K321 Cylinder Bore/Piston Clearance

[nutzboutoldies]

I just got my block back from the machine shop along with the new .030 piston. I measured the new piston with an outside micrometer (3.521) and and the cylinder bore with a telescope gauge and an inside micrometer (3.536) with a difference between the piston and bore of .015. I know the telescope gauge isn't the best way to check the bore but between that an the inside mic I think I'm close. I also checked the ring gap and found that to be .035. If I'm reading the service manual correctly for a A type piston, which I have, the clearance should be .007/.010. If my measurements are correct the cylinder is at least .005 to large. Should I assume the shop didn't bore the cylinder correctly and take it back for them to verify? If so should I be looking for another block or maybe a sleeve?

[R Bedell]

When you had the block bored, did you send along the Piston so they could use that as a "bench mark"...??

[J-Mech]

You measure skirt clearance with a ribbon gauge. The way you did it will not be accurate. You can order a ribbon gauge, or you can use a feeler gauge. Only difference between the two is a ribbon gauge is about 12" long. Piston clearance should be .0035"-.007" on a new bore. I like mine about .005"-.006". .007"-.010" is too wide.

[TNTs CC]

Originally Posted by nutzboutoldies

I just got my block back from the machine shop along with the new .030 piston. I measured the new piston with an outside micrometer (3.521) and and the cylinder bore with a telescope gauge and an inside micrometer (3.536) with a difference between the piston and bore of .015. I know the telescope gauge isn't the best way to check the bore but between that an the inside mic I think I'm close. I also checked the ring gap and found that to be .035. If I'm reading the service manual correctly for a A type piston, which I have, the clearance should be .007/.010. If my measurements are correct the cylinder is at least .005 to large. Should I assume the shop didn't bore the cylinder correctly and take it back for them to verify? If so should I be looking for another block or maybe a sleeve?

Where on the new piston did you measure? Pistons are tapered the top is smaller to allow for expansion. There is a specific height from up from the bottom of the skirt where you need to measure at.


I usually send the new piston in so they can match it properly for the correct clearance and then check it with a feeler gauge after the fact.

[J-Mech]

Originally Posted by TNTs CC

I usually send the new piston in so they can match it properly for the correct clearance and then check it with a feeler gauge after the fact.

A good machine shop won't bore a cylinder without a piston. It's not really an option to get it bored without the shop having it.

You should always double check your machinist. (Just making that statement in general, not to anyone specific.)

[TNTs CC]

Originally Posted by J-Mech

A good machine shop won't bore a cylinder without a piston. It's not really an option to get it bored without the shop having it.

You should always double check your machinist. (Just making that statement in general, not to anyone specific.)

Very true, I wouldn't trust a shop that didn't.


Mine will rough bore a damaged cylinder just to determine what piston size is required. Once that's done pistons are required and then they finish up boring and honing with a torque plate on the engine.

I'm usually doing Mopar V-8 builds. I have a 360 small block that's getting a 4" stroker crank build coming up for my car. A 408 stoker engine is a great driver, my last build was a 410 that had 488hp with 520# of torque that would run on 87 octane pump gas.

[nutzboutoldies]

Originally Posted by R Bedell

When you had the block bored, did you send along the Piston so they could use that as a "bench mark"...??

The shop that bored the block furnished the piston and the gasket kit. So I would think he didn't bore the block until he had the piston in hand.

[nutzboutoldies]

Originally Posted by TNTs CC

Where on the new piston did you measure? Pistons are tapered the top is smaller to allow for expansion. There is a specific height from up from the bottom of the skirt where you need to measure at.


I usually send the new piston in so they can match it properly for the correct clearance and then check it with a feeler gauge after the fact.

The service manual indicates to measure "Thrust Face to Bore Clearance" an the Type A piston just below the oil ring groove. Is this not the correct location?

[nutzboutoldies]

Originally Posted by J-Mech

You measure skirt clearance with a ribbon gauge. The way you did it will not be accurate. You can order a ribbon gauge, or you can use a feeler gauge. Only difference between the two is a ribbon gauge is about 12" long. Piston clearance should be .0035"-.007" on a new bore. I like mine about .005"-.006". .007"-.010" is too wide.

I'll try to check it with a feeler gauge and see what that shows. In the mean time I'll get a couple of ribbon gauges. How far down do you slide the gauge?

So are ribbon gauges the preferred method for checking clearance or would a bore gauge be and why are telescope gauges not accurate?

[cooperino]

The caliper I use has 4 points of contact and a dial gauge. I've seen some telescoping ones that do not use a gauge and only 2 points of contact. Which style do you have?