Piston-To-Bore Clearance

[1993_VG30E_GXE]

Originally Posted By: 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


Trav did you type an error again? The max bore taper in the service manual is 0.0006" / 0.15mm - that's seems INSANELY small. Yet the text book I have says anything less than 4 thou is OK. - that's a massive difference. We are talking 6 tenths here. What should I go by when I go to the machine shop tommorow to use his bore gauge.

By the way I already measured EACH piston skirt dia at the proper spot. I don't have to bring the piston skirt do I- to set the bore gauge? Can one just manually set the bore gauge by using a micrometer and my printed specs that I measured and know were correct?

 

[mechtech2]

When a factory bores and finishes a cylinder, there had better not be any taper. You'd almost have to try and get taper with modern manufacturing methods.
This is why the new spec is so tight.

 

[1993_VG30E_GXE]

OK That's cool Mechtech - so I can go by the 0.004" Limit when I see his bore gauge moving.

More importantly because I did the bore I.D. with snap gauges, I should re-measure each of those. I don't trust the snap gauge reading - I was getting all sorts of skew.

 

[1993_VG30E_GXE]

I'm trying to think what on earth then could have been causing the blow by to coat the domes on the bottom of the cylinder heads, and why the exhaust valves were crusted black, plus my spark plugs were fouled with black gummy stuff all the time.

 

[andyd]

Rich and cold running would cause just that. What kind of driving put on the miles that were on it when you bought it? Did you take a compression test before tearing the engine down?

 

[Trav]

My mistake i forgot one of the zeros in 0.0006.This is what i have in one of the service manuals. Cylinder Bore Taper Service - 0.015 mm 0.0006 - 0.020 mm 0.0008 max.If your book is 0.0006 as the maximum i would stay with that spec.

 

[1993_VG30E_GXE]

ANSWERS:

Andy: City driving in Canada - ie. Severe Service Maintenance Schedule. Also, the O2 sensor was broken for a long time and I had a hole in the exhaust header pipe (Down Pipe), and the car was running insanely rich burning tons of fuel for some reason. I did a leak down test before engine removal - but not compression test. The Leak Down Test results were as follows:

#1 4%
#2 1%
#3 1%
#4 0%
#5 4%
#6 1%

Tom: I think I can buy just 1 piston MAYBE, but it would be from the dealer and it would be a rip. The price for 1 would likely be for the all of them after-market.

Buick GN: I can't check the ring lands yet until I get the new rings being shipped. I threw out the old ones, plus the old ones will give a false reading anyways.

I want to avoid buying new pistons because of the expense. This overhaul is costing MAJOR dollars at this point - also keep in mind I'm replacing at the SAME TIME as the overhaul: Steering Rack, Tie Rod Ends, Fuel Lines to Tank, Brake Lines ALL even in engine bay, Rear Calipers, Welding Rust Holes, Broken Suspension Part, it keeps climbing, and now that I have to pay the machinist to remove that stud too.

RE: Setting the Dial Bore Gauge - Can someone answer this question:

I don't have to bring the piston skirt do I- to set the bore gauge? Can one just manually set the bore gauge by using a micrometer and my printed specs that I measured and KNOW were correct?

 

[andyd]

I'm not an expert on leak down stats. Isnt anything less than 10% considered OK? I would have tried performing a hot soak or maybe just water to shake the carbon loose from the valves and then some sorta flush/hot soak to free up the rings. Also take some steps to cure the rich/cold runnings that caused this. I never would have torn this motor down. I admire your energy, but sometimes less is more.
Too late for that, re-ring the pistons, maybe replace the main and rod shells, seal it up and drive on.

 

[1993_VG30E_GXE]

The engine had to be transfered to the stand for me to repair the exhaust manifolds - that's when I decided to do the re-ring and bearings. As well as every other inspection while I had the motor out, as well as the steering rack and plumbing underneath, as the empty engine bay gave access.

As well, the LH cylinder head was making a horrible noise so I am replacing the camshaft and lifters on that side.

 

[BuickGN]

Sounds like a fun project. I've been there before and ended up putting more money into it than the car was worth.

I may have missed the original post but it sounds like you're just freshening it up, not building for performance, right?

Those leakdown numbers are very impressive. I almost have to wonder how accurate. I never had better than 4% on a newly broken in engine until I went with Total Seal gapless rings and hit 1-2% but that may just be the difference in cast vs loosely fitted forged pistons.

 

[1993_VG30E_GXE]

Hi Buick - your right, just freshening it up and I wanted to do the fun job myself this summer as I have A LOT of enjoyment doing this stuff. It's a hobby really, not a job. I just do it at home.

Car is moderately high mileage - 400K KM but pretty old 16 yrs, and your totally right, I've put in way more money in parts than the worth of the car, but the car is just a vessel to fuel my own fun, ie. repairs.

Me too, I was wondering how accurate. It's made by Milton.

I do want to ask you though, I know when doing the Leakdown that I'm supposed to do it at TDC on the compression stroke (both valves closed), where as it's possible to do wrong on the 'between the intake and exhaust strokes'.

Can you explain this part to me.

When I looked at the rotor pointing to the numbers, that's the firing order right? Then I stuck a long screwdriver into the spark plug hole and as I rotated the crank bolt clockwise, I would see the screwdriver raise or lower. It would typically raise to it's highest point close to the rotor # for that cylinder sometime BEFORE pointing at the number. But then, I notice as the rotor turned around to do 360 degrees, it would raise AGAIN (the screwdriver)a second time, before it even got back towards the number. How do I know which raising of the screwdriver is the right one to stick in my leakdown tester in that spark plug hole?

Also, if I did it wrong, do you think that is how I would get those erroneously low % leakage readings?

I always did the test at the raising of the screwdriver that was closest to the rotor/distributor pointing at the #, usually before or close to on or slightly after, I can't remember.

 

[BuickGN]

If you had it at the wrong place, it would instantly go to 100% leakdown. It takes next to nothing to make it go insanely high so I'm sure you're doing it right. I actually try it with the piston halfway down the power stroke and at the top of compression. It's just my OCD, I've never heard anyone recommend to do it halfway up but I was hoping it may show bore taper. Of course, I'm not sure if a little taper would even make a difference in a leakdown.

 

[mechtech2]

You can get a flash reading with a leakdown tester that is very good if there is a lot of oil in the cylinders - same as with a normal compression tester.
STill, zero with used valves/seats, and worn rings/bores/lands is tough to do.
I assume gauge error is the reason. Possibly line pressure or operator error.