Cam Bearing and Journal Size Questions

[Jocko_51_B3B]

I'm doing a total rebuild of my 218 engine. The engine is completely disassembled except for the cam bearings. Before I remove them, I thought it would be a good idea to measure both the existing cam shaft journals and cam bearing inside diameters to see what the clearances are. (The #4 bearing is cast into the block and is not replaceable so I'm not including it in the discussion.) I found some things that surprised me:

Q1. The #1 and #3 bearing and journal sizes were different by a whopping .064 (not .0064). Shouldn't the #1, #2, and #3 bearings and journals be almost identical in size?

Q2. Measuring bearing inside diameters is not easy. I used an inexpensive Chinese telescoping gauge along with a top notch Starett micrometer for the measurements, but getting consistent readings using telescoping gauges was hard to do. Using the telescoping gauge, consecutive readings of the same bearing often differed by anywhere from zero to .002 or even .003. What's the best method of measuring cam bearing diameters? Has anyone else had trouble with making these measurements.

Q3: How can I order the correct size replacement bearings when I have these crazy differences in sizes between the three bearings and journals? Should I just replace the cam and all the bearings in order to obtain consistent sizes and clearances among the three?

 

[Dodgeb4ya]

The cam bearings do graduate from bigger to smaller towards the rear of the block. That's typical of all the MoPar flatheads.

Getting the correct cam bearings from a reputable supplier should be no problem.

[Jocko_51_B3B]

Dodgeb4ya,

Thanks for that info. I had no idea that the cam bearing and journal size differences were on purpose! (I should have looked up the numbers in the shop manual.) I checked my measurements again and after a few quick subtractions, it looks to me like the cam bearings and journals drop in size about .030" or .031" from front to back. From the measurements I made, If I had to assign rough values to the journals and bearing inside diameters, I'd say front to back...

#1: 2.000

#2: 2.0000 - .0300 = 1.9700

#3: 2.0000 - .0300 - .0300 = 1.9400

#4: 1.2500

These are just close ball park values I took from my 218. I'm sure they off by a few thousandths. They might be useful as starting point numbers / what to expect when buying new bearings / measuring journals.

Edited April 30, 2017 by Jocko_51_B3B

[Jocko_51_B3B]

In my last post I mentioned that I should have looked up the cam journal specs in the shop manual. Well, now another mystery popped up; the cam journal sizes I measured on my 218 (see previous post) are way different from what the shop manual calls for on page 254. For example, the manual says that the #1 cam journal is 2.248 to 2.249. My #1 cam journal measures very close to 2.000. My #2, #3, and #4 are also quite a bit smaller than what the shop manual specifies. Has anyone else taken measurements of their 218 cam journals and bearings??? If so, do they match the book?

Edited April 30, 2017 by Jocko_51_B3B
improve clarity

[Jocko_51_B3B]

And here's another mystery I just ran into: the oil galleries feeding the cam bearings are all the same size - close to 5/16" in diameter. All four of my 218 cam bearings have holes in them that match the 5/16" size of the galleries. So far, perfect. Trouble is that the #2 and #3 bearings weren't using the 5/16" hole. Instead they were using the much smaller 1/8" hole! Could a previous owner have installed the bearings using the wrong hole? In the picture below (#3 bearing) the large hole in the foreground matches the galleries, but the one in the background was the one being used. Is there some reason for a previous owner to have used the smaller hole or was a bad mistake???

Edited April 30, 2017 by Jocko_51_B3B
improve clarity

[Matt Wilson]

I spoke with George Asche a few months go about oil system mods for the flatheads, and he said that the cam bearings should be installed so that the small hole in the bearing is lined up with the gallery hole in the block.  He said this is helpful for maintaining better oil pressure/cushion for the crankshaft bearings, and still provides enough oil flow for the cam bearings.  I'm not sure why the one in the front on your engine didn't use the small hole, but the others probably did for the reason I described above.  George didn't mention what size these holes are, so I can only assume that the ones he deals with are the same sizes as the ones you are seeing in your bearings.

Another oiling system tip he gave me is to open up the size of the hole in the little nozzle that feeds oil to the timing chain.  He didn't remember exactly what size drill bit to use, but just said "it's the first one in the set, I think it's 1/16" diameter."  Anyway, I'm in the midst of a rebuild, and I plan to open it up a little, and I plan to install all 3 replaceable cam bearings with the small holes over the gallery holes.

[Jocko_51_B3B]

Matt,

Thanks for that very reasonable answer to my question. I will use the smaller holes for the #2 and #3 cam bearings. I realize now why the larger hole was used for #1. If the large hole in the #1 cam bearing is not aligned with the oil gallery, then a small hole (about 1/2 inch away) won't line up with the passage that conducts oil to the chain oiler.

Glenn

Edited May 6, 2017 by Jocko_51_B3B
correction

[Matt Wilson]

On 5/6/2017 at 6:31 AM, Jocko_51_B3B said:

Matt,

Thanks for that very reasonable answer to my question. I will use the smaller holes for the #2 and #3 cam bearings. I realize now why the larger hole was used for #1. If the large hole in the #1 cam bearing is not aligned with the oil gallery, then a small hole (about 1/2 inch away) won't line up with the passage that conducts oil to the chain oiler.

Glenn

You know, I was just thinking about that the other day and came to the same conclusion, but didn't think to post here.  Glad you figured it out.