After FAR too long, the MG is back on the road-or at least I drove it a couple of miles yesterday. I'd have driven it more, but it was raining, hot, and I needed to tweak some things that I didn't really feel like fixing at that point.
So, here's sort of the summary, even though I've mentioned specific stuff. This is what you call "project creep."
First of all, I started noticing low compression on one cylinder(#3). This particular cylinder is notorious for burning exhaust valves on B series engines, so I pretty well wrote it off then and put fixing it on the to do list. With that said, I started losing a couple PSI of compression a month, so figured it was a sooner rather than later job.
From there, I had a decision to make. B heads are also notorious for cracking. The particular casting number on my head isn't as bad as some others, but there was probably still a 50% chance of it being cracked. A rehab on it(magnaflux, skim, hardened exhaust seats, and lapping in new valves) would be $250 or so, and there was a chance I'd still be out ~$75 if I took it to the machine shop and magnafluxed it and found a crack.
So, I took the chance to both head that off at the pass and make an upgrade. I located a freshly rehabbed(new valves and hardened seats) 12H2923 head, which is the desirable "big valve" head. These were used on '73-75 engines. Those particular engines have lower compression than the earlier ones(8.0:1 vs. 8.8:1) but that was achieved by actually reducing the combustion chamber volume and fitting deeply dished pistons. What that means is that if you put a late head on a high compression engine with shallow dish pistons, you end up with roughly 9.5:1. That's a nice quick and dirty power gain-it gets the engine into the 110-115 hp range vs. 90 stock.
It SHOULD have been a quick and easy swap...famous last words. I'd figured on about a week, working in the evenings. I should have had the old head off in one evening(there's not a lot going on with a 4 cylinder OHV tractor engine), been able to do the clean up and prep work in one evening, get the new one on and everything back together, and then tuned it up.
Getting the old head off wasn't overly eventful other than forgetting just HOW low I needed to drain the antifreeze, but it came off without a lot of trouble. Similarly, clean up and prep went off without much trouble. I got the new head dropped into place and torqued down, then went to get things set up. Unfortunately, that's where the trouble started.
When I went to tighten down the rocker shaft and then set the valve lash, I found that the even with the micro adjusters backed all the way out, I couldn't get ANY valve lash. In fact, I couldn't get any compression out of the engine as the valves weren't closing all the way. A little bit of measuring showed that the valves on the replacement head have their stems sitting about 200 thousandths higher than stock(I measured two heads I have kicking around).
That sent me down another path of replacing parts to try and make the head I have work. MGB engines came with two different styles of lifters and pushrods. The early ones had "tall" tappets(about 3" tall) with "short" pushrods. The later ones used tiny "bucket" tappets with long(10 1/2") pushrods. The later design is preferred as it's much lighter.
So, I ordered a set of bucket tappets along with some fancy hollow racing pushrods(at $13 each) that came disassembled. The tubular design and the fact that they're hollow meant that I could trim them. Given the valve height difference, I stuck them in the lathe and knocked .250(1/4") off each and put it together.
Unfortunately, THAT combination put me at the other end of valve adjustment and I couldn't get compression. Even if I'd been able to, I wouldn't have been able to put a lock nut on the adjusters, which is a bad thing.
So, out of desperation, I ordered a set of stock 18V(late, long) pushrods to the tune of ~$20 for the set. That put me right back to where I was with not getting any lash.
I went to other options-one of which was to mill the rocker pedestals shorter and use my(too short) push rods, or the other of which was to use the stock push rods and shim the rocker pedestals. I went with the latter.
With some 5 thou brass shim stock and a few minutes on the drill press, I had a set of shims and at last had good compression. I went ahead and bolted everything back together and was able to start the engine on Monday evening. I didn't run it long as I hadn't refilled the cooling system at that point, but I did start it.
With that said, I still wasn't happy with the valve adjustment, as I essentially had no room left for wear. I went hunting for thicker shim material, but couldn't get what I wanted locally. Finally, I broke down and bought some 20 thou brass from McMaster Carr, which came Tuesday. I got the shims made yesterday(they weren't as easy to make as the 5 thou ones), installed them, got the lash set correctly with plenty of room, and fired it up(this time with coolant). That was a joyous moment.
I had to trace down some serious coolant leaks that left steam pouring out of the car
, but was able to drive for a couple of miles. Of course, it was raining like crazy last night so it overall was a miserable drive(it was too hot to run the heat for very long and I couldn't roll down the windows due to the rain, which meant that I was constantly using a towel to wipe the fog off the inside). Granted it was 7:30, but I'm sure that I had a bunch of banks scratching their heads over their security cameras this morning as that was where I stopped to make on the road tweaks(so I could stay under cover).
About halfway through, though, I started getting a terrible popping sound when I stepped on the gas from the passenger side of the engine. I was afraid it was pinging, so jumped out and backed the timing off but that didn't make a difference. A fresh look today fortunately revealed that one of the grub screws sealing the exhaust ports that would have been attached to the air rail had fallen out meaning that I was hearing an exhaust leak from cylinder #1, so fortunately that was a quick thing to fix.
I still have a fair bit of tuning to do to get it where I want it, but it's amazing how much more pep the car now has. It was just a thrill to get back in the driver's seat of it, though.
Unfortunately, I've also fallen victim to a typical bad part that I'm rather miffed about. I couldn't get my old temperature sender out of the head(I broke it off) and didn't lose much sleep over spending $7 on a new one. I noticed when I was driving it last night that the engine almost at "H", although a check with my IR thermometer showed that temperatures were right where they should be(~200ºF). A bit of searching showed that apparently Moss Motors(largest British parts vendor) has apparently for the past year or so has had the specs reversed on the 68-76 and 77-80 senders. Apparently I "should" have ordered a 77-80 to have it read correctly. NAPA carries the part, so I'm probably going to get it from them and then let Moss hear about the bad one. Let's face it-I've spent a small fortune with them in the time I've owned the car and this is the only part I've really had an issue with(or enough of an issue to complain) so I don't think that's too much of a concern. I just dread changing it as I know I'll make a mess doing it(it screws directly through the head into the water passage under the thermostat).
