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Cylinder head

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Old Nov 8th, 2019, 08:45   #1
Thowdfella
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Default Cylinder head

I did a compression test on my 1966 122s and this showed 125 lbs on each cylinder so decided to take the head off and have the unleaded conversion done.
first question is - should there be washers on the cylinder head bolts? there was one washer on one bolt. I looked through Brookhouse and there is no mention of washers for the bolts.
Second question is about the compression. I was expecting around 175 for compression but only got 125, but the same on all cylinders. borrowed another compression tester and the same results. however, when I took the head in for conversion they whipped a couple of valves out and they looked quite good and the springs look ok although the exhaust guides were a bit worn.
Where has my compression gone? All the pistons feel tight.
I was advised to put the pistons so they were all half way down the cylinder and fill to the top with diesel fuel and watch the results to check the pistons which I will try.
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Old Nov 8th, 2019, 11:16   #2
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Might be a bit late now...

But you can do a wet test: squirt a small amount of oil down each cylinder bore via the spark plug hole, and then perform the compression test.

The oil acts as a temporary seal - this eliminates dodgy piston rings being part of a potential low compression problem (if the readings stay the same with oil the issue is with non-sealing valves/if the readings are higher, the issue lies with the piston rings not sealing.

Like I said, probably a bit late for you, but will hopefully be of value to others.

Mike
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Old Nov 8th, 2019, 11:56   #3
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I actually tried that one one cylinder and it made no difference and as all cylinders were reading the same I didn't bother with the rest.
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Old Nov 8th, 2019, 12:00   #4
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Another interesting point is that when I took the head in to the workshop, they did a vacuum test around each valve. This built up a vacuum and then you watch it decay. All the exhaust valves decayed faster than the inlets but according to the tester they weren't too bad - not enough to explain the poor compression.
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Old Nov 8th, 2019, 13:23   #5
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Washers under the bolt heads aren't standard. Not in the parts list. If they are used they should be hardened steel. If converting from bolts to studs, studs often used on competition engines, hardened washers are used under the nuts.
Before fitting the head run each bolt fully down into the block. They should all go down very closely to the same amount and obviously far enough to enable them to take the standard torque with a bit of room to spare. All bolts are the same length. If rust or dirt builds up in the bottom of the hole it can happen that a bolt will appear to be at proper torque but not actually down far enough to clamp the head, So, if you find a bolt that is higher than the others when screwed in you will have to see what is causing that.

Did you do you compression test with the carbs fully open? Sometimes omitted when you are by yourself. It then doesn't get enough air in to compress. That can make a lot of difference. Maybe the low 125 pounds is because of this but the engine will go for thousands more miles even at that figure. It's very rare for each cylinder to be exactly the same, as yours were, up to 10% difference is acceptable.
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Old Nov 8th, 2019, 14:59   #6
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I think that if your compression is equal on all cylinders you should be pretty happy. But for best results, I think you want to do the test with engine hot and carbs wide open.

You can also do a leak down test, and that will help identify location of any problem. But you need an easy way to get each piston up to top dead center and a special piece of test equipment (a dual gauge tester) that you can probably rent from an auto supply store. Google "leak down test" and many videos will come up.

Exhaust valves can be expected to "decay" faster because that is where you get valve recession. The exhaust side is much hotter than the intake side, because the intake side is cooled by the incoming fuel. The hardened valve seats are often just done on the exhaust side for this reason. Lead free gasoline runs hotter, exacerbates the problem and that is why one installs the hardened valve seats.

Since you already have the head off, you might as well do a valve job. It's not that big of a deal for a machine shop; and you can do it at home if you can get a valve spring compressor to remove the valves. Once again, something you can likely rent at an auto supply store. Some lapping compound and an reversible, variable speed electric drill is what I've used to make short work of the valve job. Some would not like that method, preferring hand lapping. Google "lapping valves" and you will see how it is done.
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Old Nov 8th, 2019, 17:51   #7
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The test was done with my wife doing foot to the floor, and on a hot engine.
Thanks for the info on washers and running down the bolts before assembly.
I was in fact going to do that on the front bolt and back bolt on the offside because both these were very tight coming out and are rusty. I will get new bolts for these and make sure I check all the bolts.
The head is now in the workshop to have the exhaust valve conversion done for unleaded fuel. They have done a pressure test and it has passed.
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Old Nov 8th, 2019, 19:54   #8
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Also wise to chamfer any threaded bolt/stud holes in addition to running down with a tap.
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