Last year on a trip up to Aviemore for the ‘MG European Event of the Year’ I broke a rocker shaft on my TA. We did about 580 miles on the 4 day trip up to Aviemore, it was a bit windy and wet at times so I wore my cap with the ear flaps down. About 10 miles from Aviemore I took my cap off and could not believe the clatter from the engine.
On arrival at the hotel, I checked the tappets and found them all OK except No 2 exhaust, which was open to 25 thou, but the locknut was tight! I reset it to15 thou and thought that was the end of my problem.
On the first day we did a 100 mile run and the clatter returned, so again that night I checked the tappets, sure enough No 2 exhaust had opened up to 20 thou. I thought it must be a cam follower breaking up, nothing I can do about that so for the remaining few days of the event I reset No2 exhaust every evening after the runs.
It was the best MG event I have ever attended, so after all the final day celebrations we set off home, ear flaps down! I was fully expecting to call the recovery truck out but 3 days later I drove the old girl into the garage at home and immediately drained the oil. I have one of those magnetic drain plugs and was fully expecting to see bits of the broken cam follower stuck to the magnet, but there were no iron filings it was clean as a whistle!
Next morning I started to remove the cam-followers, I took off the rocker cover, undid the 8 bolts on the four rocker pillars and lifted the rocker shaft off. It fell into two parts having broken through the oil-feed hole for No 2 exhaust rocker (see photo 1).
Photo 1 – showing the broken rocker shaft.
Goodness only knows how old the rocker shaft was, but I had never heard of one breaking before, only wearing out. Over the years I had accumulated a number of complete rocker shafts from old engines that I had stripped, so I sorted out what I thought was the best one with least wear on the rocker pads. It was then that I noticed it did not have the small holes in the top of the rocker arms blanked off.
There was a Morris Engines Service Sheet published in March 1938 that told dealers to blank these holes off by hammering 1/8 inch ball bearings into the holes. If you look at photo 2 you can see various rockers with these holes either open or blanked off by various means, including using pop rivets! In photo 2 from left to right the hole is blanked off by a ball bearing, a grub screw, metal rod (too long sadly) and a pop rivet!
Photo 2 – showing various past ‘solutions’ for blanking off the small holes in the rocker arms.
I found an article by Don Jackson in an old ‘Octagon Bulletin’ #198 dated July 1986. It explains that “these holes should have been blanked off in production, so you are only correcting a fault that shouldn’t have occurred in the first place. The result will be proper pressure-fed lubrication of the rocker bushes, improved lubrication of the tappets, and a vast reduction in the amount of oil swilling around in the rocker cover. You will also gain a few pounds per square inch oil pressure as a bonus!” The diagram that Don drew is reproduced below (diagram 1) and explains the problem. If you have ever run your engine with the rocker cover off, the eight holes look like mini ‘oil geysers’, with loads of oil oozing out!
Diagram 1 –Don Jackson’s diagram referred to in the text.
I stripped the rocker shaft assembly down and started to rebuild it with a new rocker shaft. Be aware that the rocker shaft can only be fitted one way. On an MPJG engine the oil feed to the rocker shaft is via an external copper pipe from the main oil gallery in the block to the cylinder head. The feed goes directly under the rear rocker shaft support post and then up through a hole in the post to the rocker shaft. You must ensure that the oil-feed hole in the rocker shaft is at the ‘six o’clock’ position for the rear pillar support, if you get this wrong there will be no oil fed to the rockers. Once this is done you can then start to build up the rocker shaft assembly, starting from pillar number three make sure that the locating spacer washer fits in the key groove in the shaft. I have several rocker shafts with the key groves on different sides, all this means is that you need to turn the rocker pillar though 180 degrees.
In my case, before building up the assembly I had to clean and inspect each rocker arm and then block off the small hole in the top. The method I use is to tap the hole out with a 4BA tap, blow through with an airline to remove any swarf, then screw in a very short 4BA Allen headed grub screw secured using a bit of Loctite. I used 1/8 inch long Allen key headed grub screws purchased off eBay.
I found that I could tap the hole out without drilling it out to the correct 4BA tapping size (see photo 3).
Photo 3 – showing the equipment and materials used for blanking off the holes in the top of the rocker arms (which shouldn’t have been there in the first place!)
This mod is well worth doing on XPAG engines as well, I spoke to my old Aussie mate Mike Sherrell, he told me that he does this mod on all XPAG engines that he builds. Since fitting the new rocker assembly the engine is now a lot quieter, but sadly not as quiet as it should be! Maybe a new set of cam timing gears and a new timing chain will quieten it a bit more, but that can wait for the time being!
Ed’s Note: Brian is Chairman of the MG Octagon Car Club and is also Registrar for the Tickford models, a job he does for the ‘T’ Register of the MG Car Club.