Any machinist on here?


Daniel Black

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I've been driving my swapped Ranger for a while now and it runs pretty good. Definitely better than any stock Ranger I've drove but I'm running a pretty much stock carbed 302 and I'm looking for more. I planned on building another 302 or maybe 289 over the winter and swapping it in the spring. Here's my problem. I have a really nice set of forged dome pistons I want to use but they're standard bore. I have a standard bore 302 block but my machinist says it's wore .005. He said he could just do a hone job and I could run it but it would be "loose". What does that mean? Does it mean massive blow-by and a motor I'll have to tear down in 10,000 miles and bore anyway? I know the rings will take up some of the slack but how much? I already have the pistons so that would save me at least $400 and if it doesnt have to be bored then that's another $160 saved that could be spent elsewhere.
 


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PetroleumJunkie412

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I've been driving my swapped Ranger for a while now and it runs pretty good. Definitely better than any stock Ranger I've drove but I'm running a pretty much stock carbed 302 and I'm looking for more. I planned on building another 302 or maybe 289 over the winter and swapping it in the spring. Here's my problem. I have a really nice set of forged dome pistons I want to use but they're standard bore. I have a standard bore 302 block but my machinist says it's wore .005. He said he could just do a hone job and I could run it but it would be "loose". What does that mean? Does it mean massive blow-by and a motor I'll have to tear down in 10,000 miles and bore anyway? I know the rings will take up some of the slack but how much? I already have the pistons so that would save me at least $400 and if it doesnt have to be bored then that's another $160 saved that could be spent elsewhere.
IIRC from when I built my dad's 440, you're going to be much better served to start with a fresh block, or sleeve the one you have.
 

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I'm not a machinist but I will throw my thoughts in anyway. Perhaps someone will change my mind.
If it was me I'd run them "loose". But I don't put on a lot of miles with my hi-po stuff and I'm cheap...sometimes too much so. When it needs a rebuild then you know everything else but the bore is probably good and you can get new pistons and a bore job at that time. You're probably right at the outside edge of acceptable tolerances. IIRC my 460 was good up to .007
Did the machinist say anything about blowby or other concerns?
 

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At .005 over, the rings will likely seal fine. The main issue is that .005 is enough to allow the piston, specifically the piston skirt, to rub the cylinder wall. It will eat the pistons, and the block. The tolerances, when correct, keep the piston from being able to get diagonal, and sliding against the block.
 

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If you were going to race the engine I'd say run it, race engines are generally run "loose". But race engines don't last long. The issue isn't blowby, the rings would still need to be gapped to spec no matter the bore. It's piston movement like rusty said.

If you're using it as a daily driver. Sell those pistons and get .030 overs would be my advice.
 

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I would probably hone it... buy some Total Seal or Zero Gap rings and run the snot out of it. But it would be more of a weekend cruiser then a daily driver if it were mine. I think if I wanted a daily driver... it wouldn't get domed pistons in the first place.
 

Daniel Black

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The only thing he mentioned was maybe some piston slap from the bottom of the bore as y'all have stated. That's not an uncommon noise to hear in a 289 or a 260 but I know that was part of the design of a 302 block was longer skirts. If it seals up good at the rings then a little noise doesnt bother me, I'm probably gonna run a solid lift cam anyway. It is a daily driver but in my defense it's only a 3 mile round trip to work and back so even if it only lasted 10,000 miles that could take decades. I picked up the pistons for $50 because they were standard bore. I have a set of '70 Widsor heads I want to use and it works out to 10-10.5:1 with the larger chambers. I have a nice set of 289 heads but they'd end up around 12.5:1 and that is too much for a street motor. I'm super cheap and I hate paying for parts I already have.
 

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I have a nice set of 289 heads but they'd end up around 12.5:1 and that is too much for a street motor.
Can't double up the head gaskets?
 

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Or drill some holes in the pistons to relieve compression...
 

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12.5 and propane go really well together.
Just saying.
 

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I've been a UAW trained automotive machinist since 2000, and a salvage yard lurker since 1990. There are plenty of good EFI 302s in the yards that still show the original honing cross-hatch in the notes. Those are fine to hone and run without boring.
Can't double up the head gaskets?
The loss of quench would cause more detonation than the compression reduction would relieve. Just get larger-chamber heads, or sell the pistons to pay for some better heads.
Or drill some holes in the pistons to relieve compression...
That's all but guaranteed to result in cracked pistons.
 

Dirtman

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So friggin big!
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Cracks can reduce compression as well... its win win.
 


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