Cam Bearings

[Skip & Lynne]

Well now I`ve done it. I took my block to the machine shop to be vatted and cleaned up. I also had new cam bearings installed only to find that they have to be reamed to size. Well I`ve never run across this before so does anyone know where I can get a reamer or can I have the cam bearings align honed to size. Skip

 

[Randy Forbes]

That's really pretty common. I always take the cam I'm going to use in with the block. The machine shop would always check the fit for me (usually not a problem, but about 12-15 years ago, all the MGB cam bearing sets would need some real reaming to fit properly).

I can also remember a period when Big Healey cam bearings were very hard to find. I would have them in hand before committing a block to the vat!

 

[Dave Russell]

Skip,
Randy is right on. I'm not sure whether you are refering to line reaming or individual reaming. Some blocks have the cam bearing bores in perfect alignment. Many others do not. It is definitely an auto machine shop operation to install undersize, semi finished bearings & final LINE bore/hone them to correct size & all in alignment.

Just pressing in new bearings in a out of line block & individually reaming them to fit will result in some bearings needing more clearance than others to prevent cam binding. Not a good situation. Likewise precision bearing shells can be fitted with no reaming & the cam just forced into place. Probably thousands of engines have been done the "easy" way & it will work for a while. The result will likely be lower oil pressure & shorter bearing life.

As always, you have to decide if the benefit is worth the cost.
D

 

[Skip & Lynne]

Guess I didn`t explain well enough. The cam bores in the block are fine and I bought new shells from Moss and had the machine shop press them into the block but the bore of the new shells is .002 to .003 to small and the cam is binding really bad. The A/H shop manual says they have to be reamed to fit after installation with a special reamer. I am thinking about align honing them for a .001 to .002 clearance but was wondering if anyone else had run across this before. I have never had to bore new cam bearings on any engine before but this is a LBC so you know how that goes. thanks Skip

 

[Dave Russell]

"the bore of the new shells is .002 to .003 to small and the cam is binding really bad. The A/H shop manual says they have to be reamed to fit after installation with a special reamer. I am thinking about align honing them for a .001 to .002" -- "The cam bores in the block are fine" -- "I have never had to bore new cam bearings on any engine before but this is a LBC so you know how that goes."
-------------------------------------
The shells were made undersize to allow for reaming as the shop manual says. Align honing would work well as the "special reamer". The .001 - .002 final clearance should be just right. I've not done this to a Healey, but to a number of other brands. Sounds like everything is as it should be. The cam bores in the block may be fine but they are never perfect. The align honing corrects small variations in block bearing hole alignment & removes any distortion to the shells caused by the pressing in operation. Lots of production engines just make the bearings loose enough to get the cam in without further machining. Some LBC,s just went the extra mile to get everything as close as possible. A well built engine for competition will always have the cam bearings fitted this way, as well as the main bearings.
D

 

[Skip & Lynne]

Hi Dave I am taking the block to Vons in S.C. to have the bearings reamed. They have a bar and a set of shell reamers that do the job right. I have a bar and the dial indicators in my machine shop to aling hone he cam bores but I am afraid of getting honing grit embedded into the bearing shells and I can stand the block upright in the milling machine and bore them but this will leave small microscopic ridges and I don`t want that either so I am just going to go and get them reamed. thanks Skip

 

[Dave Russell]

Skip,
Good point about the grit embedding. Although I think that a bored cam bearing shell is considerably harder than the aluminum or babbit lining of a rod bearing, best to not take chances.
D

 

[RF Thom]

Skip - Cam bearings are a matrix construction babbit on the surface 0.005 /0.010" thick backed up with a layer of aluminum 0.040 " thick, and then a steel shell. They are by design smaller than the finish size because with installation there is a 'crush' effect which is required to hold them in place. This 'crush' can't be predicted as the bore in the C.I. block has a tolerance within it's size range. The design concept is to then line bore/ream the bushings for size and position. Typical clearance on these bushing is usually in the 0.002/0.003" range. Latest designs are transitioning over to a copper/aluminum matrix, which is easier to manufacture and machine, but is not as tolerant to oil breakdown with extended usage.

RF Thom - retired engine design engineer.

 

[Dave Russell]

RFT,
Thanks for the info on cam bearing construction. I guess we can assume that the bearings that Skip has are still the older high embedability, fairly thick babbit variety. Either way, honing would be very apt to leave grit embedded.
D

 

[RF Thom]

Dave you are right they should be reamed or line bored, as you point out, never honing as it would leave residual abracive imbeaded in the soft matrix. The rebuild community tried to use pre-sized cam bearings that would not require finishing in place - it was a disaster. Each phase of the manufacturing process has tolerances, and the bores are not always in perfect alignment, however that said the cam is somewhat tolerant of alignment mismatch considering the final loading. I've never seen a cam bearing fail from lifter loading, but many from timing belt load, and even more from antifreeze in the oil.

Regards, Bob