John asks:
What should I tell the machinist about taking metal off
the bottom of the cylinder head? (text at the end)
Alan
apologizes in advance for the length and provides a very good answer
(text at the end)
TeriAnn
Puts her $0.02 worth in:
As Alan said how much you take off depends on what you have in the engine and
what you want to do.
The stock compression on a stock engine allows plenty of HP for fast touring
and allows you to use regular petrol. It is a dependable combination.
If you are thinking of cutting away a lot of metal to raise the compression to
get a faster car that only runs on premium and may require a can of expensive
octane booster every time you add fuel you need to do some homework first.
As Alan mentions, start with the competition prep book before you do anything.
But also take a look at the underside of the lower thermostat housing built
into the front of the head.
A head with a diagonal flat surface on the underside of this housing has more
metal between the bottom of the head and the water jacket than a housing
underside that is completely round. You don't want to trim a whole lot off
heads with rounded lower thermostat housings. Doing so will increase the
likelihood of head cracks from local hot spots.
I have done some research on TR2-4A heads and presented it in a table on my web
site. look at the table to deturmine which head you have.
http://www.cruzers.com/~twakeman
There are navagaion buttons at the bottom of the page. There is a link
directly to the table from the "recent cahnges" area.
Here is another thought. What about down the line when you have your next
valve job done and the machinist says your head should be resurfaced? Will you
have enough metal left for a reliable resurfacing job? Everything is a trade
off.
Your answer also depends upon what size pistons you are using with what head.
There are basically two kinds of combustion chambers used in the TRs. One has
a chamfer on the side to increase the compressed volume inside the head. The
camphor actually decreases compression. The heads without the camphor were
designed for use on the engines with 83mm pistons. The ones with the camphor
were designed for use on the 86mm engines. The added head volume reduces the
compression ratio on the engines with 86mm pistons to 9:1. So just using the
heads without the camphor squish area will raise the compression on an engine
with 86 or 87 mm pistons without milling the head.
Alan mentioned the possibility of having to reshape the combustion chamber
after taking a moderate amount off the head surface. The spark plug sits
inside a pocket built into the combustion chamber. If you trim a lot off the
underside of the head this lip can become very thin and may possibly become a
hot spot for preignition. Reshaping usually consists of removing the metal
below this pocket as is described in the TR competition prep manual. This
opens up the head for better flow but also carries a cost. The lip helps makes
a seal at the edge of the cylinder. You need to cut matching eyelid shaped
cuts into the top edges of the cylinder walls to match the cylinder wall shape
to the head shape. Then you need to modify the head gasket with a cut so it
doesn't stick out into this new space. You would need to use a solid copper
gasket. The gasket has two sealing rings around each cylinder, You will need
to cut into one. This eliminates your backup sealing ring. But hay, you have
better flow inside the combustion chamber. It's just a heck of a lot of work
to get right.
Everything is a trade off.
>
>John Middlesworth wrote:
>>
>> I want a good street-driving TR3A. What should I tell the machinist about
>> taking metal off the bottom of the cylinder head? He already has to clean
>> up that surface a little: do I tell him to keep cutting down to a certain
>> level using the same machine?
>>
>> John Middlesworth
>
>Hi John,
>
>In advance, sorry for the dissertation!
>
>There is a lot that needs to be determined before you decide how much to
>shave off the head. Are you going after increased compression or just
>"cleaning things up"? What else has been done to the car (cam, overbore
>pistons/cylinders, exhaust header/free flow system, carbs, porting,
>oversized valves, distributor, etc etc)??? You may not get full
>advantage of increased compression without other modifications. On the
>other hand, a mild increase of compression can help the stock original
>systems perform a bit better, and you can add other stuff later.
>
>Most importantly, you can take a mild cut off a head and come back and
>take more later. However, if you take off a lot, then don't like the
>results, you can't put it back!
>
>You must also consider what you have to start with. There were at least
>3 different heads put on the TR3 & TR4. The earliest (which you would
>probably have) was cast "thinner" and won't tolerate as much trimming,
>or you might cut into one of the water galleries when reworking the
>combustion chamber. On the "early" head, there is also some work needed
>around the thermostat housing, I believe, and may be some trimming
>needed to accomodate the manifold gaskets.
>
>You are probably aware that trimming off some of the head will also
>require reworking the combustion chambers.
>
>With those disclaimers in place, I'll say that you can select from
>"mild", "moderate" and "wild". .060 to .090 are relatively "mild" cuts.
>You should end up with compression ration around 9.5 to 1 or slightly
>higher. Any more than that on an "early" style head is getting into
>dangerous territory. My car with a later style head has .120 trimmed off
>and should be about 10 or 10.5 to 1 compression, which I would call
>"moderate". It is possible to take off .150 with the later style heads,
>pushing compression up to about 11 or 11.5 to 1, along with all the
>benefits and problems that come with it (high octane fuel, not terribly
>"street" friendly, etc... but producing the most power with reasonable
>reliability). I'd call that "wild". Higher compression ratios have been
>tried with only moderate increase in power, but a significant drop in
>reliability.
>
>Maybe you've already done so, but I would suggest crack-detecting a head
>before getting any work done. Make sure it's a good usable unit before
>spending anything on it! With any of the increased compressions, it
>might be a good idea to use an upgraded steel head gasket (although this
>is probably optional).
>
>I strongly suggest you order the Competition Preparation Manual from one
>of the vendors (less than $10, if I recall). Either the TR3 (might be
>hard to find) or the TR4/4A would give you and your machinist a better
>guide how to proceed. I have the TR4 manual and it devotes more space,
>with lots of good illustrations, to reworking the head than any other
>topic! It has lots of details, too many to include here.
>
>I hope this helps!
>
>Alan Myers
>San Jose, Calif.
>'62 TR4 CT17602L
>--
>MZê
>
TeriAnn Wakeman If you send me direct mail, please
Santa Cruz, California start the subject line with TW -
twakeman@cruzers.com I will be sure to read the message
http://www.cruzers.com/~twakeman
Walk in harmony with the earth and all her creatures and you will create beauty
wherever you go.
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