I've found the cause of the low compression on No 4!

[pkr 0]

So, on removing piston No 4 on my CF1 Twinspark I noticed all was not well between the piston ring recesses.

 

This is what it looks like

 

 

after I'd picked out the loose bits of metal!

 

The bore itself is ok. No scoring and although there's evidence of some wear it doesn't seem too bad. I still need to remove the other 3 pistons to check them ( compression tests are fine ) and measure all the bores thoroughly.

 

What are my options to fix this? Is it possible to buy just one piston or do I need to buy a set of 4? The piston marks indicate that they are Mondial pistons and class 'B' size ( 83.010mm ).

 

As an aside, what's the difference between Mondial and Borgo?

[Smaky 73]

If youclean the crown off then you'll see a 3 digit stamp, it's that you need to match with a replacement, I'd check the head is OK, but looking at that piston I'd say all the bits went south so it should be OK, the cause, I'd guess low oil at some point which allowed the piston to cook and it's just shattered from the stress, I'm surprised there's no scoring as I doubt the piston rings would have been there.

 

I'd be checking the crank for damage, emptying out the sump and striping off the rear section, then at the very least honing the bore and repacing the rod, piston and big ends.

[pkr 0]

Thanks for the response.

 

The broken off bits were still in place. They were far too chunky to fall between the piston skirt and the bore.

 

They can be seen in the background in this photo

 

When I first took out the piston, I was expecting a broken piston ring(s) so I was a little bemused when all looked ok. Then I noticed a small crack between the rings. On removing the rings, I was then able to pick out the two bits of metal. I'm probably lucky they didn't disintegrate futher and cause a greater level of destruction!

 

The only bad thing that's happened to this engine was a number of years ago when the radiator decided to dump all its water on the road when I was on the M1 one night. The weather was foul and the motorway was very busy so I wasn't looking at the instruments too much. First thing I noticed was the temperature in the car fell as the heater was no longer working. By then, the damage was done and the top end needed a rebuild from the overheating.

 

That was a number of years ago, but mileage in the interim hasn't been very much. I suppose its possible it has taken until now to fail.

 

The engraving on the piston crown is 2E1 with the 1 crossed out and another '2' punched in. From ePer I'm taking that as Mondial ( first '2' ) and Class B ( second '2' ).

 

I'll be taking out the other 3 pistons later this week to check them and their big ends as well as the crank journals. I'll also measure everything then to ensure the block is still good. The big end bearings on 4 look quite good with very little wear evident. No point not replacing them though - that'd be a false economy.

 

I notice in ePer that they don't seem to give a part number for a class B Mondial piston - just class A or C

http://eper.fiatforum.com/eper/navi?MOD ... &DRW_NUM=2

 

Are there alternative suppliers rather than ARUK? Motor factors?

 

There don't appear to be different ring sets for the different classes of pistons. Just +0.4mm to go with the +0.4mm piston sets. I assume that the standard rings work with all piston classes ( A, B & C )and types ( Mondial, Borgo ). I just need to ensure the ring gap is correct by putting the ring in the cylinder and measuring it with feeler gauges.

[Smaky 73]

Unfortunately most 2.0 CF1 TS engines were B class and Alfa have run out of stock and with the engine out of production for more than 5 years there iss no obligation to supply them, if you post the full code on the crown then I'll scout my parts bin to see if I have one.

 

The rings come oversize and require trimming on new builds, but most 100K+ engines accept them straight from the bag, but checking is essential.

[pkr 0]

That would be wonderful. Thank you.

 

Here's a photo of the crown. You can see the Class 'A' code '1' has been crossed out and '2' stamped in instead.

 

 

Thanks again.

[Smaky 73]

Whoever did that caused the problem.

[pkr 0]

What's interesting is that I've just measured the piston skirt as per the manual and although my micrometer is only good to 0.01mm, I get a reading of a smidgen over 82.95mm. That would put it at the bottom of the Class 'A' range of 82.952mm to 82.962mm ( using the Twinspark 16v 1970cc manual ). Class 'B' overlaps 'A' ( 82.959mm to 82.971mm ) but visually judging the micrometer to 3 decimal places I'd say the piston skirt is pretty bang on 82.952mm and nowhere close to the Class 'B' range.

 

So quite why AR crossed out the '1' and stamped it '2' is strange. I've not measured the other 3 pistons yet but they are all marked identically.

 

A mystery!

[Smaky 73]

AR would not cross out and IMPACT stamp a piston, that is the handywork of someone who does not understand the impact of STRESSES throughout a material, and I'd double check your figures as it may be oval due to piston slap.

[pkr 0]

Engine is now back together again and compression is good and equal across all 4 pistons. New rings were measured and I'm at the good end of the gaps on all the rings i.e. narrowest gap but still over the minimum. However, there is still some oil burning going on.

 

Suspecting the intake gasket has suffered from high crankcase pressure and oil is getting to the intake from the variator channels, I took off the plenum today and indeed found oil. I ran out of time so didn't go further and take the inlet manifold off - that's for tomorrow or Friday when I get time.

 

What's odd to my mind though is that the oil is in all four ports - not just 1 & 2 which is where I'd expect oil contamination if it was the variator oilways leaking into the intake.

 

I've run the engine with the breather hoses disconnected ( covering up the throttle breather valve and intake pipe holes ) and no oil is coming out of the cam cover.

 

So, when the variator leaks oil into the intake, can oil be in all four ports or is it normally just 1&2? Is there any other valid explanation? If the valve guides were badly worn or the new valve stem oil seals were a bad batch, would the oil find its way all the way back up to the plenum?

 

Thanks in advance.

[Smaky 73]

Engine is now back together again and compression is good and equal across all 4 pistons. New rings were measured and I'm at the good end of the gaps on all the rings i.e. narrowest gap but still over the minimum. However, there is still some oil burning going on.

 

Suspecting the intake gasket has suffered from high crankcase pressure and oil is getting to the intake from the variator channels, I took off the plenum today and indeed found oil. I ran out of time so didn't go further and take the inlet manifold off - that's for tomorrow or Friday when I get time.

 

What's odd to my mind though is that the oil is in all four ports - not just 1 & 2 which is where I'd expect oil contamination if it was the variator oilways leaking into the intake.

 

I've run the engine with the breather hoses disconnected ( covering up the throttle breather valve and intake pipe holes ) and no oil is coming out of the cam cover.

 

So, when the variator leaks oil into the intake, can oil be in all four ports or is it normally just 1&2? Is there any other valid explanation? If the valve guides were badly worn or the new valve stem oil seals were a bad batch, would the oil find its way all the way back up to the plenum?

 

Thanks in advance.

 

If it's in all 4 ports then you have a heavily breathing engine, possible causes are blow by on the pistons or excessive gaps round the exhaust valve shafts