Big Valve Twin Cam Tappet Clearance Problems between Cam Lobe and Cam Follower

[4129R]

I have just swapped over the valves, springs, collets etc from a Federal Zenith Stromberg head to a second hand Weber head.

I have done this 3 times before without any problems.

This time there is negative clearance between the valve buckets/cam followers and the cam, even with the thinnest shim.

I measured the gap between the inlet valve stem top and the cam, and it was 195 thou. Add 160 thou for the thinnest cam bucket, add 60 thou for the thinnest shim, and 6 thou for the clearance and you need a minimum of 226 thou to start with. I am 31 thou out.

The exhaust valves have enough clearance at 235 thou, so add 10 for clearance, 160 for the thin buckets, and 65 for a shim, and that should work.

The valves are all the right length. I phoned QED and they suggested the valve seats have been put in too low, or the Federal cam has bigger than standard lobes reducing the clearance.

It just does not make sense to me.

I have tried thinning a shim below 60 thou, and at 50 thou it gets very tricky.

I thought of taking 20 thou off the valve stem end. Any more and it weakens where the collet grips the valve stem. Those 2 could get me another 30 thou which is just on the limit.

1) Has anyone else had valve clearance problems?

2) Has anyone any suggestions as how to deal with this?

I have ordered the thinnest cam buckets from QED which are 160 thou thick instead of the normal 218 thou. 

Holding the shim while grinding the face was a challenge. I tried a 16mm hole in an ally plate. The shim kept falling out and was a bitch to find. In the end I have used a heavy duty fishing magnet which grips the shim very firmly so not even a grinder moves it.

I have just practised taking 20 thou off the valve stem on an old exhaust valve. It is far easier to do than reducing the shims as it is much thinner, and easy to hold. I used 2 x 2x1s in a vice to grip the valve to grind off most, then drilled a 8.5mm hole in a 2 x 3 to hold the valve perfectly square, and used a bench sander with 120 grit paper in to get it perfectly flat. There was about 90 thou above the collet top, so reducing that to 60 or 70 thou IMHO won't upset the integrity of the collet fixing long term, with about 60-70 thou left on the end. Any thoughts?

[Kendo]

The valve seats being low was my first thought. If one had to regrind the cam bearing line, could that lower the cam?

[4129R]

The valve seats being low was my first thought. If one had to regrind the cam bearing line, could that lower the cam?

Grinding cams is expensive. The Europa inlet cam is a rarity, so I thought reducing the valve length and the shim thickness was something I could do FOC.

When the thin buckets arrive from QED tomorrow, I will try 1 valve to see if my theory works, before doing all 4.

Luckily I have spare new and old valves and a variety of shims, so I can play a bit. 

[Kendo]

I’m not sure of the terminology here. The cam rides in bearings. If the bearing line had to be rebored in that head, could it have been done in a way to lower the cam rotational axis, losing some valve clearance? Alternately, do cam bearings with offset bores exist, to raise the rotational axis?

[69merc]

Thinner buckets and reducing the valve stem length is where I would go.  I had to take a bit off of one of my valve stems to get the clearance I needed on the TC I just put together.

[4129R]

I’m not sure of the terminology here. The cam rides in bearings. If the bearing line had to be rebored in that head, could it have been done in a way to lower the cam rotational axis, losing some valve clearance? Alternately, do cam bearings with offset bores exist, to raise the rotational axis?

All new standard cam bearings. No change of position. Easy to see on a TC head.

[EuropaTC]

I'd also go with reducing the valve stem although it's not something I've ever had to do myself. Personally I'm not so keen on thin shims, especially if you have to DIY them.

I guess it's worth comparing the existing valve stems and looking hard at the weber head to see just where the inserts/valve seats are.

Brian

[4129R]

I'd also go with reducing the valve stem although it's not something I've ever had to do myself. Personally I'm not so keen on thin shims, especially if you have to DIY them.

I guess it's worth comparing the existing valve stems and looking hard at the weber head to see just where the inserts/valve seats are.

Brian

The position of the valves seated in the combustion chambers looks quite normal. All the inlet valves look identical, and are all the same length.

When you are trying to find out where say 50 thou went, it is not easy. 40 thou is only 1mm.

[Pfreen]

My only comment that may help you is are the valves stock thickness or have they been sparked off excessively.  If not, then the valves seats have been ground too deep. 
I think you can buy different length valves if you need to shorten the too much.
What do the valve seats look like?  Are they original cast iron?  If so, you probably want to replace them anyway.
Shortening the valve is a possibility but obviously not too much to start weakening the collet grooves.

[4129R]

My only comment that may help you is are the valves stock thickness or have they been sparked off excessively.  If not, then the valves seats have been ground too deep. 
I think you can buy different length valves if you need to shorten the too much.
What do the valve seats look like?  Are they original cast iron?  If so, you probably want to replace them anyway.
Shortening the valve is a possibility but obviously not too much to start weakening the collet grooves.

New valves. Standard size.

Valve seats new.

I need them about 1.25mm or 50 thou shorter. I can get this by taking a small amount off each valve stem, and grinding thinner than available shims. Hard work, but FOC. It would leave about 60 thou above the collet groove.

[jbcollier]

As the valves bed into the seats, this will get worse so special narrower shims is not the best way to go.  It sounds to me that your valve seats are low.  I would have a good machinist look at it.  When doing my Ducati desmo head, I had the same problem.  I had to have the valve seats replaced and that sorted it.  You reallly need a machinist who is experienced with TC heads.  There are lots of issues as the heads are quite marginally designed and it is easy for an inexperienced machinist to toast a head.

When line boring a head you always take metal off the caps and the cap side of the bore, virtually nothing from the head side… if you know what you are doing.

[4129R]

Whoopee, success.

DHL delivered the thin 160 thou cam buckets today less than 24 hours from ordering from QED.

I thinned a shim to 54 thou, ground about 30 thou off the head of a valve, and I now have a gap which I can measure.

It takes a while to grind the shims and valves down, but this is all something I can do carefully, without regrinding the cam, or fitting new valve seats.

There is still about 60 thou above the collets on the valve stems, the shim is not too thin at 54 thou, so I cannot see why this won't work.

The challenge was holding the shims while grinding them, but a strong fishing magnet bought off eBay a while ago holds them rigidly in place. The problem was the fishing magnet could not be held up to my bench grinder as the magnetism affected the grinder motor, so I have to use a handheld De Walt grinder with the magnet held in a vice. I then smooth the face with 120 emery paper backed with a piece of 2x1.   

Another challenge apparently conquered.

[jbcollier]

This will magnetize the shim which will then attract metal debris from the oil.

[EuropaTC]

When I've taken a few thou off a shim to get within spec my method is to use double sided tape and stick the shim to a rubber bung or similar. Then use wet'n'dry supported on a sheet of glass, water lubricant. That way you can rotate as you slowly grind the surface so it stays reasonably flat. That's very similar in principle to preparing metallographic samples 50+ years ago at university - who said it was a wasted education 

Brian

[Richard48Y]

You will need a Degauss machine to demagnetize your shims.