[Fot] TR6 Piston / Head Clearance

Fri May 27 08:45:00 MDT 2016

Since most of you like to play the “death with pistons” game keep in mind one additional factor as you are building your engines.  Over the years I have found crankshafts which were reground and the throws were not very precisely indexed.  When a crank is setup on the grinding machine for grinding rod journals it is set and reset by the number of “throws”… on a 4 cylinder engine the grinder has to be indexed twice and on the 6 cylinder, 3 times.  If the grinder, either the person or the machine is not perfectly accurate you will see variations of TDC for the throws!  I have seen these deviations be as much as .006” when a good grinder will hold dimensions within .001”.  

So, if you are running close squishes…. Measure TDC on #1 and #2 on the four cylinder and #1, #2 and #3 on the 6 motors….. this way to can make sure your throws and TDC’s are spot on.

Steve Yott

From: John Styduhar [mailto:johnstydo at gmail.com] 
Sent: Thursday, May 26, 2016 4:32 PM
To: Gregmogdoc at surfnetusa.com
Cc: Charly Mitchel; Steve Yott; Jim Gray; Triumph 'Friends of Triumph
Subject: Re: [Fot] TR6 Piston / Head Clearance

My forged pistons are .006" in the hole at TDC and I use a .030" thick solid copper head gasket and stock crank at 6200 rpm. When removing the head, the piston tops are clean in the squish area and it looks like a slight indentation at the ledge of the combustion chamber, probably from piston rock when the engine is cold.    

On Thu, May 26, 2016 at 3:23 PM, Gregmogdoc at surfnetusa.com <Gregmogdoc at surfnetusa.com> wrote:

IN our experience with TR-4, and English Ford Kent
racing engines the safe clearance between the head
and the piston crown is .035".  A TR-4 with billet
rods will just "touch" the head at 6800 rpm with
.026" clearance. It will "hit" the head hard enough
to make an imprint in the piston crown at .029"
I would say that the TR-6 crank probably flexes more
than a TR-4 does at high rpm. You would like to have
a little safety margin, I would think. I would not
run less than .035", maybe even a couple more if the
engine is going to run over 6,000 rpm.

Greg Solow
The Engine Room
Sports Car Specialists
Santa Cruz, CA 95060
831 429-1800

------- Original Message -------
>From    : Charly[mailto:charly at mitchelplumbing.com]
Sent    : 5/25/2016 6:52:35 PM
To      : tr4 at wi.rr.com; toodamnfunky at comcast.net
Cc      : fot at autox.team.net
Subject : RE: Re: [Fot] TR6 Piston / Head Clearance

 I tried b u idling a TR6 motor with a pop-up on the
piston on c e. Instead of having the pistons cut
down to zero clearance I left them about .005" high.
It eventually bent the rod and broken it. The
pistons had an imprint of the combustion chamber on
them. I learned that at 6000 rpm's, there is
enough deflection in the crank to let them hit the head.
I believe most head gaskets are .0035" aren't they?
Charly Mitchel
TR6 #44

On Wed 25/05/16 11:31 AM , toodamnfunky at comcast.net sent:
> The rattling is likely piston slap on cold forged
pistons. Did it go
> away when it warmed up and came back on the next
cold start ?jim
>
> -------------------------
> FROM: "Steve Yott"
> TO: "Robert Lang" , "FOT List"
> SENT: Wednesday, May 25, 2016 11:27:27 AM
> SUBJECT: Re: [Fot] TR6 Piston / Head Clearance
> I doubt that your .030 squish would cause the
rattling as you
> describe.  The deflection caused by the crank, rod
and piston in order
> to hit the head at .030 would be way up in the
rpm’s like 8000!
> Did the rattling go away after all the parts warmed
up and oil was
> well disbursed or is it still rattling?  My first
guess would be
> either the engine was not pre-oiled or you have
loose piston pin
> bushings….  Were the rods rebuilt with new bushings
fitted to about
> .0003?
> Steve Yott
> FROM: Fot [ mailto:fot-bounces at autox.team.net ] ON
BEHALF OF Robert
> Lang
> SENT: Wednesday, May 25, 2016 11:29 AM
> TO: FOT List
> SUBJECT: Re: [Fot] TR6 Piston / Head Clearance
> Hi,
> Where I was going with the query was this: I had a
late block
> "decked" with the intent of making the deck flat.
The machinist took
> of .032 to accomplish the goal. With a head gasket
installed and the
> head torqued, the clearance in the squish area is .030.
>
> I had initially wanted only about .025 removed, but
we had to shave
> an extra .007 to get rid of the fire ring cuts.
>
> On initial startup, the motor sounded like it might
be "rattling" and
> I'm trying to figure out why.
>
> When I used a stethescope on the block, everything
sounds okay - no
> rapping and the valve train sounds "normal" and
there were no weird
> sounds coming from the crank.
>
> I guess where I'm going with this is: if piston /
head clearance is
> too tight, I'll see some sort of indication on the
piston, right?
>
> TIA for any sage advice.
>
> This is my first experience "decking" a block. I'd
like to get it
> right, I don't have the resources to start a new
block from scratch.
>
> Regards,
>
> Bob Lang
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