After starting this thread I must firstly thank all who have contributed then apologise for not responding sooner. I was delayed for a short time by a large snow drift that had built up against my garage door then during the thaw other priorities emerged. So the other sides of the Pistons , one is the same with the damage but the other one is perfect. The engine had been "repaired" and only covered that short mileage before nearly seizing up. The previous work included the fitting of a replacement conrod. When I checked it out the engine had one 500 rod and one 126 rod with 126 big end shells. So the piston with the 500 rod got no oil squirt on the bores at all. The other should have been ok but I wonder if one jet was blocked or the overheating affected the other cylinder. The engine had also been running with just over half of the correct oil level which would not have helped with any splash effect inside the crankcase. I wanted to get opinions on possible causes as I found the beginnings of similar signs on a piston from a slightly more exotic 795cc engine that I just stripped down pending a rebuild. The engine has been in store for over 30 years and had done less than 3000 miles so I want to use the original Pistons and barrels so when I have cleaned all the parts up I will be checking clearances and oil ways for sure

Toshi 975 said:

After starting this thread I must firstly thank all who have contributed then apologise for not responding sooner. I was delayed for a short time by a large snow drift that had built up against my garage door then during the thaw other priorities emerged. So the other sides of the Pistons , one is the same with the damage but the other one is perfect. The engine had been "repaired" and only covered that short mileage before nearly seizing up. The previous work included the fitting of a replacement conrod. When I checked it out the engine had one 500 rod and one 126 rod with 126 big end shells. So the piston with the 500 rod got no oil squirt on the bores at all. The other should have been ok but I wonder if one jet was blocked or the overheating affected the other cylinder. The engine had also been running with just over half of the correct oil level which would not have helped with any splash effect inside the crankcase. I wanted to get opinions on possible causes as I found the beginnings of similar signs on a piston from a slightly more exotic 795cc engine that I just stripped down pending a rebuild. The engine has been in store for over 30 years and had done less than 3000 miles so I want to use the original Pistons and barrels so when I have cleaned all the parts up I will be checking clearances and oil ways for sure

Click to expand...

In your picture at start of thread was piston on left paired with no oil squirt hole in con rod? And was that the piston with damage on both sides?

jackwhoo said:

1 inch is 25.4mm.
25.4mm divided by 1000 =
0.0254mm.
one thousandth of an inch is 0.0254mm

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Haha.... Takes foot from mouth again. In my defence, I was writing that at 4:30am after finishing night shift.

jackwhoo said:

In your picture at start of thread was piston on left paired with no oil squirt hole in con rod? And was that the piston with damage on both sides?

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Unfortunately I had already removed the Pistons from the con rods before I spotted the problem with the rods and end shells so I am not able to say. I will post a picture of the bigger piston when I get it cleaned up.

Many years ago I worked with a young Australian gent who, when he was in America, put an "American Specs" 850 engine into the back of his 600D. Other than replacing the 850 camshaft and its bushes with 600 ones (so that the engine would turn the correct way--they turn in opposite directions normally), and a 600 distributor, he did practically nothing else to the engine---and then proceeded to run it around a large part of America and Mexico---and even with the pistons in "backwards" it never missed a beat. Just for interest, this was the car we later converted into a full-blown "850tc radiale" racer.

the hobbler said:

Many years ago I worked with a young Australian gent who, when he was in America, put an "American Specs" 850 engine into the back of his 600D. Other than replacing the 850 camshaft and its bushes with 600 ones (so that the engine would turn the correct way--they turn in opposite directions normally), and a 600 distributor, he did practically nothing else to the engine---and then proceeded to run it around a large part of America and Mexico---and even with the pistons in "backwards" it never missed a beat. Just for interest, this was the car we later converted into a full-blown "850tc radiale" racer.

Click to expand...

I assume he also fitted the 600 oil pump and oil pump/distributor drive shaft?

I had a friend who fitted several 850 engines complete with 850 gearboxes into 600's - made nice 'wolves in sheep's clothing' cars. Eventually he graduated to 850 Coupe engines (903cc, standard twin choke carb, 4 branch exhaust)- even rallied one with an 850 van gearbox fitted (lower final drive ratio) - 8000 rpm in every gear - pretty fast on Irish special stages over here.....

AL.

Greetings All, have been visiting family in the North of Ireland, so I have had a break from all things 500 and 'engine' related (no internet) I have been thinking about my past problems with piston to bore clearance and found the attached info again. I have also contacted the maker (CPS Italy) of my 80.5mm pistons to ask for their recommended clearance figures - I await an answer. It is interesting to note that the well known 'Athena' kit quotes the 'space between cylinder and piston as -
0.06 to 0.08mm !!!!!

Ian.

Hi Ian,

I wonder if these aftermarket pistons for use in tuned engines have any inbuilt expansion control e.g. 'Invar' Struts. Could you take a look inside standard and your aftermarket larger bore pistons around the gudgeon pin boss area?, sometimes you can see these struts semi-embedded, one on each side.

