Finding my 996 bearings

Do you know about bearings? No, not IMS this time. We have a thread about 996 belt pulley bearings but I hope this thread will attract other members and one of you might know the answers. The thread https://911uk.com/porsche/idler-tensioner-bearing-replacement.89457 looks at the bearings but there is a huge range of variants available and some people have had serious bearing failures. The bearings are 3202 double row angular contact bearings (35mm OD 15mm ID 15.6mm width) and there are many variants from different manufacturers, the key issue seems to be that they must be sealed and manage the high rotational speeds which many do not. The pulley sizes gear up the rotational speed from the crank by about 2:1 so we need a bearing capable of 14,000 RPM. Sadly some of the bearings run out at 12,000. The original (?) Porsche part has 'rubber' seals. I've found an SKF bearing (3202 TN9) that seems to run to 18,000 RPM but only 14,000 with rubber seals, are the seals a limiting factor and are metal shields ok in this application. Can you advise please?

Sorry 15.9mm width

You need to take in to consideration when a bearing is spec'd at 12,000rpm, that's a constant 12,000 for hours on end.. This never happens on your car unless you're streaming down an autobahn at 175mph. For the infrequent blip of over 12,000 rpm, I doubt it would ever be a problem. I think I'd be more concerned about loads. For example, the tension pully has a greater load on it than the other 2 pulleys.

Hi Alex, the 12,000 RPM is defined as the "limit speed" does that really mean the max constant speed or the actual 'don't go above' limit. The bearing with metal shields have higher RPM ratings, are metal shields as good rubber at keeping out *****? I agree that the pulley's are side loaded but I wouldn't know where to begin to calculate that and they seem to have fairly similar limits. I would hope that the belt tension is not too great! The SKF bearing does have a 14k limit with rubber seals and 18k with metal so I wonder whether its a heat related issue when running (as you say) at constant high revs....

I've found the answer: Reference Speed Thermal Conditions : The reference speed is the speed at which a bearing can operate continuously under standardized thermal conditions without overheating. ISO 15312 Standard : Defines the calculation of reference speed under controlled test conditions. Primarily for Open Bearings : Since open bearings rely on external lubrication and cooling, reference speed mainly applies to them. Limiting Speed Mechanical Threshold : Indicates the maximum speed dictated by the bearing’s design (cage stability, rolling element forces, lubrication). Advisory in Nature : Reaching the limiting speed means careful design checks are required. Sealed Bearings : Bearings with contact seals often only specify limiting speed, as seals add friction and heat, reducing maximum speed. Bearings with Contact Seals Sealed bearings typically do not list reference speeds in catalogs. Their performance depends mainly on limiting speed , which accounts for the extra friction introduced by the seals. While seals protect against contamination, they lower the speed potential compared to open bearings. I'll pay a bit extra and go with the SKF bearings with the rubber seals which are better at keeping our muck than metal shields. about £33 each including VAT and delivery, the 14k limit should be fine!

For reference, the C number is the clearance size on the bearings, so the higher the number, the higher speed. Mr AI Google bot says it needs to be a C3.

I have just bought the 997 cup pulley as I’m doing and AC and PAS pump delete on my turbo, here’s a pic of the bearing used on that part so you can compare specs.

Here: https://www.wmtbearings.co.uk/FAG-575168E-PORSCHE-IDLER-PULLEY-BEARING

Anyone who is unsure or doesn't have the expertise to ensure the tolerance of the aperture on their existing pulley is in spec, then I wouldn't attempt to fit a new bearing, I'd just buy the correct pulley from my local OPC.