Toyota Landcruiser 2F rods. They have the same width big end as GMC and a smaller journal diameter but are a lot longer (7.7") and wont work in a Chevy. They might work in a GMC with custom pistons.

I have heard of blocks bored out to accept wet liners. I am sure you will find some way of doing it.

Well fellows, this topic is getting very interesting!
And for sure, after all the research is done and if I it all turns feasable, I'll post here the results. This is in fact a long term project, not for me but for a good friend of mine, a guy that loves a lot the 261 and the one that talked to me about this great site!

Chevyman: I see the sleeving process like You described also. I mentioned boring (and sleeving) as a way to see what I'm dealing with. I do not have a sonic tester to check the walls thickness, so only boring them and taking notes on what diameter we reached water jacket will give me a final answer as for the boring limit.
As for the crank, I want to use the 292 rods just because they have a smaller big end, then I'd take advantage of the difference to increase the stroke, in this case, opening the arm .106" for each side, total of .212" increase in the engine stroke.
Also the 261 piston comp height is around 2.000", the 283 is at 1.800", as long as the new stroke is .212" longer, it will take the piston .106" higher in the bore. This will require to shave the deck around .100" to have the compression set back to a level a li'l higher than the 261 had before.
I knew the 292 rods and bearings were a bit narrower than the 261's, but not by how much. Now I know, You told me! All the 261 cranks here are forged steel, so welding and regrinding them is no big issue to me, the only difference is that I'll have to build up the sides of the rod pins at the crank, in a way that the rods will have a surface to ride on their sides all the time after the crank is done.
I like a lot this process of rebuilding it another way. In the eighties, I mockep up an engine using parts of others here, and the results were rewarding.
We had here 2 L4 chevies, the 153(like the Nova ChevyII of the 60's, 3.875" X 3.250", 5.700" long rod, 2.300" and 2.000" crank pins) and a 151 just like it, except for: 4.000" bore, 3.000" stroke, 6.000" long rod, but the same crankpins,2.300" and 2.000". So I just got a 153 crank and put it in the 151 block using a 5.700"rod. As long as I didn't had a taller 4.000" bore piston(the 151 uses one as tall as a 350 V8) and at that time we were not allowed to import auto parts here, (so I could not get 4 pistons from a 327 engine) I just shaved the deck .100" and also milled .100" from the head, and it was done. There was a fashion here to drop this engines into cehvettes, and by that time the bigger 181 CID was not available.
Also we had 2 types of 250 L6 here, an early version like the ones in US, 3.875" X 3.480", 5.700" rods, 2.300"X 2.000" crank, and a second desing, all the same, except for a 6.000" long rod and a piston with a comp height of 1.340" instead of the normal 1.640" ones.
Well, some wise drag racing fellows knowing that there were no tell tale tools available, decided to do the following: get a 292 crank, grind it down from the stock 2.450" X 2.100" to the 2.300" X 2.100" 250 set up, a set of 5.700" long rods and a set on pistons for the long rod engine, 1.340 comp. height. The result: a 292 low block. Looks kikw a 250, but a 292. Ok, with a rod/stroke ratrio of 1,38/1 it does not spin high, would say that 5.500 rpms are THE TOP, but it has such a ridiculous amount of torque that just made this no revving just a detail. And also, this 1.340" pistons uses a 2mm, 1,5mm and 4 mm ring pack.

Tony: Your comments are pretty cool also. Both in the Buick pistons and about the bike stuff. As for the bike, ever visited the Ian Drysdale page? They used to run a mailing list, but no longer available, in this list the guys there discussed not only bike stuff but also motor engineering and project. Never found a place where some deep subbjects like engine balancing and other related project itens were so well discussed . Ian's page is about his bike, a V8 powered suprbike he builts in Australia. Drysdale Motorcicle company. Seriously cool dude, this guy still finds time to reply e-mails from some curious guys from brazil, taht was likely to discover why his bike has a flat crank instead of the 2 planes version. Got really amazed by his projects, the way He project and build his bikes, and of course how he handles it all.
This Buick stuff gave more options, but still has to deal with the pin diameter stuff. But bushing the rod to a floating pin is no big deal.

Kiwi Mike: I also had the option of using the forged 292 piston, but did not want to make it just because of the thicker rings. These pistons have the ring pack way down and also have a very thick head, so milling them to get the right height is no issue either.
I've already had some thoughts about using wet liners, but it may be very well observed. Don't have any idea on the block's bottom end strengh to handle it all. Making it fit is easy, the process is not impossible to be done, but I have no idea on how and if and for how long it will work! I used to assemble some engines with wet liners, and always noticed that the water jacket bottom is pretty strong for the engine size. Both on small pass car gasoline engines and in diesel engines for trucks. At least machining and researching is easy!!

Alex, do you think the stroked crank may cause the connecting rods to hit the camshaft?I have talked to guys from Inliners who raced 261 drag cars.They estimated the maximum power from a 261 is around 300HP.A drag engine can be stressed a lot since its runs for only a short duration.If you can devolp the power without high RPM ,the engine stands a better chance of staying together.

Tony,

Don't think so. Not with this increase, because the big end of the 292 rods are smaller than the 261 and I'm not going to weld to make more stroke than I could achieve just offset grinding.
The idea of getting power without revving too high is quite clever in this case. I do not plan spinning it more than 5.000 rpm anyway(this will not be a drag racing engine and this rpm may be sustained a short while, but not all day long, of course). Would even consider sacrificing some numbers than over revving and breaking. Also plan using a shorter duration higher lift cam.