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MG MGB Technical - No compression on 2 and 4
|Thanks to everyone who helped on my GT rough running thread.|
I got the problem sorted so I decided to adjust the tappets while the tools were out. All adjusted great. Much quieter. I noticed that the adjuster had been changed on valve 4. hmm maybe it had a problem in the past which might explain the oil in that cylinder?
So out for a drive, a mile down the road and ....BANG TAP SLAP TAP TAP TAP. Back home rocker cover off and the adjuster nut on valve 7 had broken. Took one off my midget and all was fine once more, but I had another misfire right through the range. Thought I'd do a compression check. both 2 and 4 go up to 120 ish but don't hold the pressure, as soon as the enighe stops turnng the compression dies. A wet test had the same results, so I guess the valves are not seating?
1 & 3 have 150 and hold it until I let it off.
I now have the head ready to come off, but it will not budge! Soft hammer, leaver on alternator bracket, it won't bloody budge.
What a day!
|You may need to turn the studs out using locknuts. Before that try giving it a whack from alternate sides using a large hammer via piece of wood.|
|I'll try that Stan, thanks.|
I am doubting my compression check now. It is right that the gauge should hold the reading isn't it? I did do the test twice on each cylinder with the same results, held on 1 & 3, did not hold on 2 & 4.
|Graham. Slip some rope through the spark plug holes of number one and four cylinders, then, turn the engine over using a spanner. Should lift the head off for you sufficiently that you can remove it. Learned that trick here and used it on my last rebuild. Worked well.|
As to the compression leaking down, that can be either a bad gauge or leaking cylinders. Since the gauges I own have a push release that has to be pressed to release the pressure on the gauge, I might suspect a gauge problem and would have cross checked the readings with a second gauge and based my judgement on that.
However, the broken parts in the rocker arm would, to me, be sufficiently distrubing that I would have pulled the cylinder head, examined to see with, if any damage had taken place, and checked the pushrods for straightness. I would use a Payen or Fel-Pro resin type head gasket when reassembling.
Let us know what you find if you have time.
|The gauge should hold the reading, mine has little chrome button you push to release it. Once the air is pumped in there it should stay, nothing to do with the cylinder. You should see the dial swing round the gauge with every compression stroke, eventually cranking the engine does not increse the reading, and thats the number you use.|
|Stan, Les, Thanks.|
I thought that the gauge should hold the reading. Odd that it didn't hold on the same two cylinders on both tests then?
I'll try the rope trick this morning and update you on what I find.
I also broke the head off one of the bolts which runs through the heater control value(?) the one which comes out from the dizzy side of the head.
|The head is still stuck fast. I can lift the back by about 1 cm, but the front is solid. There are three studs which I can't remove. the head moves free of two of them (that is if I rock the head by my 1cm, the head moves the studs don't) but one stud moves with the head.|
I have tried tapping it, lots of WD40 and mole grips. I have tried putting two bolts on the stud and undoing the bottom one, it just strips the nut thread.
How does a stud extractor tool work? Any more ideas?
If you have 1cm of movement you are doing OK. Make sure now that you keep the head a little more level and try to "walk" it up the studs. It will come, but it's heavy.
|AT LAST! My new favorite tools, stud extractor and power bar. The head is off (30mins to remove all surounding items and head nuts - 7 hrs to remove the head from the block!)|
OK, back to the compression problem....
The head and piston tops are quite coked up, I have removed the valves and although they are coked none look really burnt out. The valve seals seem ok, at least they all gave a nice pop as I pulled the valves out.
No 2 has obviously had some oil in it and the inlet valve is the worse of the lot but there is nothing jumping out as to the problem? Just how bad does the coke need to be on the valves need to be for the compression to escape? The valve seats all look ok.
Cylinders look clean and there is no signs of scoring.
