Technical Engine bored over?

So I'm rebuilding a sbc 4 bolt main, I'm just throwing new pistions, rings, bearings, and oil pump. My pistions don't have markings that say it's bored over (I heard some used ink) I ordered a mic because I don't have a 4" but I have some standard bore rings that I threw in there. The gap is 25 thousands. Yeah. So does this indicate that it has infact been bored over. I know for a fact the engine has been dismantled because the rods have been stamped. And the top part of the cylinder doesn't have a ton of ware on it maybe .005 or somewhere around there


Thank you

 

If the motor truly does have .005 taper you are about at my max for a rering job

 

you need to get someone with the proper measuring equipment to come over and check things out. guessing and estimating usually doesn't work out well. i like to measure the bore 2-3 directions both top and bottom of the bore

while you are at it you might as well measure the crankshaft mains and rod journals

 

How much taper you can tolerate, depends mostly on how long you want it to last.

But as for ring gap vs. overbore: keep in mind that the circumferance (the "length" of the piston ring) is a bit over 3 times the diameter. So if the top of the bore were worn .005", the ring gap would increase about 0.015" above normal. If the rings have .015" gap at the bottom of the bore, and .025" at the top, there's only about .003" taper, which is not too bad.

A dial bore gage would be handy. But I can usually tell if a block has been overbored by using a tape measure...if you can't see 1/32" inch on a ruler with your eyes, something's wrong.

 

Connor
Step away from the engine and get this book!
You don't even need to buy a new one but you need this book.

https://www.google.com/search?q=how...ECBAQAw&biw=1280&bih=752#imgrc=VaRKVTEpIigN8M:

 

Most over sized pistons are marked on the tops but there may be some that aren't.
A 20 dollar digital caliper from Amazon or Harbor Freight ( I think they have one on coupon this month should be accurate enough to check the bore and check the piston and even the crank journals with Probably not as perfect as a Micrometer but years of teaching students to use micrometers showed that three people can measure the same item with the same micrometer and get three different readings. You are looking for Difference large enough that a few hundredths of a thousand isn't an issue.

 

3 out of the last 4 rebored sbc's I have looked at have not had any legible oversize marked on the pistons.

 

Some are marked on the side by the pin boss, newer ones are electro-scribed, but I see less marked on the dome itself.

 

Do they still make radical-ring sets? These were a softer ring that conformed better to tapered and out of round cylinders. Myself, I use telescoping gauges and a micrometer to get a ball park idea of a cylinder. Checking 90* to the crank centerline. If I need something better, I borrow a dial bore gauge.

 

The pistions look pretty stock, there is a mark on the side that just said 17, I have a bore guage but no mic to check it with.

 

Follow what is said in post number four and get back to us on what you find.

 

A cheap dial caliper will get you pretty close.

Sent from my Trimline

 

place a new std ring in the very bottom of the bore measure gap, then place it in the top just below the ring ridge & measure gap the difference is the amount of wear or taper. new pistons in worn cyls is a waste of money.

 

Un marked pistons are getting more common. I bought a Ford 428 short block cheap at a swap meet got it home and found it was .060 over with wear no mark on pistons luckly I paid less than the crank was worth. Taper kills rings and pistons by the ring moveing in and out every stroke wearing ring lands and sides of rings.

 

yup, taper is hard on rings. A straight bore will let rings last well over 100k miles, a tapered bore more like 20k-40k, depending on how much taper there is.

But several engines I've worked on lately only needed to last 5k-10k miles, so the taper remained.

 

If its a 350 you can have it bored and new pistons for around $200. https://www.ebay.com/itm/Speed-Pro-...=item2610101044:g:1yQAAOSw8TJcNhNz:rk:41:pf:0

 

Greetings fellow HAMBers!...it has been my understanding over the years ( things learned from pretty sharp oldtimers!) that bore taper causes the rings to move in and out of the ring grove as the piston reciprocates causing a " pumping " effect and thus oil consumption. What are your thoughts on this? I have no first hand experience as I have bored anything I disassembled and found much bore wear.

 

Get the proper tools to check the engine out. Otherwise, take it to a machine shop and have them check it.
r


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No, it doesn't do that...it mostly just causes faster wear on the bottom of the rings. A few years ago I overhauled a 292 ford engine without reboring it, just put new rings in, and it had a significant amount of taper--at least .005 wear at the top of the bores. Went over 3000 miles on a quart of oil after the overhaul.

 

I see that you are 17 and I hope that you are still in school. Take a piston to school and get it measured there if you have one out of the block. If you are getting .025" end gap with a standard piston ring you have a standard bore. Using a feeler gauge and measuring the gap at the top and bottom of the bore will allow you to find the amount of taper.
Divide the difference by 3.14.
Pete

 

Assume nothing.

If you're assembling this yourself, measure everything you're not 100% sure of. Get a notebook and keep accurate records of your measurements. Don't get in a rush. Might be a good time to get a few ribbon gauges to check piston to bore clearances.

 

Spend $20 and get a dial caliper. Take care of it and it will last for years. And lots and lots of measurements. Telescope gauges are good to.

 

I bought one of those POS Harbor Fright calipers.... forget about those. Mine keeps wanting to switch to metric no matter what I do. But for $9 I don't feel bad about smashing it with a BFH...

 

LOL something is wrong with my eyes. Actually bad wrong now according to my eye doctor. About 15 years ago (give or take) I discovered that I could no longer read the vernier, now I cannot see across the room and just order two pair of glasses. One for driving and one for reading.

I use snap gauges and a mic. I actually have a set of inside mics but they are antiques and I have never checked the accuracy of them. They are cool to look at though. I have also been known to use a piston and feeler gages to give a quick check for taper. It is kind of back yard but it will give you an idea.

Normally you can read the oversize on the top of the piston. Not always. A micrometer in the proper range is your friend.

 

Maybe you missed the part about "dial"? Dials don't switch, only digits do that!

 

Which is why I have a dial caliper. No batteries needed. Can't switch to metric. Didn't come from HF. Be careful to keep chips out of the rack. Like Beaner, I still have my vernier. Can't read it. Bore gauge is nice. But setting it for your size isn't always done right. I have gauge blocks to set mine. But where does it stop?

 

For men like us @RichFox , @squirrel (a longer list for sure) there is no end to the tool list. But we have been collecting for decades. For your average rodder all that is necessary is tools to get you ball parked so that you can go to he machine shop with enough knowledge to chat with your machinist.

 

I use the cheap dial caliper for about 95% of my precision measuring needs....but I have a bore gage, snap gauges, mics, inside mic, depth gauge, etc. for when I need them.

 

Iv got a set of inside mics made by Sarrett. I set the mics at the bore size, double checked with my out side mics, then see if the inside mic will “ turn “ in the cylinder. Then increase the size of the inside mic till it won’t “ turn “ in the cylinder. Usually the size at the bottom of the cylinder is the original, correct size. Standard or oversized. Then how many thousands you have to add to the inside mic,in the top of the cylinder will give you the taper. Kinda like the “ go,no go” gauges. Also if three people mic a shaft and get three different readings at least two are the wrong readings.



Bones

 

“I actually have a set of inside mics but they are antiques and I have never checked the accuracy of them. “
Are those the ones you borrowed from me?
r


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