Piston-To-Bore Clearance

[1993_VG30E_GXE]

The FSM states the limit to be 0.0006" to 0.0014" for clearance. If the after my measurements (after honing), the clearance is slightly off, because of piston skirt wear, am I able to just replace the suspect piston with replacement standard size piston, or do I have to re-bore and replace ALL pistons?

What is one to do, to rectify the situation. The machinist who did the hot tanking and honing told me the clearance is 4 thou, and the limit is 2 thou. I'm going to do the actual measurements at home myself tonight.

 

[Johnny248]

I think it depends on what you are doing. Last time I have an engine bored, I had it bored to the set specs with the set of new pistons I supplied the machinist...

 

[XS650]

0.0006" to 0.0014" clearance sounds like new clearance with cast pistons. The manual should have also have wear limits listed. A little looser would be OK since it sounds like you are using used pistons.

Sounds like your "machinist" is giving you some old rule of thumb numbers that don't belong in a modern engine. 0.004 is out of line for a modern car engine with cast pistons.

 

[tom slick]

can you buy just one piston for this engine?
If you can then it really depend on if the bore is to spec for the new piston.

 

[Kruse]

I know things don't always work out like you want them to.
But the block should have gone to the machine shop and then you find out if you can reuse the old pistons. Buying all your parts before you take it to the machine shop can come back to bite you, money-wise. This train of thought does not apply if you know you are overboring your whole setup.
Getting back to your original question, if you can find a single oversized piston, go for it. There should be no weight difference between a standard and an oversize piston, but in a perfect world you would want your whole assembly balanced.

 

[ARCOgraphite]

You can lighten the piston, there are usually pads on the underside for weight management and balancing.

 

[Trav]

Originally Posted By: 1993_VG30E_GXE
The FSM states the limit to be 0.0006" to 0.0014" for clearance. If the after my measurements (after honing), the clearance is slightly off, because of piston skirt wear, am I able to just replace the suspect piston with replacement standard size piston, or do I have to re-bore and replace ALL pistons?

What is one to do, to rectify the situation. The machinist who did the hot tanking and honing told me the clearance is 4 thou, and the limit is 2 thou. I'm going to do the actual measurements at home myself tonight.


If the bore is within specs (pay particular attention to bore taper) then just replace the worn piston.Check the clearance with another original size piston.
When all bores are within specs there is no need to re bore the block.If the block is only honed and not bored Iron rings seem to seat easier.

 

[bonnie john]

make sure the original pistons are not of a "select grade"

 

[BuickGN]

Depends on whether they're cast (or hyper) or forged, the type of use (nitrous, blower, NA), and bore size.

My GN is setup quite a bit looser (to JE specs) in the piston to cylider clearance due to the forged JEs and 30psi of boost creates lots of heat. I also have a little cold piston slap as the techs at JE told me I would have.

Honestly, I think the worst thing you might end up with is a little cold piston slap but why not have everything perfect from the beginning?

 

[1993_VG30E_GXE]

Thanks guys, I got the block, crank, and bearing cap back, and it turns out in the end he didn't even have caustic, even though he told me he did in the beginning. I'm going to re-measure the bores and pistons tonight and post the results, and I'd really like your opinions on what my next move should be after I post the results. I need your help at this point, and consider that I already bought the STANDARD size piston ring set (I am able to return it though because it's un-opened), and everything in my engine is OEM original right now (400K KM).

 

[BuickGN]

Originally Posted By: 1993_VG30E_GXE
Thanks guys, I got the block, crank, and bearing cap back, and it turns out in the end he didn't even have caustic, even though he told me he did in the beginning. I'm going to re-measure the bores and pistons tonight and post the results, and I'd really like your opinions on what my next move should be after I post the results. I need your help at this point, and consider that I already bought the STANDARD size piston ring set (I am able to return it though because it's un-opened), and everything in my engine is OEM original right now (400K KM).


Just a word of advice and I'm sure you already know this but make sure the pistons and block are at the same temp when measuring. I've seen guys do the pistons inside or leave the block in the sun and then measure the bores.

At 400KM, you might want to think about a set of stock replacement pistons. They will fatigue over time and while 400KM is no where near enough, if you do another 400, you may be too close to the limit. Have you measured the ring lands?

 

[Trav]

More than 0.04 mm should be considered the max for bore taper,less than 0.03 mm is ok.
Anything more than that consider a mild re bore with new pistons.

In any case remember to check the ring end gap.Use a small inexpensive hand crank ring grinder,Your stock rings are probably
gas-nitrided rings and can be easily tweaked with this kind of tool.
Find out exactly what the ring material is as different materials require a different hone finish,to fine and they will glaze,to rough and they will scrub the bores excessively.