I just don't want to miss anything before I restore the head and refit it.
|Graham. I see that Chris Betson has posted today, hence, his website address is available. Chris has some better grade cylinder head studs available which I am starting to use on my rebuilds. Stronger than the Rover studs, one of which I have had break after installing. I was fortunate, it broke at the top and was easy to remove. Others have had similar problems with the stud breaking at the bottom and making removal more difficult.|
My website, which you have been to, has some notes on the bolt sizes associated with the MGB engine, specifically the 18V I most commonly work with. There is also another article that goes into what taps and dies are and how to use them. Running a tap into all of the threaded holes on the cylinder head and the cylinder head stud holes on the block is an excellent idea. The tops of the head stud holes should be then, lightly, countersunk and the tap run in again to clean up any minor damage due to the countersinking. This is a standard process when rebuilding as it helps to ensure the cylinder head gasket will remain flat and cannot be pulled slightly upwards around the studs as the cylinder head is torqued down.
Yes, under most conditions, a stud extractor is a great device. Does not always work, but works a sufficient number of times that I keep a complete set in the garage.
When a stud extractor does not work, as is common with exhaust system parts, using two pair of Vice Grips (Mole Grips), one below the other to form a T handle, will often allow you to twist the stud a little tighter and a little looser, eventually releasing it completely and allowing it to be removed. You have to break down the bond of the rust which has caused the parts to bond together. I have never broken a stud when using this method, but, it is possible to do so.
The amount of coke build up will vary. Back in the 60's, it was common to have quite a bit due to the fuels and oils being used back then. Today's fuels burn more cleanly and the oils seem to keep things cleaner. I do not find the sludge build up in the bottom of the oil pan (sump) that I used to back in the 60's. Modern oils do provide a better mechanism for removing contamination and the oil filters may well be better also.
The coke (carbon build up) can be cleaned using a wooden tool for the pistons and a powered wire brush for the cylinder head combustion chambers. When the head is clean, you can see if there have been hardened steel valve seats pressed into the head. If not, consider having an "unleaded conversion" done to the head.
Inspect the pushrods for straightness. If you had a damaged rocker arm, the pushrod might also have been bent in the process. They are, however, a strong item and are usually in good condition.
Glad you got the head off. Please keep us informed as you have time to do so.
Thanks for the tips.
I would love to get an unleaded head, but money is tight right now so I am going to have to put that one off for a while.
How can I check if the piston rings are ok? If I am loosing the compression via the piston is there any way of checking while the head is off?
|Graham. I do not know of a reliable method to check out the sealing of the piston rings when the cylinder head is off. The normal procedure is to, with the cylinder head installed, pressurize the cylinder with an outside air source, through a connection screwed into the spark plug hole, and listen for escaping air. If you hear air leaking into the sump, the rings might not be making a good seal. |
In your case, I would run the pistons downwards and look for scratching, wear, on the cylinder bores. Also, look for a lip on the top of the cylinder which is an indicator that the bore has worn. You can also run the pistons up to the top of the bore and apply sideways and fore-aft pressure at the top to see how much movement there is. But, this is only a "rough and ready" form of inspection, in no way definitive. Significant movement might indicate a broken ring, as might scoring of the cylinder bore. However, most of the wear takes place lower in the cylinder bores and is hard to check out without the proper measuring tools. If you know of anyone who has a set, or, if someone here is available to bring some over, it would be possible to gauge, accurately, the amount of wear on the bores and see if that contributes to your problem.
In your case, I think I would put the system back together and hope for the best. I have had to do this when money was tight and it usually worked out well.
|I agree , grind the valves clean all the coke out plenty of copper slip new studs and put it back together. did you have enough of the heater valve bolt left to get on ?|
|Stan, Yes just the head of the bolt broke, so I should be able to get that out ok.|
Les, there is some fore and aft movement, but it seems the same over each piston.
Another problem I had was taking the carbs off, The linkage between came off, so when I put these back will they need setting up again, or does the linkage only go back one way?
I also see there are two types of gasket at Moss, copper and composite. What are the differences, which is best?
