Junk Yard 903

Perl

Perl

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I’m working on this 903 I got from a junk yard and have it apart. The cylinders look really good still cross hatch no rust no water. The oil was black but no water so I was thinking things looked good. This afternoon I took the valves out of the head which was carboned up but looked ok. I got the valves out and see that a lot of the valve guides are at a different levels. It was nothing I did because the carbon buildup on the guides inside the ports show that they have been that way for a long time. Has anyone else seen this? I know this is not correct but do you think the guides could be pressed back into place and the head still useable?

20250105_172949.jpg
20250105_173015.jpg
 
Perl said:
I’m working on this 903 I got from a junk yard and have it apart. The cylinders look really good still cross hatch no rust no water. The oil was black but no water so I was thinking things looked good. This afternoon I took the valves out of the head which was carboned up but looked ok. I got the valves out and see that a lot of the valve guides are at a different levels. It was nothing I did because the carbon buildup on the guides inside the ports show that they have been that way for a long time. Has anyone else seen this? I know this is not correct but do you think the guides could be pressed back into place and the head still useable?

View attachment 93909View attachment 93910
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It was not unusual for 850s to die from overheating due to poor cooling system maintenance (removed fan shrouds, low coolant, etc). If it got really hot, the guides could lose their press fit and move. Check head for warping and consider all new guides while insuring they go in with the correct amount of interference.
 
I'd consider replacing all of the valve guides in that head. They should not be sunken like that (cylinder #4 - yikes!) & several look worn (enlarged center) anyway.

Also, judging by the nearly non-existent head thickness between the gasket surface (deck) & the bottom of the spark plug hole, the head itself has maybe enough "meat" left to allow a very thin skim job. Not a head to use with wedged/domed high compression pistons, flat-tops only.
 
They do that when they get really hot...

It would be a good time to replace all 8 guides and use the later style with a stem seal on all 8 too

new guides mean a new seat cut too, to ensure it all remains concentric to the guide hole

and if it's a single valve spring type, upgrade to the dual spring setup.

SteveC
 
Thanks for the info. Yes this head has duel valve springs. I will look into replacing the valve guides. Is the difference between the older and the later model valve guilds that the later model ones have a grove in the top for the stem seal?
 
Perl said:
Is the difference between the older and the later model valve guilds that the later model ones have a grove in the top for the stem seal?
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The groove in the older style guide is for the lock ring/clip:

Screenshot_20250106-094312~2.png


The newer style guide that uses a seal has the upper part of the guide cut away, like this:

Screenshot_20250106-093730~2.png
 
If the head has been skimmed down into the spark plug thread, or in your case if it's been cut with a very rough / coarse finish, and it needs guides / valve seat cut (which could also be cut too deep) it often works much better to source a NEW replacement head from overseas...

Fortunately, the 100 series engine had a very long production run, and new head from various uno/panda and 127's will fit straight on.

like this, complete and brand new (not mine, no connection to seller, just an example)

www.ebay.it

TESTA CILINDRI TESTATA FIAT 127 903cc 81-87 ORIGINALE FIAT 4427430 Con Valvole | eBay

Le migliori offerte per TESTA CILINDRI TESTATA FIAT 127 903cc 81-87 ORIGINALE FIAT 4427430 Con Valvole sono su eBay ✓ Confronta prezzi e caratteristiche di prodotti nuovi e usati ✓ Molti articoli con consegna gratis!
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SteveC
 
The question is will it be more expensive to have the machine work done and buying the parts or should I just buy the new head. The challenge is that other parts of the old head could be bad plus trying to find a machine shop that will work on a 70's Fiat 850 engine is not easy. I may just buy the new head.

Also should I be more concerned with the heat that happened in the head to cause the damage or should I be more concerned with what might have happened in the block because of that heat?
 
With the 850 heads, many times a machine shop will simply be asked to skim it flat (after an overheat incident) and a lot of the time they wont even remove the valves to do the skim, you don't need to as they are well away from the head face. Cutting the valve and seat adds time and expense to the job... and some people in an attempt to save money only do half a job.

if the bores and pistons dont have marks/ scuff from a minor seize (commonly called "nipping") then the pistons and bore will probably be OK... although a clean cross hatch could be simply a sign that someone honed it after it nipped up, the only real way to tell is to measure the piston to bore clearance to see if it's down near the lower limit for clearance.

the other thing that is affected by heat is ring tension, so it would make sense to pull the engine down to measure and then you might as well replace the rings.

