Wheeler Dealers new episode on the 996 Carrera

GARY S said:

"a couple of taps with a light hammer"!!! As Alex has already commented, what stresses/damage is this doing to the sensitive items attached or linked to the IMS? In 2012 I spoke with Hartech about whether it was possible for them to check & if necessary replace the IMS bearing on my (November 2004 registered) 997.1 with one of the common aftermarket items. They explained they only upgrade the bearing to the post 2006 larger bearing as part of an engine strip down as they have found replacing an early bearing in situ if fraught with potential problems. In their experience bearings hardly ever come out smoothly even with the "removal tool" shown being used on the internet & putting the new one in subjects sensitive internal components to unnecessary stresses.

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rigsby99 said:

Alex said:

So do you think that by fitting a pipe in the side of the oil filter that allows oil to be pumped to the other end of the shaft have zero effect on pressure at (for example) the crank bearings or cam shafts?

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No it wouldn't as long as the bearing, oil cooler, gauge etc didn't leak or, in the case of a bearing was tight enough to maintain the pressure. Worn main bearings will show a drop in oil pressure assuming that the pressure control system cannot compensate.

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bazhart said:

Ball bearings do not require oil under pressure - just a small amount of splash oil is perfect for them.

Oil pumps deliver oil pressure and flow that increases with revs until the maximum oil pressure designed is reached when a blow off or bye pass valve opens to prevent over pressuring the system.

These engines have increased oil supply points due to the inclusion of 24 hydraulic tappets, three hydraulic tensioners (unsealed so a leak path) and variable camshaft solenoid actuation.

The galleries that deliver the oil to the cylinder head area are not sealed (just alloy to alloy face closures) so also have small leak paths.

As a result the leakage potential at low revs is relatively high.

The pump is a crude double gear pump that drops both pressure and delivery volumes considerably at low revs and as the gear shafts wear the diametral clearance increases reducing low speed pump efficiency further.

The Gen 2 engines have a much more sophisticated oil pump (much more expensive).

If the engine is being stripped the larger splash oil fed ball bearing is the best solution available (and we can supply the Porsche IMS with the larger bearing for later cars or - we re-manufacture a shaft that incorporates the larger bearing in both Hivo (later engines) and roller chain (earlier engine) variants (only available during a Hartech rebuild at the moment) and it costs less than the Porsche version with the larger bearing.

Since the weaknesses in the design (bore ovality, cracking and/or scoring) may lead to an increasing number of engines needing a rebuild - and bearing in mind the small number of IMS bearing failures so far anyway - it may be better to wait and have a larger ball bearing version fitted during a rebuild.

If a rebuild is not going to be considered (or afforded) than replacing the bearing with a 6204 single row ball bearing with a stronger spindle and splash oil feed should last several years (although if the engine has suffered an IMS bearing failure it is best to strip and rebuild the engine as in our experience debris goes everywhere and can cause more serious damage and a more expensive rebuild eventually).

There can be resulting problems unless the people retrofitting the bearing kit "in the car" without stripping have special tools and take great care - so there can be a small risk in doing that and probably no less than leaving it alone anyway.

There is nothing wrong theoretically with a pressure fed plain bearing kit - it should provide a reliable system but it will inevitably have some affect on the oil pressure and delivery volumes on hot tickover. I would consider adding the new oil pump and the associated casting if oil pressure was not high on hot tickover before the change.

I don't think anyone will have enough time or money to test hundreds of cars for enough years to establish conclusively whether this additional oil supply need will result in any deterioration in average engine life but crankshafts are not hardened very deeply and shells are relatively thin - so most engines we rebuild (over say 30K) require new shells anyway and in view of all the uncertainty and relative rarity of IMS bearing failure anyway - combined with the eventual higher probability of requiring an engine rebuild - and the relatively expensive "upgrades" available - we suggest it might be better on average to wait until then and fit a larger ball bearing version.

If you can easily afford this pressurised solid bearing kit and are happy to accept the slight risk - and want the peace of mind and have no plans or expectations to have an engine rebuild anytime soon then I feel sure it will work better than an IMS small bearing replacement.

We have had a few ceramic bearing failures in (but have also had most other types to) but they are insignificant in numbers compared to other failures like bore scoring, cracked cylinders, crankshaft bearing failure and valve seats falling out or valve heads breaking.

The loads imparted to the shaft from the chain slap are very much higher than most people would imagine (hence the under-spec of the original bearings) both linear and lateral loads can be very high and not helped by the relatively crude hydraulic tensioners (compared to most of the air cooled 911 range with hydraulic tensioners) and they wear quite quickly and have a reduced effectiveness with time and mileage.

A good watch is the "rollring" tensioner shown in a video on the Internet youtube to see how bad chain snatch can be at relatively low revs.


Baz

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Griffter said:

rigsby99 said:

Alex said:

So do you think that by fitting a pipe in the side of the oil filter that allows oil to be pumped to the other end of the shaft have zero effect on pressure at (for example) the crank bearings or cam shafts?

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No it wouldn't as long as the bearing, oil cooler, gauge etc didn't leak or, in the case of a bearing was tight enough to maintain the pressure. Worn main bearings will show a drop in oil pressure assuming that the pressure control system cannot compensate.

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There's pressure and there's flow. A gauge requires pressure but not flow, so it will not affect the flow of oil (or therefore the available pressure) to other components.
An oil-fed bearing, however, requires a flow of oil and incurs a pressure drop across it, so it would potentially affect the flow of oil (and available pressure

to other components.
Of course you would expect the capacity of the oil pump and the pressure regulator to be such that the minor effect of an oil fed bearing would be inconsequential to the oil flow and pressure to other components, but it will depend to some extent on where the tapping is and what the flow characteristics of the oil fed bearing are. Given the M96 is known to be prone to surge in hard cornering I'm not sure I'd risk it.

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rigsby99 said:

I don't know as this is the first time I've seen a bush being used as opposed to a plain bearing which is why I would be interested in Baz's observations.

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rigsby99 said:

Yes but the prog was maybe a bit light on detail as to the construction of said bush. They are generally found in suspensions not in the rotating parts of engines.

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rigsby99 said:

Yes but the prog was maybe a bit light on detail as to the construction of said bush. They are generally found in suspensions not in the rotating parts of engines.

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