The following was taken from the 'MGCC Y-TYPE REGISTER' at http://www.mgccyregister.co.uk/technical-information/the-xpag-files/xpag-rebuilds/xpag-engine-rebuild/xpag-engine-rebuild-questions-and-answers/ on 7th May 2016. Numerous other questions on XPAG rebuilds are answered on the same page.
This is an old nut. I’ll copy an old article from years ago below to explain. It is VERY IMPORTANT to remember that the little brass tag on the rocker cover may, (1) not be original to your engine, (2) your cam may have been changed a number of times since the engine was new, and (3) I’m convinced MG used up lots of 0.019” rocker covers on engines that had 0.012” camshafts.
XPAG Camshaft Checking.
This is the most important part of the engine in terms of performance and drive-ability, though good cylinder head design does help. As years pass, people modify their cars, or fit replacement items that have been improved by the manufacturer. One problem that often occurs is trying to decide which camshaft your XPAG has fitted, without stripping it down. Many cars have the 0.012″ clearance camshaft, but some still have the older 0.019″ clearance version. By far the best method I have seen was in the MGCC Safety Fast magazine, sent in by David Clark of Westminster, Vermont, USA. It relies on the fact that only the later cars had 5;45;45;5 cam timing, known as equal-overlap, (or split-overlap if you speak American). That is both inlet and exhaust cam have the same timing, but in ‘mirror’ fashion. The earlier ‘T’ types had 11;57;52;24, very un-equal. The first pair of numbers read as inlet opens BTDC, closes ABDC, then exhaust opens BBDC and closes ATDC. The XPAG is not a quiet engine anyway and one to worry about is one with no tappet clatter, as it is better to hear it than not. In today’s lead-free petrol age, no noise means the valves are pocketing, ie eating away the seats, and closing up the clearance.
If David Clark’s check is carried out, and you decide you have a 12 thou cam, but performance is awful, you actually have one of the mid-way 5;45;45;5 1140cc timed and ground camshafts, but with an 8mm lift at the valve for the M.G. 1250cc XPAG. See modification list. This cam still has a 0.019″ tappet clearance. Conversely, running a 0.012″ cam at the 0.019″ clearances sounds almost like a diesel engine.
“I would like to share with you a cheap and cheerful method for making an accurate determination of high verses low camshaft with your feeler gauge, a screwdriver, and a five-sixthteenth’s BSF spanner.
Because of the equal overlap of the valve timing, of the 0.012″ clearance XPAG 5;45;45;5, they are split evenly about top-dead-centre ( TDC ) and bottom-dead-centre ( BDC ). This is called split-overlap.
Now, to check the valve timing, adjust the valves to the recommended valve clearance, in this case 0.012″. Turn the engine on the starting handle until the fan belt pulley on the engine indicates TDC for number one cylinder, with the valves ROCKING. This means we are at the end of the exhaust stroke and beginning of the inlet stroke, with both valves partially open. At this TDC loosen the locknuts and turn the tappet adjusting screws all the way up and then down again until there is exactly no clearance at the valve. The valve will now be shut, and the adjusting screw just touching it. Now turn the engine one more crankshaft revolution ONLY, ( till you are now at the top of the compression stroke,) turn until the TDC marks on the pulley once again align exactly. Using your feeler gauges measure the resulting valve clearance. If the clearances match, you have a split-overlap camshaft, possibly with 0.012″ design. If you are out five to ten thou, with the differences being from keyway tolerances, etc, it is still a split-overlap cam. If it is an earlier, or fast cam, the clearances will differ a lot.”
Camshaft lobe design is a very precise art, and the shape is very important. Morris & M.G. strove to get the best with silence, but a compromise was the result. The .012″ gap was it. The gap is part of the camshafts design and has to do with the ‘ramp’ and its acceleration of the valve lifting gear. Stick to the clearances given.
| Model | Cam Timings | Lift | Part No.(if known) |
| YA 1 1/4 ltr. Saloon | 11;57;52;24 | 6.5mm | MG900/106 |
| YA after SC/16831 | 5;45;45;5 | 8.3mm | AAA3096, 168553 |
| YB 1 1/4 ltr. Saloon | 5;45;45;5 | 8.3mm | AAA3096, 168553 |
| Wolseley 4/44 | 5;45;45;5 | 8.3mm | AAA3096, 168553 |
Again, these cams were fitted when your car was new, virtually any cam might be fitted now. To explain the 5;45;45;5 above see the paragraph on TDC, BDC, etc.
The inlet valve timing is always given first, in this case the inlet valve is opened 5 degrees BTDC and closes 45 degree ABDC. The exhaust valve is next and it opens 45 degrees BBDC and closes 5 degrees ATDC. So the inlet valve and the exhaust valve ARE BOTH OPEN at the SAME TIME between 5 degrees BTDC and 5 degrees ATDC, a total of ten degrees of crankshaft rotation called ‘overlap’. This is a very mild camshaft timing, sports cams have much more ‘overlap’.
© David James 2016 Last updated: 7th May 2016