mikey_audiogeek

I don't understand your question. Increased clearances cause oil flow to be diverted away from the intended path. In this case, worn thrust bearings cause #2 bearing to be starved of oil just when it needs it most.

Cheers,
Mike

TexasRider

One thing back on the oil to use thing.

I use the Mobil 1 5W-40 diesel oil as it is made for turbos and still has the additive package in it that the car oils dont.

It holds up well and has the ZDDP. Flows well and lubes well and stays clean. As hot as it is here and running hard 50W might could be used, but the 15W-50 doesnt have the additives as they are more for "regular" car engines .

I am sure the Penn and Gibbs stuff does too. Just adding that for folks that may not have the others available.

mikey_audiogeek

You sure about that?

http://www.mobiloil.com/USA-English/...duct_Guide.pdf

Lex_GTX

People change rod bearings as a preventative measure for spinning the #2 rod bearing. The thrust bearing is not changed, only the rod bearings so I don't see the reasoning behind changing the rod bearings alone.

mikey_audiogeek

Me neither!

Cheers,
Mike

TexasRider

Interesting - thanks. Not sure I would change but good to know. One thing the chart does say for the 5-40 is "for engines requiring more ZDDP" or soemthing to that effect. I assume they are calling the 15-50 a race oil only although, while I am not sure I thought that was Porsche approved for Panamera etc.

In a race application, no cat, etc I would certainly use the racing oils.

After 30 years of racing mostly Yamaha motorcycles I really like the YamaLube. I just cant get them to sponsor a Porsche. But I still have lifetime supply of both 2 stroke and 4 stroke YamaLube for my other engines in the warehouse.

mikey_audiogeek

Cheers,
Mike

Tedro951

I'm guessing that changing rod bearings may increase the oil pressure to the rest of the engine, especially at lower RPMs. Changing the mains would have a greater impact, but that's not a Saturday project.

Lex_GTX

If oil pressure is below spec then changing rod bearings alone won't help - a full rebuild is needed.

There is a risk to taking stretched rods and fitting new bearings without checking clearances and whether the big end of the rod is still within spec. At worse, you could screw up the install or stretch a rod bolt to within failure.

At best, replacing the rod bearings along will not address the root cause of spinning a rod bearing.

I've just seen the rod bearing change spoken about here quite a bit and I wanted to understand why the community feels so strongly about it.

In regards to flat tappet motors - all motors I have worked with (4 cylinder Japanese) use hydraulic lifters and flat tappets. These are motors from the 80s to the current generations. They have no issues running current oils. Further, my current direct injected Mazda DISI uses a flat tappet on the high pressure (1800psi) fuel pump on a 3 lobe cam. The tappet from the pump sees lots of pressure and is actuated three times per cam revolution. There are no issues when using modern oils. So where is this need for a special oil coming from?

TurboTommy

Actually, alot of them use rollers now, as far as I understand.
Even the ones that don't are small cylinder engines with four valves/cylinder. So the valves are quite small (light) and only soft springs are needed. Therefore much less stress on the cam/lifter interface.

Lex_GTX

4 cyl overhead cam engines don't generally have roller cams - never seen any. You are correct in that they are 4 valve motors but 2.5l is not an unusual size.

As mentioned the Mazda and VW motors that use direct injection (I assume BMW as well) use a flat tappet to build pump pressure which is anywhere from 1800-2200psi.

What is the valve spring pressure for our 951?

I am also curious about ZDDP since I know VW users chew through their cam followers but Mazdas have no issues.

Here's a picture of one, this one is damaged due to a seized pump:

LFA951

Or Kendall 20W50 ($1 cheaper per quart) just as good

DanR 1201

Thrust bearings normally are splash fed. No connection with main and big end bearings that are pressure fed. IE unless Porsche has done something unusual with these 944 engines.

They wear out due to foot pressure not clutch pressure. Especially when drivers leave their foot on the clutch while stopped at lights etc for a long time, rather than selecting neutral and keeping any pressure off the clutch pedal.

mikey_audiogeek

Yep foot pressure is what I meant. Thrust bearings are often fed from the effluent flow from the main bearing. I don't know if I'd call that splash-fed?

I'm not an engine builder (or destroyer!) so I'll step away now.

I found this article helpful: http://www.4secondsflat.com/Thrust_b..._failures.html

and another http://www.mahleclevite.com/publications/CEB-1-1208.pdf

Cheers,
Mike

TexasRider

Here is a Tech Brief talking about lifter loads etc and ZDDP that I was pointed to by a buddy.

http://www.zddplus.com/TechBrief8%20...0on%20ZDDP.pdf

For what it is worth I did change out my rod bearings, rod bolts (nuts really) etc when I was in my 951 for rebuild. My cam, lifters and pistons and bores etc looked excellent and the shop said "lets leave it alone." But as so many #2 bearing failures (statistically) had been around 100,000 or a more, and mine had 90,000 I elected to do it while I was there. Seems good so far - cross my fingers LOL.

My oil pressure has always been good - but I am thinking a baffled pan is soon in the future too.