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Fitting camshaft bearings - one for the Spitfire gurus!


JohnD
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No!   The bearings fit the standard journals, so the block must be bored.

Yes, no doubt that ever-pennypinching Triumph abandoned them to save money.

So to return to the question, how much smaller than the bearings' OD should the block be bored, to achieve a satisfactory interference fit?

In case you haven't seen one, they are complete rings, unlike main and big end bearings.   The steel backing has a 'jigsaw' shaped joint - see below.   This cannot provide much movement :

 image.png.b062303930415bf853656f683feb5c31.png

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Hello John

                  The WSM gives 1.9659 - 1.9664 for end journals  

                                             1.9440  - 1.9654 for middle ones

Haynes gives the same and also 1.9680  - 1.9695 for the block

Roger

ps large journal cam take one from the other and you get 0.125" give or take a bit so 0.0625" shell thickness! (I mean small journal cam)

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If you had to bore the block out, why would you have small and large journal camshafts? They would all be the same size??

The bearings that fit with a standard 6-cyl camshaft are possibly designed to repair a block that has been used with overly strong springs and it has ovaled the journals.

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the reason for small and large journal was bean counters at a guess. Originally the cam was a small journal that ran in the block. They then decided to put bearings in the higher performance spit I guess for reliability, but didn't want 2 sets of block tooling so they bored all to the same and had small journals cams for with bearings and large without. I'm guessing that they had a deal from the cam suppliers on a per unit basis. Then they decided the bearings were unnecessary cost so just went with the large journal cam from about 1972. 

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Your are trying to mimic the Spit Mk3 arrangement with the larger cam-way bore (50mm nominal) and smaller cam journals with wrapped bearing shells between them. There the block needs boring out to 4 cylinder block specs as shown below in this extract from a 1300 FWD O&M

3E44C71C-0CD6-44EA-B09A-3DDE526FAAD8.jpeg.6b668123ab2514be003784f85c169ab5.jpeg

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A lot is going to hang on the accuracy of the machine shop equipment and the skill of the operator, getting close tolerance. My take, looking at the Spec; table is that the Fitted Clearance tolerance is around 0.002"?, unless I am missreading.? With that in mind the Interferance fit to the Case, would need to be such that the design clearance is not reduced to the point where the "Oil wedge" cannot form and point contact takes place?. My "gut" suggests to me that 0.001" would be the optimum, enough, but not too much that the bearing "fit" would be "tight"?.

Pete

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Thank you, All!

I fear that Spitfire fitment is a red herring, as AFAIK, all six cylinder camshafts had the same sized journals.

The camshaft I want to fit is with the machinist, but again, AFAIK, it has the same sized journals as a standard camshaft, of which I have several, and they all have journals the same, all along the shaft.    I've measured the ODs of this one, and the ID of a set of camshaft bearings I just happened to have on the shelf.   They agree at  1.865".  See pic.   So I don't where 1.9... comes from except the Spitfire engine.

Thnak you, Pete, for your gut instinct of one thou tight.   I have found a Rule of Thumb on the site of a non-Triumph engine builder, 0.7-1.5thou per inch of diameter.    This means 1.3 to 2.8 thou and maybe 1.5 thou tight will do the job.   If it's too tight, then I must ask the machinist to hone the bearings.

John

 

IMG_20210718_193304.jpg

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they only have one set of figures for the cam journal diameter for all mk3 herald 1360 etc I believe that this is wrong the spit mk3/1300 fwd cam fitted with bearings has the same diameter as the early herald cam journals so 1.8402 - 1.8407. I have a block in the garage that had a mk3 cam in the early 1147 block and was a factory recon unit. But agreed that still doesn't match the figures that you have from the 6cyl cam

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Hi John, 

we had the same issue when putting cam bearings into the GT6 Engine. 
I had a mate boring a 30mm thick iron plate with a 50,000mm hole. Yes, precision Workshop.

We pressed a test bearing into it. Worked fine. Thus the engine workshop was instructed to bore the cam journals to 50.000.

I installed the bearings with a long threaded bar. I made some mandrels for this. In addition some Loctite was used.

Engine works fine towards 7.000/min.

Bearings came from CC or Rimmers. Cant remember. County stuff.

Martin

 

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I fitted a genuine Mk3 cam to my Spitfire which needed the bearings as it has a 13/60 GE block.

I used the old GE cam as a drift to fit the bearings.

Worked perfectly, just check the oil holes line up well.

Would the 6 cylinder be any different? Could be worth a try. If the cam failed to drop into place after fitting the bearings it would point to issues....

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