Wilsons Sandbox

Here is a technical question that may relieve some of the stress of the
purist/antipurist debate [or it may not].

I am having an XPAG engine rebuilt and I find the head has been planed
a number of times in the past so that it is now 73.4 mm thick. This is
quoted by some as the limit or just beyond the limit of thinness for
a head.
What is the wisdom of the list on this? Is it beyond? I know
there are questions of the load from high compression on the bottom end
and of the clearance of the plugs, etc. It is [and here is the 'or may
not' part] a TF head with the larger valves and I would like to use it.
Could we have some discussion on this? Thanks, Barc

Barc:

At 73.4mm you will be running a little over 9.3 to 1 comp. ratio. This is
very suitable in this day and age. I have been running 9.5 to 1 for over 35
years. I do run premium gas at 92 octane. I use 5 degrees static advance,
and all is well. I have put 130,000 miles this way on the car. These
engines run very well on this compression. THis is stage three in the
factory manual.
My car has been from coast to coast, and Canada to Mexico with no bad results.
I do realize that this compression is not the original 7.25 to l as it left
the factory, but I like it, so does the car..

Cheers:


Skip...................


At 09:35 AM 3/10/00 -0800, Barc Cunningham wrote:

>Here is a technical question that may relieve some of the stress of the
>purist/antipurist debate [or it may not].
>
>I am having an XPAG engine rebuilt and I find the head has been planed
>a number of times in the past so that it is now 73.4 mm thick. This is
>quoted by some as the limit or just beyond the limit of thinness for
>a head.
>What is the wisdom of the list on this? Is it beyond? I know
>there are questions of the load from high compression on the bottom end
>and of the clearance of the plugs, etc. It is [and here is the 'or may
>not' part] a TF head with the larger valves and I would like to use it.
>Could we have some discussion on this? Thanks, Barc
>
>
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Easiest way to tell is to look at the spark plug holes. If they are now
a part of the bottom of the cylinder head, you are nearing the limit. If
they are oval shaped, you are definitely reaching the limit of using the
head without a double thickness head gasket (www.headgasket.com). As
long as spark plug electrode tip does not protrude below bottom of
head, head is still useable. Other variables: how high do the pistons
come to the top of the block. If recessed below block top, head is still
useable. Old trick (and I'm not an old timer) is to use Play-dough, or
other modeling clay, to check clearance between head and piston.
John Seim

Message text written by Barc Cunningham

>I am having an XPAG engine rebuilt and I find the head has been planed

a number of times in the past so that it is now 73.4 mm thick. This is
quoted by some as the limit or just beyond the limit of thinness for
a head.
What is the wisdom of the list on this? Is it beyond?<

You should be fine with decent fuel - this will give a compression rate of
about 9.3 to 1 with standard pistons and is just normal stage two tuning if
you polish the head and ports - I used to run the TB with this amount off
the head and high compression pistons - hate to think what the compression
ratio was!

What you do have to watch out for though is that you round off the sharp
and thin areas which will be around the base of the plug hole, (only for
about half an inch or the gasket wont seal well) or they will become
incandescent and cause pre ignition and run on.

Its also worth fitting bigger diameter valves, and importantly to fit
spacers under the rocker shaft or you will screw up the geometry of the
pushrod/rocker/valve gear assembly.

Clive Sherriff

On Fri, 10 Mar 2000, Clive Sherriff wrote to mg-tabc@egroups.com:

> Its also worth fitting bigger diameter valves, and importantly to fit
> spacers under the rocker shaft or you will screw up the geometry of
> the pushrod/rocker/valve gear assembly.

Decreasing the thickness of the cylinder head doesn't alter the
relationship between the rocker arm and the valve stem, and that's the
most important "geometry" issue. What planing the head does do is
decrease the distance between the tappet and the rocker arm, which in
extreme cases makes the pushrod too long to allow rocker adjustment. That
is why spacers under the rocker arm pedestals used to be recommended for a
planed head, but it's the wrong way to solve the problem. It does give
you back the ability to adjust the rocker clearance, but it also alters
the geometry beteen the rocker arm and the valve stem. That's not usually
a good thing.

Ideally, with the valve at half lift a line drawn through the centerline
of the rocker shaft to the tip of the valve stem should be exactly
perpendicular to the valve stem. That does two things: (1) It ensures
that you get as much total lift as possible at full lift, and (2) it keeps
the side-load on the valve stem to a minimum.

Assuming this bit of geometry is close to correct in the first place,
putting spacers under the rocker shaft pedestals will obviously change the
angle between the rocker arm and the valve stem. The result is increased
wear in the valve stems and guides due to increased side load, and
decreased power due to decreased valve lift.

If after planing the head you can still adjust the rocker clearance to
specs, leave well enough alone. If you can't, then the correct solution
is to install shorter pushrods. Shorter pushrods are sometimes available
from the usual T-Type parts suppliers, or you can have a machine shop
shorten the original pushrods. As a general rule of thumb, shorten the
pushrods the same amount as you decreased the depth of the cylinder head.

The only time you want to put spacers under the rocker shaft pedestals is
if you are "blueprinting" the engine and want to get the geometry between
rocker arm and valve stem as close to ideal as possible. In that case you
may have to add spacers to raise the shaft, or you may have to plane the
bottoms of the pedestals to lower the shaft.

--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E
fold@bcpl.net

Chip,
Thanks for a fine explanation on shimming/not shimming rocker pedistals!!
Finally I understand _obvious once one has had a good explanation like
yours!!!

Barc,
If you do put in shims under the pedestals, make SURE the rearmost one
has a hole for oil to flow into the rocker-arm shaft. Easiest way to be
certain of this is to have oil holes in ALL the shims. Then you won't find
yourself wondering : Did I put the right shim in the right place?
Regards,
Carl Fritz

Message text written by "John T. Seim"

>Other variables: how high do the pistons - come to the

top of the block. If recessed below block top, head is still
useable.<

This is not a function of head usability, the piston crown is
flat (except for the High Comp piston) and should be level
or below the block face anyway - it shows either
incorrectly made pistons (too high a crown height) - or
that the block has been faced off too much.

Clive Sherriff

I have found thaT most XPAG engines that I have built have the pistons
sticking up too high. Anywhere from 5 or 6 thousands to as much as 17
thousands. I pre-assemble the pistons and rods without rings, and use an
arbor dial indicator to measure each one. Then I disassemble and cut the
appropriate amount off of each piston. Then I balance the piston set, and
re-assemble. I try to get them flush with the top of the block. When
installing the cyl head, I always check the clearence between the fully
open exhaust valve, and the block. It should be approx 60 thou or better,
otherwise the exhaust valve will contact the block with resulting problems.
This is especially important if the head has been milled, and or if a
higher lift cam is used than stock.
Also new valve seats will exacerbate the problem.
On my own engine I had to cut small recesses in the block to clear the
valve, due to large head clip, and a .357 thou valve lift.

Cheers:

Skip Kelsey............................

At 03:48 AM 3/11/00 -0500, PMS GB Ltd wrote:

>Message text written by "John T. Seim"
>>Other variables: how high do the pistons - come to the
>top of the block. If recessed below block top, head is still
>useable.<
>
>This is not a function of head usability, the piston crown is
>flat (except for the High Comp piston) and should be level
>or below the block face anyway - it shows either
>incorrectly made pistons (too high a crown height) - or
>that the block has been faced off too much.
>
>Clive Sherriff
>
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