Re:- piston to cyl. bore clearance. Iirc, Fiat, in their workshop manuals used to show very long thin feeler gauge strips to which were attached a spring balance to use when measuring the piston to bore clearance.

It would be an interesting exercise to try heating up a standard and an upgraded (big bore) piston and compare the bottom of skirt measurements on both pistons from cold to normal operating temperature and see if they expand by similar amounts. Idk what temperature to heat them to but I'd guess that the bottom of the piston skirt is possibly at or slightly hotter than the oil temperature - anyone got an oil temperature gauge fitted? (piston crowns would run at higher temperatures). If the maximum piston skirt diameter could be determined by the above test, then possibly an additional 0.001 Inch or so would give a suitable cylinder bore size for safe running.

AL.

Greetings, had a dig around for my 'past' fitted pistons to look at (pic) the piston on the left is 80.5mm from CPS Italy the one on the right is a 126 BIS piston 80.0mm from Hepolite. Both are flat topped, Not sure about Invar struts though

Ian.

Hi Ian,

The piston on the left looks to have been machined from the solid i.e. a billet piston. If so it won't have 'invar struts' to control expansion.

The piston on the right looks to have been cast and might have 'invar struts' built in -I just can't see from your pic.

Could you possibly take another pic, this time at approx 45 degree to the vertical looking at the gudgeon pin boss inside the piston. These 'invar struts' can sometimes be partially seen in such an oriented pic. If so, they look like a strip of different coloured metal (i.e. not aluminium coloured) embedded in the aluminium.

For further info on the material 'Invar' or it's use in pistons try a google search or similar.

A piston without these expansion control struts will expand more than a piston that has them. If you change from standard pistons with invar struts to ones without and you adhere to the same standard piston - bore clearance, it's quite possible you'll have problems with piston damage as described earlier in this thread.

Seeing as you have two pistons, one without invar struts and one possibly with invar struts, maybe you'd like to try the test I described earlier i.e. measure both piston skirts at room temperature, then heat both pistons equally (even to 100C./ boiling water might be enough) and re-measure both, and report back on your findings. you may be surprised at the difference in expansion resulting from including these invar struts.

AL.

A hint of sunshine and I finally got outside and gave the worst piston from no 2 cylinder a bit of a soak in petrol. The 795cc engine was mothballed in 1985 and some oil poured into the bores that had now gone to the consistency of a very thick grease. The piston from no 1 is virtually unmarked but no 2 has 4 very distinct vertical bands of rubbing front and back of the piston. The bands do not seem to correspond to any feature inside the piston.

The engine was said to have done less than 3,000 miles but I have every reason to believe that it would have been well thrashed in the time. In freeing off the rings on that piston, middle one still to go, the oil scraper ring came out in two pieces plus the spring inner piece. No 1 cylinder looks fine. I am wondering if the ring was broken on assembly and caused the damaged stripes in some way.

Hi Dave, the attached pics show the damage done to one of my new pistons with the corresponding vertical markings the third pic shows the same piston with the position of the marks. I can only surmise that the damage is due to overheating and I am investigating the exact piston to bore clearance used during rebore. I am aware that 'standard' oversize pistons should have a clearance of 0.020 to 0.040mm - 'Athena' use 0.06 to 0.08mm in their modification running at 10.3:1 C/R. With a bit of luck the Italian maker of my pistons should get back soon with the 'approved' clearance ?????
Ian.

Toshi 975 said:

A hint of sunshine and I finally got outside and gave the worst piston from no 2 cylinder a bit of a soak in petrol. The 795cc engine was mothballed in 1985 and some oil poured into the bores that had now gone to the consistency of a very thick grease. The piston from no 1 is virtually unmarked but no 2 has 4 very distinct vertical bands of rubbing front and back of the piston. The bands do not seem to correspond to any feature inside the piston.

Click to expand...

Hi Dave,

Was No.2 cylinder the one that had no oil splash from the con. rod due to the oil hole being blocked by the fitting of the wrong bearing shell? I suspect the score marks on the piston skirt were as a result of lack of oil splash, not a broken ring. I've stripped many old well-worn Fiat engines, some have had broken oil control rings - none had bore damage or piston damage as a result - they just burned oil

Looking at No.2 piston, there appears to be a lot of carbon build-up on the ring lands (spaces between the ring groves) for an engine that supposedly only covered 3000 miles. Does No.1 piston look the same in this area?

It's possible, as you suggest, that the oil control ring was broken when engine was being assembled and this would likely allow too much oil to bypass the oil control ring resulting in excessive carbon build-up. If this was the case, it would probably result in some oil being burnt and some blue smoke from the exhaust.

Point of technical interest :- notice the expansion control slot in No.2 piston in the oil ring groove. This is to divert heat from the piston head away from the thrust faces of the piston skirt, thereby reducing expansion across them.

If the oil spray issue has been corrected, I think you could just clean everything up and reassemble with new rings - assuming that neither piston has collapsed across the skirt - if both piston measure the same I'd be satisfied. I've cleaned up worse damage to pistons in the past - they all worked and lasted just fine - ask if you want details on how-to, but it's not rocket science....

