ACL STD Rod Bearing Clearances (with Bridgeway Rods)
08-10-2024, 12:02 PM
#1
Instructor
Thread Starter
ACL STD Rod Bearing Clearances (with Bridgeway Rods)
Hi Folks,
My main thread is here - 944 Turbo Mini Restoration - Rennlist - Porsche Discussion Forums
I've started assembling my bottom end and have stopped as I'm concerned that my rod bearing clearances are too tight.
The table below shows my measurements taken with a micrometer and telescopic bore gauge.
My clearances are within Porsche specification (0.034mm - 0.092mm) but outside of the recommendations from ACL - these give around 0.0020" or 0.0517mm minimum.
"ACL Race Series performance engine bearings can be assembled with .00075-.001" per 1" of journal diameter (0.020-0.025mm per 25mm of journal diameter) plus .0005" (0.013mm)"
Looks like the rods for cylinders #2 and #4 have slightly smaller BE diameters, I switched the rod bearings from #1 to #4 to see if it changed the clearance but got virtually identical measurement (which appears to be telling me it is the rod).
I think I already know the answer, which is probably to get the rods to the machine shop so they can check the big end diameters and machine them to spec.
Just looking for any knowledge or pointers from the more experienced on here, thanks!
My main thread is here - 944 Turbo Mini Restoration - Rennlist - Porsche Discussion Forums
I've started assembling my bottom end and have stopped as I'm concerned that my rod bearing clearances are too tight.
The table below shows my measurements taken with a micrometer and telescopic bore gauge.
My clearances are within Porsche specification (0.034mm - 0.092mm) but outside of the recommendations from ACL - these give around 0.0020" or 0.0517mm minimum.
"ACL Race Series performance engine bearings can be assembled with .00075-.001" per 1" of journal diameter (0.020-0.025mm per 25mm of journal diameter) plus .0005" (0.013mm)"
Looks like the rods for cylinders #2 and #4 have slightly smaller BE diameters, I switched the rod bearings from #1 to #4 to see if it changed the clearance but got virtually identical measurement (which appears to be telling me it is the rod).
I think I already know the answer, which is probably to get the rods to the machine shop so they can check the big end diameters and machine them to spec.
Just looking for any knowledge or pointers from the more experienced on here, thanks!
08-10-2024, 05:15 PM
#2
Instructor
Thread Starter
I've had a bit of a think about this, and I understand you can mix the extra clearance bearings with the standard clearance bearings according to the ACL information.
It is common practise to adjust oil clearances by mixing bearings of adjacent grades i.e. HX-STD with H-STD or H-STD with H-001 or H-0.025, to obtain desired oil clearance. Using the HX-STD with H-STD will give .0005" (0.013mm) additional clearance than using two H-STD shells, and similarly using H-STD and H-001 will reduce clearance .0005" (0.013mm). This, in effect provides the engine builder with 5 potential clearance grades* of 0.0005" (0.013mm) increments.
https://aclperformance.com.au/faq
Going with the following combinations of H-STD and HX-STD bearing halves gives me the following clearances which are much better.
I've packaged my rods up anyway for the machine shop to double check for me, but looks like I will be ordering a set of the HX-STD bearings so I can tune my clearances.
It is common practise to adjust oil clearances by mixing bearings of adjacent grades i.e. HX-STD with H-STD or H-STD with H-001 or H-0.025, to obtain desired oil clearance. Using the HX-STD with H-STD will give .0005" (0.013mm) additional clearance than using two H-STD shells, and similarly using H-STD and H-001 will reduce clearance .0005" (0.013mm). This, in effect provides the engine builder with 5 potential clearance grades* of 0.0005" (0.013mm) increments.
https://aclperformance.com.au/faq
Going with the following combinations of H-STD and HX-STD bearing halves gives me the following clearances which are much better.
I've packaged my rods up anyway for the machine shop to double check for me, but looks like I will be ordering a set of the HX-STD bearings so I can tune my clearances.
Last edited by dlknight; 08-10-2024 at 05:16 PM.