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Problem with shims 101 1600.

1K views 9 replies 8 participants last post by  velocedoc 
#1 ·
Hi all,
I am finishing an engine rebuild - polished cam, new timing chains, cleaning off the silicone used instead of gaskets in a previous (unrecorded) rebuild, reconditioning carburetor, amongst other things.
We have encountered a final problem which is that, having replaced the tappets we found that the valves weren't closing properly. I bought two sets of 1.5 and 2.5mm shims. The mechanic has played with these and discovered that he can solve the problem in all cases except one final exhaust valve. Is there a smaller shim available? Is there a work-around? Can a 1.5mm shim be machined to reduce the thickness without jeopardising the strength?
All comments gratefully received.
Regards,
Jim.
 
#2 ·
Jim,

I grinded down some on sand paper when I did the same work on my Spider many years ago.
Don´t know if that was that "professional", but like you, didn´t have any thinner when I needed them so I had to resort to these makeshift ones (that are still there).

Dennis
 
#3 · (Edited)
Well, this is much more complicated as I read this.

It sounds like you discovered this AFTER the engine was assembled.

That be bad. The head should be totally documented with what shims and lash was in it before you started...and all valve gaps set before you assemble the engine. If the valve seats have been reground and/or worn too deeply in the head and the valves have been reground then the obvious step is to replace the valves with new and replace the valve seats. This will result in a gap that can be shimmed with the thicker shims in the range and is the most preferred solution.

If you started with a head that was bare, good experienced machinists can literally tell what has to be done by looking at it and the valves. Valves can be trial fitted without valve springs to make it pretty easy to set gaps/lash and raise red flags if you are close to the limit..

Let's assume you knew that.

If one valve fails to close, I assume you have NO gap with even more to sacrifice before it closes. That is a tall order to reach .540mm or somewhere around .020 to .021 inches for the exhaust.. You could go as low as .019 across the board but I still think you might have a problem with one valve.

I've heard of all kinds of remedies but I won't go there. None of them are very good and are total shortcuts that would keep me awake with one eye open. I don't like any other alternative direction from what I have read but to start from scratch with the head and do it right.
 
#4 ·
^^^^^

I have experienced a similar problem, but was fortunate to discover it while the head was on the bench, not in the car. I cannot improve on Rick's advice. Remove your cylinder head, and sort this out on the bench. Consider finding a new machinist to redo your valves.

P.S. Do not alter shims with sandpaper or a grinding wheel. The risk that the upper and lower faces will be out of parallel is too great, and it will be difficult to measure.
 
#5 ·
The above posts are 100% correct. Seat recession or inadequate valve head margins is what is wrong here. As mentioned, it gets worse, as spring seat pressure is involved, as well as correct geometry, but like Uncle Rick, I don't want to go there in a short BB post. You may make it run, for a while, but the valve work as mentioned, is incorrect for long term use.
 
#6 ·
Thank you all.
Rather depressing as I was convinced that the mechanic and machinist know what they were doing (having just finished a 1750 for a friend who is delighted) but I fear that you are right. I don't know of anyone else in the area who can help so it looks like a 500k ride to Madrid where I do have a good alternative.
Many thanks all.
Jim.
 
#7 · (Edited)
I wouldn't throw him under the bus if you want to ever use him again. If he has never changed valve seats and looks at you with a quizzical look when you suggest it with new valves .. then you might want to get in the drivers seat of the bus. I have a feeling he will grasp what you are talking about and get on the bus.

It's not that big a deal if he knows the steps and procedures. The ideal valve seat cut is a three angle cut instead of the factory one cut. This gives better flow and cooling to the valves. Reread paragraph one..

Then there is the business of "polishing the cams" and the cams you are using. I'm not too fond of the idea of "polishing" the cams. If they needed polishing, what is to say they are as the intended profile after polishing. It's always somethin'... and then we die. Cheer up, this is a minor set back. PS most small engine shop/machinists don't have the machines to do valve work. They all send them out to specialists. If he thought it was only a matter of assembly.. he is not your guy.
 
#8 ·
The sets start at 1500. But shims are available starting at 1300 and going up by 0025.
 
#9 ·
1.5mm is .060" in US terms, I've used shims down into the high .050" range, and have heard of them into the .030 range, which would be like .75mm. That's getting pretty thin. Could one trim a tiny bit off the top of the valve stem to gain some working space for a thicker shim? I realize that's treating a symptom, not a cause. Sounds like the valve seats are cut way back.

Andrew
 
#10 ·
The solution might be short term to get the valve in clearance, but what happens down the road after the valves are seated and the seat wears. You will find that you have to take the head off as there are not any shims that will fit. Best to do what Uncle Rick suggested, take the head off to do this on the bench. I just dealt with this on my Motoronic head and it took a very competent machinist to get all of the seats set properly so the valves had some life.
 
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