Strip-down - Bore and piston measurement checks.

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Strip-down - Bore and piston measurement checks.

Post by Guy Croft » August 4th, 2006, 12:58 pm

Here a sequence, fairly self explanatory, about the checks I do when stripping down an engine (in this case an Evo 1 Integrale 16v) to ascertain the true dimensional condition and 'fit for duty' of bores, pistons and rings.

The bores vary in size during machining at the factory (from any manufacturer) in this case from class A 84.00mm bore (Class A smallest piston) in 0.01mm steps per grade up to Class E 84.05mm bore (Class E biggest piston) and they used (still do to this day) differing size pistons to match the piston skirt - bore fit, which in this case is 0.05mm - 2 thou" or 0.002". On the skirt of the crankcase where the sump bolts on the bore size classes will be letter stamped, A thru E.

OE factory TC supplied pistons for all 84mm bore models were size classed to suit these bores according to the 'across-skirt' measurement at the gauge height (see below):

Piston sizes
Class A Piston is 83.93-83.94mm
Class C Piston is 83.95-83.96mm
Class E Piston is 83.97-83.98mm

with the other grades B, D in between.

In the case of this Integrale unit the bores were, when new:
1. Class C (84.02mm bore)
2. Class C
3. Class B (84.01mm bore)
4. Class C

You can see from the photos below that the bores are way over those limits now. Now they are:
1. 84.04-84.06mm
2. 84.04-84.05mm
3. 84.04-84.06mm
4. 84.05 -84.07mm

I measured the pistons and got:

No1 83.95mm - would be OK for the old Class C bore.
No2 83.96mm - would be OK for the old Class C bore
No3 83.965mm - Not right even for the original Class B bore - should be 83.94 -83.95mm. Too big, Fiat don't make mistakes, most likely been swapped from another engine by the seller.
No4 83.93mm - That's a Class A piston and again, doesn't originally belong in a Class C bore.

If you want a proper evaluation of 'fit for duty' you cannot guess the history of a used block, you have the right measuring equipment. Otherwise you run the risk of high ring gaps and loose skirt-bore which can crack the skirts. There is no significant wear-ridge at the tops of the bores so some wise-guy in the 'trade', I know who it is, has been up and down the bores with a hone and 'passed it off' as a rebuild, which is the kind of cheating thing I see often.

In summary the skirt to bore clearances are currently (at strip-down):

1. 0.11mm ( about 4.5 thou")
2. 0.09mm (just under 4 thou")
3. 0.095mm
4. 0.14 (just under 6 thou")

All these pistons would crack their skirts in service!

The piston ring end gaps were so big I did not bother measuring, see below, nearly 1.5mm, should be about 0.45mm. This is a combination of very badly worn rings (see photos) and bores too big. NOT FIT FOR DUTY! If you like to run engines like this never phone me!

The rings are worn both radially and vertically - huge ridge in upper face where the ring bangs against the top land at tdc, not easy to see in the photo because it is so highly polished, usually typical of very high mileage/poorly maintained oil. But - add in the scoring and scuffing and impact damage around the top land and on the skirts in a vertical pattern and the razor sharp ring edge condition and we have all the sure signs of bore washing from over-fuelling from, yes you guessed it - ECU fuelling chipped and generally messed with.

So we have a block with massively oversized bores, pistons not matched for size, rings worn out all over and way over size on end gap and the whole thing basically a pile of junk.

(To add insult to injury, the crank had been badly reground and was already tearing the new bearing to bits, a replacement rod had been fitted where it had run a bearing, there was no attempt to match rod weights, one balance shaft and the oil pump were seized, the crank had not even been unplugged and cleaned. Madness. And that's just the BLOCK!)
0 017.jpg
Measuring the piston skirt at the gauge-height distance, about 1/2" -1" skirt tip, you have to swing the micrometer to find the true width, piston skirts are elliptical (top view) and barrel shaped (side view).
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No 4 is the worst size, 84.05-84.07mm diameter, huge ring gap and piston-skirt clearance of 0.14mm - needs only 0.05mm!
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Bore gauge is calibrated against a micrometer and set at a nominal, say, 85mm size. With the gauge in the bore, smallest reading is true diameter, hence the +/- arrows on the dial to tell me the deviance over/under set size.
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0 014.jpg
Piston in bore where it would see max cylinder pressure, about 16 deg ATDC, huge ring end gap. I build to 14-16 thou"...This engine would be a 'pumper' with no power at all. If the gaps are too big the rings won't 'choke' and seal, ever.
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New engine and well maintained one you're going to get 1.5 - 2 thou" side clearance, we have 6 here and it's NOT the worst. The ring and groove are battered to bits.
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