[E-Type] piston design for engine rebuild

In reply to a message from PeterCrespin sent Wed 20 Jun 2007:

Thanks to everyone for your help and outstanding advice! I
can’t tell you how grateful I am to have you all out there.

I ordered the pistons this afternoon. I’ll report to you
how this all turns out.

I learned that cometic gaskets compress to within ~0.001’’ of
specified thickness under load. This makes their selection
a lot easier!

I also learned that JE Piston is a HUGE fan and active
advocate of cometic gaskets.

My piston design should be a boring one (sorry I couldn’t
resist). I’m trying to return the engine to 9.0:1. This
won’t be a super car. But I do hope it will be a very long
and smooth running car. My dream would be for Ray to jump
in, drive for a while, and then comment on how smooth
everything is.

The head now looks really, really nice (and deboneheaded).
The connecting rods and crank have been all checked and
balanced. By the end of next week I should have the block
prepped, as if I was actually doing the work: hah!, and
ready for mating to the new pistons.

Interestingly when checked out: the weird wear on the
bearings was due to the connecting rods having been
overtightened. The sizes were about right.

My goal is to complete the job by July 4 (a great day for
driving a british car in my view).

More to come,

Larry–
Larry Wade 62 OTS, La Canada, California USA
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from PeterCrespin sent Wed 20 Jun 2007:

I believe Rob Beere sells some 3.8 only pistons with the asymmetric
domes designed by Cosworth. For this to work on a 4.2 you would
need 6 different pistons accounting for plug, valve, and combustion
chamber offset. I would think one would have to fit these pistons
very closely to the combustion chamber shape to get the benefit. I
wonder if by firing the mixture offset from the center of the
piston one creates some front to back loading on the piston.
Pistons are generally designed to handle only side to side thrust
and might wear abnormally.

Another way to address this issue would be to change the plug
location to one more centrally located. Rob Beere tried this with
10mm motorcycle plugs almost touching both valve seats. But as with
the pistons it is a huge amount of effort for incremental gains.

Here are a couple of things to think about when ordering custom
forged pistons.

  1. What ring grooving are they going to have and what rings are you
    going to use? I’ve had customers show up with shiny new pistons
    that no one makes rings for.
  2. Typically forged pistons are designed for .005’’ to .007’’ bore
    clearance. That works for full throttle racing. For street use it’s
    a little loose. A worthwhile upgrade is the application of an anti-
    scuff coating to the piston skirts. It cuts the clearance down to
    around .003’’ which works fine and you don’t have to worry about
    scuffing because of too little clearance.

BTW, Cometic or any MLS (Multiple Layer Steel) type gaskets don’t
compress. They have embossed beads at critical areas that compress
but the body doesn’t at all. If you buy a .060’’ gasket, your head
will be .060’’ from your block. The beads stay compressed which is
why you can’t reuse MLS gaskets. I feel they are better suited to
long stud engines. They were designed to be used with stretch
bolts. With a short stud engine, when the head expands a composite
gasket will give whereas a MLS gasket won’t. Usually the head nuts
and washers end up recessing into the head bosses. At least the
long studs will stretch more. MLS gaskets also require a very
smooth finish on the head and block to seal properly.

Regards,
Rich Olson '67 OTS–
The original message included these comments:

In reply to a message from Petrarch sent Wed 20 Jun 2007:
This is true, but I got the impression Larry was after a good rebuild with some blueprinting, not a full rework. Somebody in the UK (Cosworth? Rob Beere?) sells a set of asymmetric pistons with the domes fitting each hemi to correspond to the off-centre plug position but a) they’re not cheap & b) I think they aree only for 3.8 cars because of the bore centre issue on 4.2 cars. I suspect there’s a whole different order of work/price between shaving a standard type slug to adjust squish and creating a completely bespoke set of pistons. But I agree with your comments in a general sense.

making it better. A clue here would be to have a design that pushes
the gases to the spark plug area, so that the mixture ‘‘tumbles’’.
Also, try to make the piston top so that the spark plug sees all
the non-swished areas easily (i.e. no hidden pockets). This will
greatly reduce the octane requirement for your engine, so you could
bump the compression up even more. This is if you are interested in
more gains, as you are getting custom pistons made.


Richard Olson
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Richard Olson sent Thu 21 Jun 2007:

Hi Rich,

It is very kind of you to share your experience and with
luck save us from re-experiencing it!

The Cosworth designed pistons are supposed to be great. I
understand that SNG Barratt can get them. But they cost
about twice the $200 each that JE pistons cost. I’ve no
doubt that racers would benefit from the difference. But
I’ve even got stock exhaust!

