Gudgeon pin knock?

NickRP · Post by **NickRP** » January 16th, 2007, 10:38 am

Hello,

during last 5000km, it seems like my engine has been producing more noise than usual, and maybe a bit more vibration. It is Delta 1.5 SOHC (831) engine, stroked to 1.6. Full rebuild 30000km ago included rebore, new pistons, new conrods, crankshaft regrind, lightening and ballancing, flywheel machining, lightening, ballancing (together with crank and clutch pressure plate), cylinder head rebuild, even new aux. shaft bearings (old ones were badly worn, due to oil pump drive gear damage).

The engine has been producing some vibrations on idle ever since that rebuild, due to reduced rotating mass inertia. However, what I expirience in last 5000km is different a bit. First, it is followed by noise (like light tapping somewhere, like if cam lobe was worn), it is not present at idle, but rather on mid range revs, regardless of load. Around 3000RPM, it seems like it goes through some kind of resonance, it gets loud, and then get quieter again.

Oil presure is as good as when the engine was new, from tickover to redline. Cam lobes are not worn. Compression test showed constant readings on all the cylinders, similar to the ones found when the engine was new. However, on cyl #2, there are some deposits on the spark plug that are not present on the others (looks a bit ash fouled, deposits have brown colour). The engine has normal oil consumption (say 0.75l on 5000km oil change intervals).

As soon as I spoted the problem, I went to the workshop that often deals with tuned Fiats (can't really say how good or not they are), and they suspected conrod small end bushes. I was said that I should drive and ignore the noise. So I made 5000km, the noise is still at the same level... but I can't sleep tight until I, at least, know the real cause.

Hints?

Regards,
Nikola

Post by **Guy Croft** » January 16th, 2007, 11:14 am

In my exp 3 things (in the main) cause that noise;

- loose tappet - easily checked. But it would have to be at least 10 thou loose to be that noticeable.
- worn small end bush - unheard of except on 'GpA' Integrale (ie: high boost, very high rpm and rallying 10 times a year), except to say that if new bushes were fitted and honed/reamed loose they would rattel especially when you lift off throttle.
- loose flywheel bolts
- piston hitting valves head or spark plug (only with big dome pistons)
- very badly worn big end bearing - the most likely cause.

Get the sump off first and check the bearings urgently. Then take the head off.

GC

NickRP · Post by **NickRP** » January 16th, 2007, 12:36 pm

Thanks Guy!

I will go through the points:
1) loose tappet - good point, I will check that over weekend.
2) worn small end bush - I bought a set of factory new conrods at the time of rebuild. All the piston pins needed solid force to go through the bushes. I was even concerned that the fit might be too tight. I will listen how it sounds when the throttle is lifted.
3) loose FW bolts... never really thought about this. Might be very good point. Not so easy to check, though.
4) piston hitting valves head - there would have to be something very wrong if this happens, as there was over 3mm of clearence all round (I have very mild cam).
5) very badly worn big end bearing - wouldn't this result in low oil pressure, as well? And wouldn't it be load related? My oil pressure is about 2 bar at tickover (800RPM) and 4.5-5 bar at 3500RPM with oil hot. If that's to be suspected, I'll order a gasket kit and check it as soon as the weather gets nicer... would have to resort to public means of transportation in the mean time.

Regards,
Nikola

Post by **Guy Croft** » January 16th, 2007, 1:01 pm

Nick, thanks

be careful, in the first instance a bearing collapsing/seizing does not always cause low oil pressure, it may even go up as the bearing material blocks the crank oil galleries on the crankpin.

GC

NickRP · Post by **NickRP** » January 16th, 2007, 1:08 pm

Guy,

good that you said that! I won't use the car then until I find the problem. Actually, I may order a set of big end bearings, as well, and replace them just in case... Is there a reason to suspect mains, as well?

Thanks,
Nikola

Post by **Guy Croft** » January 16th, 2007, 1:56 pm

The mains rarely cause a problem but if the bearing is knocking, the crank will be so badly damaged it will have to come out anyway.

Check the big-end brgs - if they are fine - no need to inspect the mains, the problem lies elsewhere.

GC

NickRP · Post by **NickRP** » January 16th, 2007, 2:15 pm

Will do, tnx!

Nikola

Post by **Guy Croft** » January 16th, 2007, 2:34 pm

Nick, just looking back at your post.

