273 Bore Taper

[Rustyfish]

Hey Guys,

I've got a used 273 short block in the back of my garage begging to get back into service but it's condition is mostly unknown. It has almost no ridge at the top of the bores (just a little bit of carbon build up, not enough to catch a fingernail), but haven't measured the bores yet. When I do, what's an acceptable taper to just dingle ball hone and reassemble?

I've got some good surfaced heads ready to go with the short block to replace to tired 318 in my '66 Barracuda.

 

[brian6pac]

If there is a lip even a small one It is usually rounded and even if you hone it the sharp edge of the new ring will hit it.

 

[6PakBee]

How many miles are you planning to put on it? I've re-rung 383's with 0.010 taper but they weren't 100,000 mile engines. If you have 0.003 to 0.004 I'd say you are good. And brian6pac is right, it doesn't matter how minor the ridge is, it has to come out. What I always found interesting about re-ringing engines with a lot of taper is that people think the ring seal to the wall is the concern. If you realize the ring is moving in the piston grooves with every cycle, you'll see the ring lands wearing more than anything else.

 

[Rustyfish]

How many miles are you planning to put on it? I've re-rung 383's with 0.010 taper but they weren't 100,000 mile engines. If you have 0.003 to 0.004 I'd say you are good. And brian6pac is right, it doesn't matter how minor the ridge is, it has to come out. What I always found interesting about re-ringing engines with a lot of taper is that people think the ring seal to the wall is the concern. If you realize the ring is moving in the piston grooves with every cycle, you'll see the ring lands wearing more than anything else.

After micing, looks like 5-7 thou on most of the cylinders. Sounds like a bore job is necessary.

 

[Krooser]

After micing, looks like 5-7 thou on most of the cylinders. Sounds like a bore job is necessary.

Run it....use a ridge reamer to remove the ridge and a dingle ball to hone it.

Tens of millions of engines around rhe world have been rebuilt this was and have given good service.

When I was a kid its how we were taught.

 

[AJ/FormS]

The ring gap moves .0031416 for every one thou of bore growth. .... so .019 for .006 borechange.
and most of the growth happens in the last inch, ............ which is where the compression makes the most difference. So yeah the ringlands are taking a chitkicking from the sideways scrubbing, but they might be seeing a good oil supply on the underside and they are 2 to 3 times as wide as is the edge-contact where the temperature is gonna soar, especially as the hot gasses try to scoot past the now-opening ring gaps. Pow! the hot gasses hit the oilrings, and that is why your pistons are coated with varnish.

I know, fixing this is gonna be expensive. 273 Pistons are not cheap. And yes you can just slam it together, if yur not looking for a long-term engine. But;
IMO, you are better off building a hi-compression anything, than just slamming a re-ringed 273 with .007 bore taper, in there. The ringlands are already scrubbed from running that way previously, and there is no good bandaid for that taper-situation.
But hang-on; not only is the compression leaking away, consider what is happening on the intake stroke. The ring gaps have to be adequate as the piston is falling so that the gaps do not butt on the power stroke. So that means the gaps at the top will end up at least .019 too big at the beginning of the intake stroke. That's on top of the probably .028 they need to start with. So now you have a top gap of .047! And on the intake stroke, there is a good chance that the low pressure in the chamber, at idle to low-rpm and mid-throttle settings, is gonna pull up air from the crankcase. Air with oil vapors in it. It doesn't take much oil in the chambers to destroy the octane rating of your fuel. So if you get into detonation with your 8/1 273, and have to run 91 instead of 87E10, with retarded timing; How long can that go on before it wouldda been cheaper to just rebore it. I mean IDK, it depends on how many miles a year that you are planing to run it. I mean, $1000 buys a lotta gas difference.
So, as you know, the 4-stroke engine has 4 or more parts to it, none of which are helped by bore-taper.
Besides compression and intake, are power and exhaust. On the power stroke at about 25 to 28 degrees past TDC, is where the design max pressure should occur. But if the pressure has been leaking past the rings for these 25/28 degrees, how much is left over to actually push the pistons down? IDK, just asking.
The top one inch of piston-travel is the most important part of piston travel in the 4-stroke cycle, except maybe on the exhaust stroke with a short-period cam. Leakage here can break your performance/economy expectations, not to mention the time between oil-changes and adding oil.

 

[Rustyfish]

The ring gap moves .0031416 for every one thou of bore growth. .... so .019 for .006 borechange.
and most of the growth happens in the last inch, ............ which is where the compression makes the most difference. So yeah the ringlands are taking a chitkicking from the sideways scrubbing, but they might be seeing a good oil supply on the underside and they are 2 to 3 times as wide as is the edge-contact where the temperature is gonna soar, especially as the hot gasses try to scoot past the now-opening ring gaps. Pow! the hot gasses hit the oilrings, and that is why your pistons are coated with varnish.

I know, fixing this is gonna be expensive. 273 Pistons are not cheap. And yes you can just slam it together, if yur not looking for a long-term engine. But;
IMO, you are better off building a hi-compression anything, than just slamming a re-ringed 273 with .007 bore taper, in there. The ringlands are already scrubbed from running that way previously, and there is no good bandaid for that taper-situation.
But hang-on; not only is the compression leaking away, consider what is happening on the intake stroke. The ring gaps have to be adequate as the piston is falling so that the gaps do not butt on the power stroke. So that means the gaps at the top will end up at least .019 too big at the beginning of the intake stroke. That's on top of the probably .028 they need to start with. So now you have a top gap of .047! And on the intake stroke, there is a good chance that the low pressure in the chamber, at idle to low-rpm and mid-throttle settings, is gonna pull up air from the crankcase. Air with oil vapors in it. It doesn't take much oil in the chambers to destroy the octane rating of your fuel. So if you get into detonation with your 8/1 273, and have to run 91 instead of 87E10, with retarded timing; How long can that go on before it wouldda been cheaper to just rebore it. I mean IDK, it depends on how many miles a year that you are planing to run it. I mean, $1000 buys a lotta gas difference.
So, as you know, the 4-stroke engine has 4 or more parts to it, none of which are helped by bore-taper.
Besides compression and intake, are power and exhaust. On the power stroke at about 25 to 28 degrees past TDC, is where the design max pressure should occur. But if the pressure has been leaking past the rings for these 25/28 degrees, how much is left over to actually push the pistons down? IDK, just asking.
The top one inch of piston-travel is the most important part of piston travel in the 4-stroke cycle, except maybe on the exhaust stroke with a short-period cam. Leakage here can break your performance/economy expectations, not to mention the time between oil-changes and adding oil.

Thanks for the detailed response. I'm not likely to slam it together just to run a short term 273 in favor of a gasping 318. Too much effort could be wasted for a short term gain. I'll probably break it down and have machine shop do their magic. I like the idea of building an engine on my own now that I've got more time on my hands at this point in my life.