Cyl Bore - Piston Clearance

[3d914]

The last step in checking out my 3.2 motor before rehoning the cylinders and getting new rings is to measure the pistons. SO I measure the pistons at the pin centerline, 90 deg off the pin - but this just isn't jiving. Now forgive me cause I may over-thinking this but here goes.

The piston to bore clearance is spec'd to be .0012 - .002". With a service limit of .003". Now the out of round limit is .0008" which eats up most of the low side of the .0012 clearance. Are these limits for real - I mean is anybody seeing these number on used pistons and cyls?

Here's what I'm getting for differences in bore & piston:

1. 0.0095"
2. 0.0110"
3. 0.0107"
4. 0.0136"
5. 0.0080"
6. 0.0073'

The best ones are more than double the serviceable limit. And of course after honing these numbers will be slightly larger. So does that mean I'm buying new pistons along with rings?

 

[probe_racer]

hmmm i don't know for sure but i think once the actual difference between the piston diamater and bore measurements is calculated, then that number needs to be divided by 2 so u get the actual gap measurement.

I will check my repair manuals and will see if there is anything different there.

 

[msteiny]

Been pondering this a couple hours. For ***** and grins, throw a ring in and see what you get for a gap. If you get over, I think its 0.018 then you might have issues. Normally a person would have to file down the rings to get the spec. If the new rings are out of spec when you put them in the you might want to consider something else. But see what they look like, one thing you can rule out anyway.

 

[3d914]

Been pondering this a couple hours. For ***** and grins, throw a ring in and see what you get for a gap. If you get over, I think its 0.018 then you might have issues. Normally a person would have to file down the rings to get the spec. If the new rings are out of spec when you put them in the you might want to consider something else. But see what they look like, one thing you can rule out anyway.

Haven't got the new rings yet, but here's what I got using one of the used rings - the ring gap on Cyls 1-3:

• Cyl #1/Ring #1 = .021
• Cyl #1/Ring #2 = .026
• Cyl #2/Ring #1 = .023
• Cyl #2/Ring #2 = .029
• Cyl #3/Ring #1 = .028
• Cyl #3/Ring #2 = ? Broke removing it

I then went back and did all cyls to check the taper. The taper service limit for 3.0/3.2 is 0.0008in

On my 3.2 I got the following using the ring gap method:
Note - I'm dividing the gap difference by 3.1416 to get the diameter distance since the gap is measured on the circumference.

• Cyl-1: (.0005/3.1416) = .00016in
• Cyl-2 thru 6: (.001/3.1416) = .00032in

 

[rubydist]

so are the bores over spec or the pistons under spec?

 

[3d914]

Based on the specs in the first post - its the pistons that are significantly under. I suspect I'm just avoiding the obvious - these pistons are NOT going to work. Even if I compare them to the nominal bore dia (3.622), they're still way off.

1. 3.6138 (.0082 dif)
2. 3.6110 (.0110 dif)
3. 3.6120 (.0100 dif)
4. 3.6096 (.0124 dif)
5. 3.6150 (.0068 dif)
6. 3.6154 (.0066 dif)

OK, so it looks like I need a set of new or used pistons, but they seem difficult to find for this motor unless I want to spend $500-1000 - which I don't.

Any other options, sources?

TIA

 

[zak]

How many miles were on this donar engine? Picking up a low mile 3.2 might be easier at this point (they are inexpensive in the northeast, not sure about the southwest).

 

[jimtash]

Been pondering this a couple hours. For ***** and grins, throw a ring in and see what you get for a gap. If you get over, I think its 0.018 then you might have issues. Normally a person would have to file down the rings to get the spec. If the new rings are out of spec when you put them in the you might want to consider something else. But see what they look like, one thing you can rule out anyway.

The last block I tore down, the factory rings gave had gaps that were huge, around .002. New rings were under and had to be filed.

 

[jimtash]

I saw your ad in the WTB section. I've got two 3.0L blocks sitting around with the matching main caps. One is cleaned up, oiled and a good candidate for an overbore. The other hasn't been cleaned up yet but there are no visible issues with it.

Probably the best route for you to take and you can match the pistons to it.