All the piston pins needed solid force to go through the bushes. I was even concerned that the fit might be too tight.

I thought the pins were full-floating on that engine? I don't like the sound of too-tight wrist pins either; full-floating pins must be a (cold) sliding fit when oiled - in the rod bushes. No idea what happens if they're too tight, maybe seizure in rod and piston slap I guess, because they go tight in the piston anyway.

GC

sumplug · Post by **sumplug** » January 16th, 2007, 5:51 pm

Well i had an engine with a tapping sound coming and going and on strip down found the gudgeon pin to be seized on three pistons and this had caused the crank to float about causing said sound. It can also be a loose tappet. Had one on one of my 16v Tipo engines, but that also came and went. Just lived with it. Does it sound top end or from the bottom end? Have you used a listening stick to try and find where the sound is coming from? Brush handles make great listening tools !!

Andy.

NickRP · Post by **NickRP** » January 16th, 2007, 8:48 pm

Andy,

could you please clarify - how the piston pins were seized (i.e. to what, and how did that happen), and how that affected crank?

Thanks,
Nikola

sumplug · Post by **sumplug** » January 17th, 2007, 9:16 am

Car came into my possesion with rebuilt engine, and was ok for several hundred miles, then the knock/tapping started. Gudgeon pins were fully floating type but they were virtually seized in the rod due to ham-fisted rebuild.
Piston was being forced at an angle and the bore had wear to it. So the piston was slapping the bore slightly.The crank had movement as well, so I put this down to conrod/piston forcing the crank. Crank was beyond repair as it was bent !
Gudgeon pins must have been very tight when fitted, and so over the miles had seized up. I am not saying for a second you did a bad build, but it definately needs the sump off to check things out. Might turn out to be something innocent.
Question:
Does the knock/tapping happen when cold or only when hot ? Or is it anytime?
Mine started when warm, but eventually was all the time due to the wear of the bores and crank moving.
Andy.

NickRP · Post by **NickRP** » January 17th, 2007, 10:13 am

Interesting story indeed, Andy.

Well, I remember well that the effort to insert the gudgeon pins was somewhat higher that it used to be with old rods... but I still managed to do it by hand (twisting and pressing the pin), oiling the rod bush heavily. At the time of assembly, all the rotation of gudgeon pin occured in piston, not in bush... but I was told that this is normal for new rods, and that it will change with time, as the bush is pressed a few times by forces of piston under pressure of gasses. I did not have small bore gauge to measure the bush ID. So, this could also be my problem.

Until the engine gets a bit hotter, I do not rev it enough to hear the sound, but as far as I know, it sounds odd from start.

Regards,
Nikola

sumplug · Post by **sumplug** » January 17th, 2007, 10:23 am

Think you need head and sump off for investigation. Sounds like little end or/and piston slap you are getting. Piston slap usually quitens on warm up but not always. Measure end float by rotating crank. Might be an idea to check camshaft for tappet marks if you have a loose tappet say.

Andy.

Post by **Guy Croft** » January 17th, 2007, 10:49 am

OK, Nick, so they are full floating like I remember. Re:

At the time of assembly, all the rotation of gudgeon pin occured in piston, not in bush... but I was told that this is normal for new rods, and that it will change with time, as the bush is pressed a few times by forces of piston under pressure of gasses.

This is just not so.
With a f/f setup the pin MUST be a sliding fit in the rod bush from day one. The bushes rarely wear and certainly do not exhibit bedding-in to the extent that they will develop a (predictable) sliding fit to OE spec - ever. It sounds like that set were not honed for pin fit properly and I doubt very much if they ever came from a genuine OE Fiat source. I've never known F/L to get any fit wrong. As I said earlier, the pins do go tight in the pistons - tight in piston/tight in rod - expect problems all round there.

GC

NickRP · Post by **NickRP** » January 17th, 2007, 11:04 am

Ouch! Not good at all! As I am 100% sure about the tightness of fit and where the rottation occured on day one, it is highly likely that this is where the sound has been coming from.

I bought a matched set of rods in local factory that produces engines for Yugo cars, under Fiat licence. So they are genuine factory items, but who knows, maybe they're not selling them readily honed!

Let's just hope that my bores are ok, otherwise I will practically need completely new bottom end.

Many thanks,
Nikola

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Gudgeon pin knock?

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