The MGA With An Attitude

At 10:44 AM 1/13/2009 -0800, Max Heim wrote:
>>"If I were going to go to the trouble of pulling the cam I would certainly replace it if at all indicated -- what I am resisting is removing it in the first place."

Ah, one of my favorite subjects, saving parts that are not necessarily trash.

>>"I am determined not to get involved in "shipwright's disease", by doing the absolute minimum repair required. On this car, "might as well while I'm at it" would logically devolve into a complete rotisserie restoration".

On-line advice varies widely on such a question. The pro shops have a vested interest in selling parts and services. They also have a vested interest in doing an engine job up such as to minimize the probability of it coming back to them failed in an unreasonably short time, regardless of how good the probability is that a cheap job might last a long time. In other words, they don't like a 10% probability of downside against a 90% probability of upside, because they think the customer would blame them for the failure if it happened. Even when the pro is expert enough to fully assess the engine condition and make such logical decisions, he will likely not convey the same information to the customer. The problem there is lack of communication, or no confidence that they can give the customer a firm grip on all the facts so the customer can make a good informed decision about the risks versus costs, considering his own position of financial affairs and other personal priorities.

In between the experienced pro and the rank know-nothing amateurs lies a full range from total incompetence through slight lack of full confidence. Many of these people will be hedging their advice in similar manner as the pro, not wanting to chance being wrong in the end and looking like fools in public. So these people too will be likely to advise you to spend more money for more parts, just to be on the safe side and not take "unnecessary chances". What they are not taking into account is that it is your money that they want to spend to make themselves look good, and no consideration for the up side of spending less money and possibly succeeding. Opinion can be oddly tilted when "it ain't their money".

I come from the School Of Hard Knocks with a shoestring budget being quite familiar. As a result, even when I may have enough money I don't believe in spending big money when it may not be needed just because I might be a little worried about a small possible downside to the cheaper proposition. As a career machine design engineer, and a long term MG owner doing my own maintenance, and not being financially or professionally vested "in the business", I can stand in an objective (non-biased) position and shine a light on the truth with my best judgment of the balance between cost and value. I do this often for my own car when I think a cheap shortcut has a good chance of succeeding and saving lots of time/and/or money, especially when the possible down side is not very expensive.

From prior information, your tappets are badly worn, but the engine was running okay otherwise. If you leave the badly worn tappets in there they will surely eat the cam lobes a whole lot quicker than you would like. For a moderately small cost a new set of tappets will most likely prolong the life of the cam somewhat, no matter what the current condition of the cam. I cannot imagine any circumstances that would make new tappets wear the cam faster than old badly worn ones. The worst possible down side is that you might spend the money for a new set of tappets and a few gaskets, and then find the cam totally shot in another 10,000 miles and have to do the cam and tappet replacement anyway. If nothing else that smaller up front cost of just tappets can put off the cam replacement for a while longer while you might get your finances in order for the more costly cam job. It is therefore a reasonably cheap means of buying some more running time before you have to make the expensive decision. In any case, new tappets are not going to make the problem worse. If the cam should ultimately fail in short time, then it was going to fail anyway, and quicker if you hadn't changed the tappets.

The possible up side (aside from the running time extension) is that a new set of tappets might keep the old cam going for another 50,000 miles, at which time the entire engine might be tired enough to need a complete overhaul anyway. Or by that time you may no longer own the car or the engine (for whatever reason) and may avoid the cost of cam replacement all together.

One key to this decision has only been touched on so far, in that someone recommended giving the cam to a pro for inspection and analysis and the professional opinion. I think I already mentioned the common mind set of most pros, and they may not always have your best interests in mind. The difference between an expert and a professional is that a pro by definition gets paid for what he does, while an expert knows what he is doing. A pro is not necessarily an expert, and the expert is not necessarily a pro. I would describe myself to be an expert who is definitely not a pro. You really would like to avoid the pro who is not an expert.

Pros who are also competent experts are more rare than you might believe. I have personally gone through about a dozen engine shops in the past 40 years, maybe 6 of which I wouldn't do business with any more because they are incompetent, 4 of which refuse to do business with me any more because they don't like the implication that I may be trying to tell them how to do their job when I chat about technical issues (which is okay with me), and one who is dead and gone. That leaves one competent pro engine shop that I like and trust and can do business with on a regular basis (for about 15 years now), and I hope the shop owner does not close up shop or die before I do.

The better solution to this problem is to get enough information and understanding about cams to make a decent analysis and judgment for yourself. In other words, learn enough about it to become a minor expert in this very narrow field of engine parts so you don't need to rely on the possibly biased advice of a pro. I can help here, as it is my hobby (or addiction) to assist other MG owners in do it yourself maintenance.

The first key bit of information that you would like to have is something about the current condition of the cam lobes. Cams are surface hardened by one of a few different techniques, meaning the hardness is only skin deep, possibly as little as 0.003-inch deep. When the cam lobe is at full height it will have a very long life ahead. Being worn down as much as 0.002" is not criminal, and it may still have some good service life available. Once the cam lobe is worn down more than 0.003", the wear rate accelerates very rapidly. Cam lobes should all be the same height with in 0.003". If you find even one cam lobe apex that its more than 0.003" lower than others, then you can figure on relatively short remaining life of the cam even with new tappets, probably less than 10,000 additional miles, and possibly a lot less depending on how low it might already be.

If you had the cam in your hands you could easily measure the cam lobes with a dial caliper. Measure the width across the base circle, and the height from base circle to apex. The difference between those two measurements is the cam lift, or the amount of vertical motion it will impart to the tappet. It is generally sufficient to compare the height of all cam lobes at the center of the lobe to be sure they are all the same height with in 0.003", implying that none of them have been worm through the hard surface. If you find a variation of .005 " or more you may consider it to be junk and in need of immediate replacement.

You can also measure the cam lift without removing the cam from the engine. For this you drop the originally matched worn tappet (or a new tappet) onto the cam. Stick in a pushrod, and mount a long travel dial indicator securely on top. Rotating the crankshaft (and consequently the camshaft at half speed), you can set the dial indicator to zero at bottom of travel, then read the cam lift directly on the dial indicator when it reaches top of travel. This can be accurate to the nearest 0.001" for good comparison if you think you may have a good cam.

As an initial short cut you might be talented enough to roughly measure travel of the pushrod with a dial caliper. This might be accurate with 0.001" to 0.005" or so (+/-) depending on your ability of holding the caliper steady during measurements. It can help to have an assistant turning the crankshaft for you, and the original starting handle (hand crank) is a nice tool for that job. If you find one or more cam lobes to be 0.010" or more lower than others, then it's a no-brainer, and you can skip the long travel dial indicator measurement and scrap the cam. This is as good as the analysis gets without removing the cam from the engine, and you don't need to be a pro or an expert to do it.

If you end up finding after measurement that you have to replace the cam because it is definitely worn out, then at least you know the facts and you can enjoy the new cam with the confidence that the money was well spent and not wasted.

Without any long explanation I can tell you from long experience that cam bearings never wear out in these engines, unless the oil was never changed for 50,000 miles, in which case the whole engine might be shot anyway. When the engine is hot tanked for cleaning during machining the cam bearings would be ruined. Original cam bearings might be removed prior to hot tanking and reinstalled after hot tanking, but since they will be removed anyway, no one would skip buying a new set for a meager $20 and tossing the old ones out (except maybe me with my own engine).

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