944 Rod Bearing - Fix?
02-26-2014, 08:29 AM
#1
Rennlist Member
Thread Starter
944 Rod Bearing - Fix?
Is a performance oriented rod bearing the solution to 944 rod bearing failures? Well, I won't go that far, but a good rod bearing is a huge step forward in addressing this painful issue.
First, a little pertinent history: Years ago early 911 racetrack oriented engines started having rod bearing issues similar to the 944. An investigation revealed that Glyco had changed the bearing material and structure to something more appropriate for the perceived intended purpose - street driving. The failures continued until Scott McPherson at Auto Associates in CT (860-693-0278) commissioned a race quality bearing from a major bearing manufacturer. He made them available to the performance community and the issue went away. Thank you Scott! Fortunately there's a large enough market in the 911 world to justify that investment.
The 944 OEM rod bearing (also Glyco) is another street design, and usually does its job in that application. On the race track or when used in performance applications, however, it's out of its element. The bearings won't survive the distortion of the connecting rod big ends at high RPM, and the material is way too soft for severe service. What's needed is a performance oriented bearing like Mahle/Clevite's 'H' spec bearing. Here's their description:
Mahle/Clevite H - Series Bearings
These bearings were developed primarily for use in NASCAR racing, but are suitable for all types of competition engines. H series bearings have a medium level of eccentricity, high crush, and a hardened steel back. They have a precision wall tolerance of +/- .00015" with strong, extra thin overlays to withstand high loading and resist flaking.
I've used the 'H' bearing many times in the 944, but had to machine the crankshaft rod journals to a smaller size and use custom connecting rods. Since this is a relatively complex and certainly expensive solution, a simpler approach would make the modification more suitable for the typical 944 racer. It turns out that there's a Clevite 'H' bearing that can be made to fit a modified 944 connecting rod. We've built a dedicated fixture to machine the rod, and the wimpy Glyco rod bearing gets replaced with a performance oriented H series bearing which fits the standard Porsche 944 crankshaft. The result is an inexpensive upgrade to any of the existing 944 rods. The process includes fitting the 'H' bearing, re-machining the rod's big end to ensure concentricity, setting proper oil clearance, checking the pin bushing and balance. The rods get returned with a new set of the Mahle/Clevite 'H' bearings. Life is good, or at least better….
The 'H' bearing conversion can be performed on any cast or forged 944 rod. The 'H' bearing may be even more important with cast rods since the big end tends to deform more at high RPM than the forged pieces. The eccentric design of the 'H' bearing addresses this issue and prevents the bearing from 'pinching' the crank journal.
This modification shouldn't be considered a complete nor perfect solution to the 944 rod bearing issue, but it's certainly a major improvement to a serious problem. It should be strongly considered for any performance oriented or race 944. The basics still apply: maintain proper oil level, use an appropriate grade and weight oil, and monitor oil temperature and pressure.
Photos show (1) an unmodified rod, (2 and 3) rod modified to accept 'H' bearing, (4) new bearing installed in the rod.
First, a little pertinent history: Years ago early 911 racetrack oriented engines started having rod bearing issues similar to the 944. An investigation revealed that Glyco had changed the bearing material and structure to something more appropriate for the perceived intended purpose - street driving. The failures continued until Scott McPherson at Auto Associates in CT (860-693-0278) commissioned a race quality bearing from a major bearing manufacturer. He made them available to the performance community and the issue went away. Thank you Scott! Fortunately there's a large enough market in the 911 world to justify that investment.
The 944 OEM rod bearing (also Glyco) is another street design, and usually does its job in that application. On the race track or when used in performance applications, however, it's out of its element. The bearings won't survive the distortion of the connecting rod big ends at high RPM, and the material is way too soft for severe service. What's needed is a performance oriented bearing like Mahle/Clevite's 'H' spec bearing. Here's their description:
Mahle/Clevite H - Series Bearings
These bearings were developed primarily for use in NASCAR racing, but are suitable for all types of competition engines. H series bearings have a medium level of eccentricity, high crush, and a hardened steel back. They have a precision wall tolerance of +/- .00015" with strong, extra thin overlays to withstand high loading and resist flaking.
I've used the 'H' bearing many times in the 944, but had to machine the crankshaft rod journals to a smaller size and use custom connecting rods. Since this is a relatively complex and certainly expensive solution, a simpler approach would make the modification more suitable for the typical 944 racer. It turns out that there's a Clevite 'H' bearing that can be made to fit a modified 944 connecting rod. We've built a dedicated fixture to machine the rod, and the wimpy Glyco rod bearing gets replaced with a performance oriented H series bearing which fits the standard Porsche 944 crankshaft. The result is an inexpensive upgrade to any of the existing 944 rods. The process includes fitting the 'H' bearing, re-machining the rod's big end to ensure concentricity, setting proper oil clearance, checking the pin bushing and balance. The rods get returned with a new set of the Mahle/Clevite 'H' bearings. Life is good, or at least better….
The 'H' bearing conversion can be performed on any cast or forged 944 rod. The 'H' bearing may be even more important with cast rods since the big end tends to deform more at high RPM than the forged pieces. The eccentric design of the 'H' bearing addresses this issue and prevents the bearing from 'pinching' the crank journal.
This modification shouldn't be considered a complete nor perfect solution to the 944 rod bearing issue, but it's certainly a major improvement to a serious problem. It should be strongly considered for any performance oriented or race 944. The basics still apply: maintain proper oil level, use an appropriate grade and weight oil, and monitor oil temperature and pressure.
Photos show (1) an unmodified rod, (2 and 3) rod modified to accept 'H' bearing, (4) new bearing installed in the rod.
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02-26-2014, 12:32 PM
#10
Rennlist Member
Great post, thanks! Is it just me or are the Glyco bearings for this car seem particularly "cheap" (i.e., poorly made). Seems like half the nose bearings come out of the box with a sharp ridge where the ends dovetail together, for example.
Would this approach work on Carrillo rods?
Would this approach work on Carrillo rods?
02-26-2014, 01:22 PM
#11
Rennlist Member
Great post, thanks! Is it just me or are the Glyco bearings for this car seem particularly "cheap" (i.e., poorly made). Seems like half the nose bearings come out of the box with a sharp ridge where the ends dovetail together, for example.
Would this approach work on Carrillo rods?
Would this approach work on Carrillo rods?