You probably already know this..
An easy way to check bore taper when a telescoping bore gauge is unavailable is to use a ring in the bore at various depths until the bottom of the bore is reached,use a piston with no rings to push it down and keep it square,measure the ring end gap at each spot with a feeler gauge noting the difference.
http://www.carshopinc.com/product_info.php/products_id/59235/CSPRF

 

[1993_VG30E_GXE]

CYLINDER BORE DIAMETER (POST HONED)
OEM Standard I.D.: OEM Wear Limit: 0.0079"
GRADE 1 3.4252" to 3.4256" + Wear Limit = 3.4335"
GRADE 2 3.4256" to 3.4260" + Wear Limit = 3.4339"
GRADE 3 3.4260" tp 3.4264" + Wear Limit = 3.4343"

Result:
#1 GRADE 3 3.4264" PASS
#2 GRADE 2 3.4260" PASS
#3 GRADE 2 3.4260" PASS
#4 GRADE 2 3.4260" PASS
#5 GRADE 2 3.4258" PASS
#6 GRADE 2 3.4260" PASS

PISTON SKIRT DIAMETER "A"
OEM Standard:
GRADE 1 3.4238" to 3.4242"
GRADE 2 3.4242" to 3.4246"
GRADE 3 3.4246" to 3.4250"

Result:
#1 GRADE 3 3.4247" PASS
#2 GRADE 2 3.4245" PASS
#3 GRADE 2 3.4248" PASS
#4 GRADE 2 3.4246" PASS
#5 GRADE 2 3.4244" PASS
#6 GRADE 2 3.4246" PASS

PISTON-TO-BORE CLEARANCE "B" (CYLINDER BORE DIAMETER - PISTON SKIRT DIAMETER)
OEM Limit: 0.0006" to 0.0014"

Result:
#1 GRADE 3 0.0017" FAIL OVER LIMIT
#2 GRADE 2 0.0015" FAIL OVER LIMIT
#3 GRADE 2 0.0012" PASS @ LIMIT
#4 GRADE 2 0.0014" PASS @ LIMIT
#5 GRADE 2 0.0014" PASS @ LIMIT
#6 GRADE 2 0.0014" PASS @ LIMIT

 

[Shannow]

You'll be fine...

or swap #3 & #1 piston, and be a little more fine.

 

[andyd]

Shannow X2. another consideration is having the barely out of spec pistons knurled and swaged.

 

[mechtech2]

Bore measurements may look good, but it depends on the surface finish.
A rough bore may measure OK, but will wear down FAST to a loose fit.
Proper surface finish is tough to do. It is not easy to match what the factories can do. They are very good nowadays.
And also double check the taper. Get the bore straight with a good finish, and then size the pistons.

 

[1993_VG30E_GXE]

OK thanks guys, I'll do the taper measurements with a 0.0001 Bore gauge during the week. I can confirm that the top ring is chrome type. What's the surface finish desired for that type?

As far as the wear limit for tape, did you mix up the graduation Trav? Isn't the maximum not to exceed 0.004". The Maxima service manual states maximum limit as 0.0006" - is that limit a bit excessive and overkill, and just for the BRAND NEW engine from the factory? My text book says not to exceed 0.004".

 

[BuickGN]

Originally Posted By: mechtech2
Bore measurements may look good, but it depends on the surface finish.
A rough bore may measure OK, but will wear down FAST to a loose fit.
Proper surface finish is tough to do. It is not easy to match what the factories can do. They are very good nowadays.
And also double check the taper. Get the bore straight with a good finish, and then size the pistons.


I learned this the hard way.

To the OP, unless money is a really big issue, why reuse the pistons? I'm sure everything will be fine as is but I just hate reusing high mileage pistons. You can get some nice forged pistons from SRP for $400, maybe less. Then you're free to use any size ring you want or bump or lower the compression all at no extra cost.

What are the ringlands like?

 

[Trav]

Sorry i was looking at the wrong spec (piston to bore) this is what it should be for max Cylinder bore taper - 0.006 in 0.015 mm. Go by your service manual specs.
This may be of interest to you as far as bore finish goes.

http://www.enginebuildermag.com/Article/2520/_piston_rings_and_surface_finish.aspx

 

[mechtech2]

I want to add that a little taper is OK.
Most bores have a bit of taper after thousands of miles anyway - they work perfectly and it is normal.
So if you can't get it perfect, a little taper is not a deal killer. [Little is .001" or so new.]