I can't remember if you intended fitting new pistons or not, but regardless, unless you're going to hone the cylinder bores, check that there's no wear lip
starting to form near the top of the cyl. bores as this can damage new rings.
This is unlikely to have happened in 3000 miles of use but worth checking.
(there are also special 'ridge-dodger' top rings which have a small step to avoid contact with any wear lip, available from specialist suppliers if needed)

Al.

Bleeding Knuckles said:

---------------------------
------------------------------------------------
---------------------------------
-------------------- With a bit of luck the Italian maker of my pistons should get back soon with the 'approved' clearance ?????
Ian.

Click to expand...

The answer is probably on it's way to you right now, in a Mail bag, on a slow boat, from China

AL.

Bleeding Knuckles said:

Hi Dave, the attached pics show the damage done to one of my new pistons with the corresponding vertical markings the third pic shows the same piston with the position of the marks. I can only surmise that the damage is due to overheating and I am investigating the exact piston to bore clearance used during rebore. I am aware that 'standard' oversize pistons should have a clearance of 0.020 to 0.040mm - 'Athena' use 0.06 to 0.08mm in their modification running at 10.3:1 C/R. With a bit of luck the Italian maker of my pistons should get back soon with the 'approved' clearance ?????
Ian.

Click to expand...

Hi Ian,

Your pics are very interesting.....

No marks on piston skirts across thrust faces so piston to bore clearance is probably just fine.
Piston on left - milled from a solid billet - too much mass of material across gudgeon pin bosses compared to a standard cast piston. I think this is where the problem lies.

If this was my engine :- I wouldn't increase piston to bore clearance.
I'd remove the piston rings, file all torn/scraped material away (4 lines on piston) plus a bit extra for clearance (or luck!), clean up the ring grooves using a needle file just enough so rings are free, clean up any marks on the cylinder bores and ......

Reassemble the engine using the old gaskets (I don't like wasting gasket sets). Run engine for 2-300 miles. Strip engine, check how things are faring re. piston clearance etc. If everything is now ok, reassemble with new gaskets and enjoy the car for the summer.

Remember, whenever an engine is modified, things don't always work out the first time. Sometimes, it takes a few cycles of build/try/strip to find cause of problem/reassemble, to get things just right. Don't lose heart

AL.

F123C said:

Hi Ian,

Your pics are very interesting.....

No marks on piston skirts across thrust faces so piston to bore clearance is probably just fine.
Piston on left - milled from a solid billet - too much mass of material across gudgeon pin bosses compared to a standard cast piston. I think this is where the problem lies.

If this was my engine :- I wouldn't increase piston to bore clearance.
I'd remove the piston rings, file all torn/scraped material away (4 lines on piston) plus a bit extra for clearance (or luck!), clean up the ring grooves using a needle file just enough so rings are free, clean up any marks on the cylinder bores and ......

Reassemble the engine using the old gaskets (I don't like wasting gasket sets). Run engine for 2-300 miles. Strip engine, check how things are faring re. piston clearance etc. If everything is now ok, reassemble with new gaskets and enjoy the car for the summer.

Remember, whenever an engine is modified, things don't always work out the first time. Sometimes, it takes a few cycles of build/try/strip to find cause of problem/reassemble, to get things just right. Don't lose heart

AL.

Click to expand...

Greetings Al, I note what you say thanks and lo and behold I have heard back from CPS. Below is the content of the message -

Hallo,
my boss says>:

1° Liner 0,10 cents
2° liner 0,12
regards

Francesca
CPS s.r.l.

Now I'm not good with the Italian lingo, but I take the message to say - Early 126 cylinder 0.010mm and the later 126 cylinder 0.012mm ???????????????????
If this is so I believe that my piston to bore clearance is WAY too tight, (these pistons must expand a lot!!!!) but I shall confirm if these figures were used during the rebore anyway.
Ian.

I'm thinking that should be 0.10 mm to 0.12 mm
Which is approx 0.004 inch to 0.005 inch
Or to put it another way 4 to 5 thousands of and inch.
That's pretty close to the Athena kit figures.

jackwhoo said:

I'm thinking that should be 0.10 mm to 0.12 mm
Which is approx 0.004 inch to 0.005 inch
Or to put it another way 4 to 5 thousands of and inch.
That's pretty close to the Athena kit figures.

Click to expand...

Thanks JW, never did get decimal points correct

Ian.

Have just heard from the engineer who did the work for the rebore, he states that the cylinders would have had a 4 thou piston clearance and as the cylinders have been rehoned after the last piston damage that would make it very slightly more than 4 thou. So my cylinders do have the correct piston to bore clearance!!!! I now have to pay attention to Cooling, Timing and fuel grade. My oil cooler fan switch is being changed for one that will start the fan sooner (87DegC) and run until (77DegC):devil::devil:
Hey Ho back to running in (after rebuild - again!)
Ian.