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CAMSHAFT EXCHANGE - TC-302
(written by Mick Anderson)

Pair of MGA Twin Cam camshafts For those who are missing an inlet or exhaust camshaft and are only able to locate one of the opposite type. How do you tell which camshafts you have? Both standard camshafts are identical, except for the position of the timing slot and the bolt hole positions. The lobes on both camshafts have the same lift, the same open duration, and the same radial separation in the same order. Looking at the camshafts from the front, with the timing slot at 12 o'clock, the inlet camshaft will have the first lobe at approximately 7 o'clock. The exhaust camshaft will have the first lobe at approximately 5 o'clock. To convert a MGA Twin Cam camshaft, just mark or cut a new timing slot.

Calculation theory (example):
To convert an exhaust cam to an inlet cam the new marked slot has to be 105 camshaft degrees counter-clockwise from the old camshaft slot, viewed from the front. This is calculated as follows:

Have an engine at TDC on compression # 4 with the original slots both lined up with the bearing cap slots. If you turn the crank 130 degrees (50 degrees BBDC) the exhaust cam will just contact the tappet. If you go back to TDC on compression # 4 and turn the crank again you will have to go 340 degrees before the inlet cam contacts its tappet at 20 degrees BTDC (180 + 180 - 20). That is, the front lobe on the inlet cam is 210 crank degrees later than the front lobe on the exhaust cam, in relation to the timing slots, which is 105 camshaft degrees.

Procedure:
To modify an exhaust camshaft for dual use, mark or cut a new slot 105 camshaft degrees counter-clockwise from the old slot, viewed from the front. If cutting a new slot the old slot should be engraved with the letter E and the new slot with I.

To modify an inlet camshaft for dual use, mark or cut a new slot 105 camshaft degrees clockwise from the old slot, viewed from the front. If cutting a new slot the old slot should be engraved with the letter I and the new slot with E.

As an alternative method, do not use the timing slots at all, then inlet and exhaust camshafts can be used in either position, using a dial gauge. It is assumed that the camshafts are of the standard profile. Set the valve clearance before timing camshafts with the dial gauge method. Turn the crankshaft to approximately 90 degrees BTDC # 4 compression (check the Distributor Rotor position). Fit either type camshaft in the inlet side and turn counter-clockwise in an opening direction until it depresses the # 1 tappet .077" plus or minus .005", just opening. Fit either type camshaft in the exhaust side and turn clockwise until it depresses the # 1 tappet .077" plus or minus .005" (just open). Note that the exhaust camshaft, which runs counter-clockwise, is actually past the peak of the lobe and about to release the tappet.

Move the crankshaft forward 90 degrees to the TDC position # 4 compression, and attach the sprockets and timing chain. The crankshaft and camshafts must not be moved until the chain and sprockets are fitted. The chain should have no slack from the half speed shaft to the inlet camshaft and then none to the exhaust camshaft. Always remove and replace camshafts in accordance with the procedures in the Workshop Manual and Service Memorandum MG251, to avoid pistons hitting valves or camshafts breaking.

Pair of MGA Twin Cam camshafts
Click for larger image
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Some camshafts had a hexagon, to assist turning, between the third and fourth lobe. Some camshafts do not have this hexagon.

The camshafts must not be used unless they have rear plugs (items 2 and 4) and the front sprocket support spindle (item 7) fitted. This is necessary for correct lubrication to occur.

When fitting original cams in original locations the two bolts for the sprocket mount may not be even with the top of the front timing cover. Also when fitting an intake cam on the exhaust side or vice versa, the sprocket bolts will be dislpaced about 105 dergees (one way or the other). However this is not a problem. Because the camshaft front flange fits neatly into the back of the sprocket you can turn the engine easily with one sprocket bolt in place, even if it is not fully tightened. You can then rotate to bring the other hole near the top to fit the second bolt.


Addendum:
At 09:04 AM 7/15/04 +0200, Milan Zahradnicek wrote:
"I will produce cams, plans are finished. There are cams which allows TC engine better performance on revs up from 2000. Tell me how many cams do you need. Production need some time - about 3 months."

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