I started cleaning the piston tops and block, there is much more carbon than I first thought. In the past I have had engines where the carbon would cause pinking, but I have not seen it loose much compression.
|Graham. I use the American Fel-Pro upper gasket sets which have a resin type headgasket marked "Made in England". This either is, or is very similar to, the Payen headgasket. My experience is it is the best one to use on a road car.|
The copper gasket is, generally, simply a gasket which has been stamped out of a sheet of copper. It was much used by racers back in the old days.
The standard, composite, gasket is a blend of some material, formerly asbestos, and a thin sheet of steel. My experience is the Payen gasket is superior to both on a car capable of being driven on the road and I have used it exclusively for the last several years.
As to the carb linkage, what came off was the piece going between the two carbs and joining the throttles and the throttle cable. It is a piece of 5/16" steel with some form of attachement to connect the throttle cable to. No problem. Just re-install it and your carbs should be a well set as when you removed them. Bit of juggling to get the carbs back on while keeping the linkage in place between them. If you have a problem, take a break, then go back to the job. Frustration is your biggest enemy here. Amazing how after being ready to toss the carbs across the room, a break and a beer make things better and they go back perfectly when you go back to them.
Take your time. Expect to make a few mistakes, which you will learn from. This is not a complex job, but, can seem that way the first time you do it. I have done it. Many others have done it. Work carefully and thoughtfully. You can do it to.
I hope you picked up on my mention of replacing the cylinder head studs. There has been a lot of information flowing and it may have gotten lost.
Studs and bolts get their clamping power by being able to stretch. Over the years, and it has been 26 years since the newest of MGBs have been produced, the steel may have fatigued while in use. This results in one of two possible outcomes. First is that the stud/bolt breaks when being tightened. Second is that, even though it does not break, the steel does not have sufficient strength to stretch and provide the sufficient clamping power. Remember, that our torque settings are relatively meaningless. Torque, applied to a stud or bolt, is only an indirect measurement of stud/bolt stretch. If the stud/bolt is sufficiently fatigued, it will either break, or not stretch properly. Both cases are bad things because either will mean that the cylinder head, or other part, will not be properly clamped.
Thus, old cylinder head studs, at some point which we cannot determine, will loose their ability to stretch and provide the proper clamping force necessary to a secure seal of the cylinder head and the cylinder head gasket.
Chris Betson can provide upgraded cylinder head studs, stronger than the originals. Do not know what the price difference would be in the UK. I considered them, shipped to the US, to be a bargain as compared to the Rover studs (one of which broke in initial tightenting in one of two rebuilds) and the more expensive US made studs. Chris told me that he uses these studs in all of his rebuilds and the people who have used his engine rebuild services seem quite happy about the quality of the rebuild.
Even when money is tight, this is not something to fail to replace.
Hope all goes well. Les
|Les, thanks, just looking at the stud and nut kit on his site. I have had studs break on me during rebuilds, its soul destroying. I think I'll change the manifold studs too as these look a bit rough.|
I'm at work now, so can't see the head. I remember it having rubber(?) valve stem seals, I can't seem to find these anywhere?
|Graham. Moss, US, sells the rubber valve stem seals as part number 297-108. Every upper gasket kit I have purchased has included a set, so they should be available in the UK. Peter Burgess has mentioned the use of the umbrella type seal as being worthwhile. Why not give Peter a ring and discuss it?|
|Valve stem seals have been exrensivley discussed on here. I'm running the top hats at the moment although the recieved wisdom is the O rings are better on the exhausts. Frankly drifting out the exhaust valve guides 50K miles from now is not my biggest worry.|
|One small problem, I have mixed up the valve springs, collets etc. In the past I have always kept these in order for teh specific valves they came off of. Is it bad news to mix them up?|
On the up side I do have teh push rods and valves in the right order.
|I think the most careful builders do put springs back in where they came from, but what difference does it make now? Collets, never heard of anyone that anal.|
This thread was discussed between 16/09/2006 and 20/09/2006
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