MGB & GT Forum

A little over a month ago, I pulled the head off my car for what I thought would be a two or three evening job.

Basically, I was getting low compression in cylinder #3. I saw this as a chance to get a performance upgrade and raise the compression, so I bought a rehabbed 12H2923 head and had it ready to go.

I dropped the head back on the car, torqued it down, and that's where the problems started.

It seems as though the valve height on the rehabbed head was about .200 higher than stock, which meant that even with the adjuster bottomed out I couldn't get the valves to close completely.

So, that started a parade of parts.

First of all, I went to Basil and ordered a set of trimmable pushrods in the 10 1/2" 18V stock length along with a set of "bucket" 18V tappets(might as well upgrade those if I'm venturing into the engine). I trimmed 1/4" off, but that left me at the OTHER end of the adjusters and I couldn't fit the jam nut onto them.

So, I went back to stock 18V pushrods(fortunately I have a parts engine I could rob those from) and went from there. Once again, my adjusters bottomed out, as was not a huge surprise. So, at a couple of folks suggestions, I shimmed the rocker shaft to make up the difference.

Initially, I trimmed some 5 thousandths brass I had lying around, and that was just enough to get things to the point where they would run. I was able to get a flat 170 psi across all cylinders, and once I had everything put back together except the cooling system I figured I'd get brave. After realizing that I'd hooked the plug wires up wrong and running the battery down in the process I finally hit the key and it fired right up. I only ran it about a minute since I didn't want to take a chance with no cooling, but it ran.

I still wasn't happy, though. With the 5 thousandths shims, I had very little adjustment left in the adjusters. After hunting around at local sources, I finally gave in and ordered a piece of 20 thousandths brass from McMaster Carr. I managed to get the shims made up yesterday, and so finally I was ready to really make things happen.

I bit the bullet and fought the heat and humidity last night to get things put back together. The shims went in, and of course the head got a retorque. With the valves set at 15 thousandths, I'm about 1/4 of the way into the adjuster screws so have some room for adjustment once things settle and of course as the engine wears.

Finally I got the cooling system back together and filled, tied up some other loose ends, and proceeded to fire it up again and fired it up once again. I initially got an anti-freeze shower, but tracked down the source of that(mostly a thermostat housing that needed a bit more torque and one loose hose clamp).

After a quick timing check(I backed it off a bit for now, about 30º all in mechanical, just to be safe) and tweaking the carbs I decided it was time for a test drive. I filled the tank up with premium, went a mile up and down the road, hit some hills to check for pinging and other issues, and finally came back home. I'm not TOTALLY happy with how it's running, but it's definitely a lot peppier. I'd have driven more aside from the fact that it was pouring rain. I stopped a few times along the way in bank drive throughs(it was 7:30 in the evening) to make tweaks to the car , but it's definitely a lot easier to just be able to pull into any parking lot and do what you need to.

I have to give MANY thanks first of all to James Booker for supplying a beautiful rebuilt head in the first place, and then to Basil, John Twist, and Dick Moritz for their HUGE amount of technical advice as I fought my way through this. The MG community truly is great.

Ben,
Nice to read you are back on the road.

Is this a typo?

"the valve height on the rehabbed head was about .200 higher than stock,"

B

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Life's most persistent and urgent question is, "What are you doing for others?"

No, that's not a typo.

Ben
Were you experimenting with rocker arm ratios?

What did you end up with?

B

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Life's most persistent and urgent question is, "What are you doing for others?"