SteveC
 
Steve thanks for your knowledge on this. I am still learning about this even at my age. The day you stop learning is the day you start dying.

More investigation into this engine and the surprises keep coming so here we go down the rabbit hole.

I started measuring the bores and 3 of the cylinders seem to be along the same line basically I set the bore gage to zero on number one cylinder and then used that on all the other cylinders just to get an idea of where things were. Three cylinders were all the same top to bottom measuring both sides but one cylinder seems to be smaller than all the rest. I am still waiting for a micrometer to be delivered so I can do this right but first glance it’s weird.

Then I cleaned off the top of the pistons and number one seems to be a new piston compared to the rest that are beat up on the top. It has a 2 stamped on one side and what is a 0 or a C stamped on the other side and is clean on top. Number 3 piston has 3 cracks in the top but that is the cylinder that measures smaller than the rest. Just a mystery. Second picture below is of cylinder 3.

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20250107_172944.jpg
 
yep. probably nipped up number one and marked the piston, so they replaced it, probably homed all cylinders and re - rung it.... but the last pic showing the piston at bottom, I can see marks in the bore (or could be reflections - underlined and circled in red)

850 903.jpg


SteveC
 
fiatfactory said:
yep. probably nipped up number one and marked the piston, so they replaced it, probably homed all cylinders and re - rung it.... but the last pic showing the piston at bottom, I can see marks in the bore (or could be reflections - underlined and circled in red)

View attachment 94014

SteveC
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It is always a nice surprise to disassemble an engine and find all it needs is cleaning and new gaskets. Time to stand back and assess what the goal is for this one. Is it stock bore or oversize? If you want 50,000km of trouble free service, this one seems to want a rebore and a new set of pistons. While you are at it check the timing chain for wear and timing chain cover for worn chain witness marks. At a used car lot, they might reassemble it and sell it as is. In the early 70's when money was tight and a piston cost $15 the customer might have asked the mechanic to bore just one cylinder and replace one piston.
 
Yes number 3 cylinder is the worst. Those are not just shadows but they are not deep. My fingernail does not catch the edge of any of it but still they are there. You can look at the center of the piston in the picture and see the cracks. That’s actually raised a little in the center where all 3 cracks converge. I have the bore gauge and today or tomorrow I should get the other mic so I can figure out where things are. In closer inspection there really isn’t any cross hatch left in any of the cylinders and the marks in number 3 tell me it needs to be bored if it’s possible. So this engine now needs valve guilds, valve job, skim the head, boring, possible skim the block, pistons and rings and I don’t know the condition of the bearings but I took the pan off and there is nothing else that looks bad except the previous owners needed to change the oil years before it was parked. Anyone know of a rebuilt 903 short block in Southern California for sale? Might be cheaper than rebuilding this one.
 
I did the best I could measuring the bores. I have a bore gauge and micrometers to measure the bore gauge. This is what I have come up with. The bores are all the same at 63.63 mm. All cylinders seem to be .01 mm egg shaped. The bad part in number 3 cylinder seems to be .02 mm but with .01 mm egg shape it’s .01 deep. I measured all cylinders side to side and at 30 degrees and 165 degrees. All cylinders measured out the same with the exception of the spot in number 3 cylinder. I’m not sure what all this means so if someone could chime in and give me some opinions that would be great.

This is not a race engine. I just want a little more grunt to handle the 3.88 gears that will be going in the car and to be able to run the freeway at 60-65 for an hour up and an hour back without running at 5500 RPM. That’s the goal.
 
The Haynes manual says the stock bore of a 903 is 65mm. The stock bore of an 843 is 65mm. If the measurements are correct, it appears you have an 816 with an overbore or two. 903 stock stroke is 68.
 
Unfortunately, measuring the bores only gives you half the data you need.... you also need to remove and measure the pistons at the appropriate point, and that will let you determine piston to bore clearance.

Some manufacturers make their pistons right on marked size, so a 65.60mm piston and then the bore would be 65.63/65.64 (usually about 0.03 / 0.04mm clearance) .... other manufacturers make their pistons slightly under marked size ... i.e. the pistons would be 65.570 and then you bore the block to precisely 65.600mm.

and then there is the ring end gap.... and ring to piston groove clearances as well.

SteveC
 
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