Cylinder Bore Taper

[Westriver87]

What bore taper is within spec on these blocks? The machine shop just checked my bore and all cylinders have a taper between 0.002 and 0.003. Its got a forged steel crank with 3 billet caps. I need to get my rods and pistons. I don't want to bore it if it doesn't need it. So what is within spec? Thanks for the help

 

[Ttype6]

What bore taper is within spec on these blocks? The machine shop just checked my bore and all cylinders have a taper between 0.002 and 0.003. Its got a forged steel crank with 3 billet caps. I need to get my rods and pistons. I don't want to bore it if it doesn't need it. So what is within spec? Thanks for the help

Are you going from a stock piston to a forged?

 

[Westriver87]

Yes, stock rotating assembly to a forged crank, billet steel rods, and forged pistons...

 

[chopped39]

Do you have the new pistons yet? Mine was the same. The shop went .020 over with the block. He would not do it without having the pistons there. He measured each piston and then matched each to their respective cylinders

 

[thadeal4real]

I'm curious why taper is a concern at this point as ttype6 beat me to

 

[Westriver87]

I don't have the pistons yet. I was waiting on some feedback to determine whether to bore or not. My machinist said if it was his he would run it. He is well respected and I trust him, but I would like to hear from others here. However, if it is bored, he would need the pistons before he honed it.


Posted from the TurboBuick.Com mobile app

 

[Ttype6]

The new pistons will require more clearance than the stock ones. Honing to that new clearance should get rid of the taper. If it doesn't,no big deal as that is an insignificant amount.

 

[dr_frankenstein]

Alot of old honing machines and boring machines have a tendency to leave a slight taper at the bottom of the bore. This is where a good machinest who has had time on his equipment pays off.

 

[ek02]

The big question is how much wear is there on the cylinder walls? Ask your machinist what the size of the bore with the most wear is, and then ask the piston company the diameter of their pistons, and what the recommended piston to wall clearance is. Then you and your machinist can determine if the recommended clearance can be attained by honing to size. If you have to go .020, that still leaves enough for another rebuild.

 

[Westriver87]

Do any of the TB vendors sell a standard JE piston? I'm not seeing none on the sites. Several carry the std. Diamonds but not the JE's. I will get that info from my machinist and get a diameter on a std. Diamond and JE and see if it will work. Is one preferred over the other?


Posted from the TurboBuick.Com mobile app

 

[Nick Micale]

Since a forged piston is installed with less clearance than the stock cast piston, you will not have a proper fit unless it is bored.

Why the hell would you spend lots of $$$$ on this rebuild, and cheap out on a bore job?

If a machinist said he would "run it", I would find another shop. This is a build that will more than double the original HP, and you do not want to cut corners.

I know you will do want you want, but the idea other that members might get is installing new pistons in a standard bore block is a good idea and would expect to get a long life out of their build, and that is not right.

 

[earlbrown]

If you hav .003" of taper you have no choice but to go oversize. Depending on the actual exact shape of your bore, you can get away with honing them back round and running some .005" or .010" overs

 

[Westriver87]

I'm not cheaping out on anything. I just didn't want to bore a block that didn't need it. I just want it to be right. That's the reason I posted. If it needs bored I will bore it no big deal. It is a lot of money, that's why I ask questions and find out as much as I can. Thanks for the replys.


Posted from the TurboBuick.Com mobile app

 

[ek02]

The big question is how much wear is there on the cylinder walls? Ask your machinist what the size of the bore with the most wear is, and then ask the piston company the diameter of their pistons, and what the recommended piston to wall clearance is. Then you and your machinist can determine if the recommended clearance can be attained by honing to size. If you have to go .020, that still leaves enough for another rebuild.

Stock piston to wall clearance is .001-.003. JE recommends .0045 on their coated forged pistons, or more depending on application. I have used a stock bore with custom stroker pistons made .002 larger than stock, and then honed the block to clearance. That would cost more than new off the shelf forged standard stroke pistons and the machine work. Go with the .020 overbore and you will have no worrys. Good luck with the build.

 

[Ttype6]

The new pistons will require more clearance than the stock ones. Honing to that new clearance should get rid of the taper. If it doesn't,no big deal as that is an insignificant amount.

I see that I misread the amount of taper. My brain turned the posted .002" - .003" int0 .0002" - .0003". You have too much taper. As ek02 said you'll need to find out the diameter of standard sized forged replacements and do the math. My guess is that it won't work,but you have to check. If it won't,you buy a custom piston or go to the next available over sized off the shelf piston.

 

[turbodave231]

I would not recommend "running it " with that much taper. You won't be happy with the results. Blow by will be high and ring seal will be poor. The PCV system will not be able to keep up with the extra air entering the crankcase. You will leak oil out of every possible leak point and blow the dipstick out of the dipstick tube.

More than .001-.0015" of taper or out of round should be corrected. You could consider having the cylinders honed (with a rigid stone hone) or bored round with a boring bar. Dingle ball hones are not really hones (garbage) and will not correct taper or out of round.

The majority of wear will be at the top inch of the cylinder. Most competent machinists will tell you that .002 to .003" is way to much taper for a performance application.

Think about what is happening to the rings and ring gap as the piston travels up and down the bore. The ring gap expands at the top of the cylinder and contracts at the bottom of the cylinder. The ring is being worked back and forth every time the piston goes from the top to the bottom of the cylinder. Your rings will loose tension and will not seal well over the long term. What happens when you bend metal back and forth? It gets fatigued!


When a freshly bored cylinder is properly honed with a torque plate, the taper should be less than .0002 and the out of round should not be measurable.

You also did not mention how much the bores were out of round (also highly likely if you have that much taper). New rings should be perfectly round, right? How well will a new round ring seal in a bore that it out of round? (square peg in a round hole)

I guess you can figure out where I stand........proceed with caution and good luck!!
Dave

 

[Westriver87]

We are going to go ahead and bore it .020 over and start fresh. Like, I said, I just want it to be right. Just trying to get as much info as I can. On another note, does anyone have any experience with the Molnar rods?

 

[chopped39]

We are going to go ahead and bore it .020 over and start fresh. Like, I said, I just want it to be right. Just trying to get as much info as I can. On another note, does anyone have any experience with the Molnar rods?

They look like k1 rods. Finished in the USA means they are forged overseas like all the others . A lot of the overseas products have come a long way. The metals they are using are way better than what they were using 20 years ago. The price is right about the same as all the others.I would not be afraid to buy them.

 

[turbodave231]

I bought a set of Molnar stock length rods for a current build. They look nice but I had to do more work on (weight matching) balancing them than I expected. The big end weights varied by more than a couple of grams.

Dave

 

[750H.P.V6]

Since a forged piston is installed with less clearance than the stock cast piston, you will not have a proper fit unless it is bored.

Why the hell would you spend lots of $$$$ on this rebuild, and cheap out on a bore job?

If a machinist said he would "run it", I would find another shop. This is a build that will more than double the original HP, and you do not want to cut corners.

I know you will do want you want, but the idea other that members might get is installing new pistons in a standard bore block is a good idea and would expect to get a long life out of their build, and that is not right.

Forged pistons need more clearance due to the greater expansion rate based on the density of the forging itself vs a cast piston. FWIW I'd bore the worst cylinder and see what it cleans up at then proceed with ordering pistons from there.

Neal