924Board.org

[Jaf]

Hi guys.

History: Have had my 1978 924 for 14 years. No service history, odo stuck at 25,000 miles. In 99 the big ends went when I was going a bit fast on the motorway. Replaced big ends but then didn't use the car until 2 years ago. Of course after so long the fuel distributor was stuck open and sent gallons of fuel into the sump. After that happening twice I finally fixed it and changed the oil a few times. But was left with horrible clatter, although oil pressure fine.

Finally got round to investigating the clatter today. Number 3 big ends are copper, but the others are fine. Going to replace the set, of course, (once I find some!) but I'm wondering if anyone's had just one bearing go? Could it have been the fuel in the oil? Or one duff bearing?

What do you think?

Cheers, Jaf
_________________
1978 924na

[Martijnus]

I'm positive the fuel contaminated oil is the cause.
Can't tell you exactly why #3 died first, but it could be due to the pressure to that bearing, caused by it's relative location to the oil filter, or by debris in the oil channels.
I've got a Honda XR600 motorcycle and the camshaft has 4 lobes, oil feed at lobe 1... strangely enough lobe and rocker #3 are the ones that wear out first when there's oil starvation... been thinking about that for a few years now but I still can't figure out why this happens to #3, while #4 is further away from the oil feed.

Time to flush the engine anyway to be sure it doesn't happen again

Oil pressure won't tell you much, could be a bit lower due to the lower viscocity of fuel contaminated oil, but more important, the oil loses it's lubrication capacities when it's contaminated...and that's what caused your problems.

Make sure your CIS works properly.

edit: DO NOT underestimate the importance of oil pressure and good lubrication. As soon as your idiot light tells you the oil pressure is gone, you've got damage to your bearings. Often doesn't make your bearings useless, but they're damaged alright...

Since you've replaced your bearings before, I think you know how delicate the material is on those shells, you can easily scratch them with your nail...
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

[Jaf]

Thanks for your quick reply! It's great to have the cause confirmed.

I agree about the oil pressure. In 99 when the oil pressure suddenly dropped that was the only clue something was wrong, no clattering noise. Unfortunately I was in the fast lane, at almost top speed, and took a minute to get into the emergency lane!

When I was first trying to start her a couple of years ago, with the overfuelling she never fired up. Once I fixed the fuel distributor and checked over the CIS I changed the oil 3 times after running for just a few minutes. Had only done 200 miles looking for the noise before I stopped and put her away again.

The thing that gets me is this: When I worked at a proper main dealer garage we used to put parafin oil into the engine to clean it out! Ran it for 20mins and then changed the oil. Cowboys. I know better now!

She's getting fresh oil with the new big ends and then I'll change the oil another couple of times after a few miles.

Fingers crossed the clatter is because of the shot big end! It just doesn't quite sound like a big end. Argh, the tension! Off to the shops tomorrow.

Jaf
_________________
1978 924na

[RC]

Was it the same #3 bearing you had trouble with the first time?

When a bearing shell goes and the engine knocks the conrod big end is being pounded out of round (egg shape) and the crank is also suffering.

Strongly suggest you mike the crank and measure big end bore size. Not easy with the pistons in the block. Crank may need grinding and very likely the rod will need resizing or at least replacement.

The minimal check should be plastigauge bearing clearance on replacement, but that may just indicate you bought incorrect bearings for a damaged crank. This is the thing you either check and do properly or waste your time and money to do again. Like this guy:

http://www.924board.org/viewtopic.php?t=23649&highlight=resized

[Nobbi]

what is a big end ?
_________________
NA 924 - april/1977- 4-speed-audi gearbox.
500.000 km with fun.....and new toy:Porsche 944 S2 Cabriolet.Other car Daimler c-class cabby,brandnew..Plus : DEUTZ 4005 tractor Built 1967

[Martijnus]

[924RACR]

FWIW, last time I lost one (on the racecar), it was the #3...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[RC]

[Jaf]

[ideola]

FYI, main bearings are apparently only available from one source, and that is EBS. We recently completed a group buy, I believe the STD sets are already completely sold out (which will require another run by the manufacturer, King), but sounds like you may be needing oversize. There should be some of those still available, in fact, I will probably have an extra set of oversize, so send me PM once you've assessed your crank and I can help you out with bearings.
_________________
erstwhile owner of just about every 924 variant ever made

[Joes924Racer]

I can get bearings mains standard size.
_________________
1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

[Jaf]

Have checked the crankshaft, it's within spec. Phew!

Machine shop managed to get me the bearings from Barcelona so I put them in today.

The clattering has gone! I'm really pleased, my dad was so convinced it didn't sound like big end noise and he's normally right.

Thanks for help and bearing offers guys. Fingers crossed I won't need to do a full rebuild for a while at least.
_________________
1978 924na

[Mike924]

Hi, Jaf.

Did you have the engine out, or did you do it upside-down with the sump removed?
_________________
1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org

[Jaf]

Hi ya Mike. Did it the uncomfortable way, upside down, hair in the steering rack! Somehow forgot how tricky a job it is from the wrong angles. Amazing how much mess I made. Got to play with the bonnet hinge now as the pin holding it together snapped. Never ending eh?!
_________________
1978 924na

[RotaryMaster]

Did you check the rod as well? i hope it does not happen but dont be surprised if you spin it now. I come from the world of spun bearings all the time on 3000gts sucks i had to rebuild my motor after a few seasons of race.
_________________
For every tree hugging hippie there is a guy with a rotary polluting at 120mph