Here’s their webpage:

http://www.jepistons.com/

The JE Pistons come with wristpins and rings. They also
sell tons of rings for any application. They’ve also done a
bunch of Jag engines (I’ve got the 3.8 which simplifies
things some).

It’s my impression that they don’t normally work directly
with customers however. Instead they work through
distributors who are experience with custom piston design
and engine rebuilding. They guys I’m working with have been
using their pistons for a long time and really like them.
I’ll let you know how it turned out.

Thanks also for the notes about the cometic gaskets Rich!!
I’ll double check the finish specs needed.

Cheers,

Larry–
Larry Wade 62 OTS, La Canada, California USA
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Richard Olson sent Thu 21 Jun 2007:

‘‘I believe Rob Beere sells some 3.8 only pistons with the
asymmetric domes designed by Cosworth. For this to work on a 4.2
you would need 6 different pistons accounting for plug, valve, and
combustion chamber offset. I would think one would have to fit
these pistons very closely to the combustion chamber shape to get
the benefit.’’

I agree. Since the 3.8 has centred bores under each hemi you don’t
have so many variables and for people who race these engines (there
are classes where the 4.2 is not allowed I believe?) every little
bit helps. So having an asymmetric piston crown to crowd the
mixture around an off-centre plug probably works great, especially
at higher CRs, and with race fuel the risk of ending up with a
detonation-prone ‘orange peel’ combustion pocket at the far side of
the dome from the plug is probably not an issue and you could get a
close squish-type fit of dome to head at the far side from the plug
(presumably needing valve pockets on the crown?).

‘‘I wonder if by firing the mixture offset from the center of the
piston one creates some front to back loading on the piston.
Pistons are generally designed to handle only side to side thrust
and might wear abnormally.’’

I was wondering about this and about the overall increased mass os
a large domew (may not be the case?) and the fact it presumably
isn’t split equally either side of the con rod/piston centreline so
it might tend to twist the small end? For sure a slipper-type
piston isn’t made to take large lateral loads but I guess in
reality the effect, if any, is small and within the capability of
the motor to bear. Half the people spending 400 dollars a piston
are probably on Carillo rods anyhow (drool…).

‘‘Another way to address this issue would be to change the plug
location to one more centrally located. Rob Beere tried this with
10mm motorcycle plugs almost touching both valve seats. But as
with the pistons it is a huge amount of effort for incremental
gains.’’

Indeed - the law of diminshin returns begins to kick in for a raod
motor, but races are won on cumulative incremental gains, I guess.
A second pair of 10mm plugs is fairly common on hemi bike engines
and overcomes some of the disadvantages of extreme piston crowns
and high CR. But those were msotly aircooled motors so the head
machining was pretty simple with no water passages to intrude.
Almost a DIY proposition in fact, if you could counterbore the hole
and spot face a plug seat.–
Peter Crespin 66E, 76 DD6 Coupe, 84 DD6, 85 XJS 5sp convert
Cambridge, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Larry Wade sent Thu 21 Jun 2007:

Larry,
What dome design will you get with your custom pistons? I wonder
if it is possible to give JE a 3D model of the dome desired and get
the piston dome of choice, or do they give you only specific
options of what dome you can get?

Regards

-Don–
The original message included these comments:

It’s my impression that they don’t normally work directly
with customers however. Instead they work through
distributors who are experience with custom piston design
and engine rebuilding. They guys I’m working with have been
using their pistons for a long time and really like them.
I’ll let you know how it turned out.


Petrarch
Los Gatos/CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Petrarch sent Fri 22 Jun 2007:

Hi Don,

They can make anything you can dream of. I think that my
design will be pretty boring. Since I’m only going to 9:1
and have standard cams, the clearances are still pretty
large. With high lift cams and higher compresson they do
some really cool designs.

I’m told they also have some pretty sophisticated flow
modeling programs and can do laser topography measurements
of your combustion chambers. With those tools they can
develop solutions that are quite clever.

For an example of a more extreme design you could check XKs
Unlimited. They sell a 12.5:1 piston made by JE. They look
much more like the high performance domed two valve designs
that JE makes for more modern cars.

Mine will just be a small variant of a normal jag dome. The
only real difference is that the dome height will be
selected to compensate for the various shavings the block
and head have previously experienced…and accounting for
our selected deck height and head gasket thickness.

I’ll post pictures of them when they arrive.

Cheers,

Larry–
The original message included these comments:

Larry,
What dome design will you get with your custom pistons? I wonder
if it is possible to give JE a 3D model of the dome desired and get
the piston dome of choice, or do they give you only specific
options of what dome you can get?
Regards
-Don


Larry Wade 62 OTS, La Canada, California USA
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Larry Wade sent Sat 23 Jun 2007:

Hello Larry,

Thanks. Sounds like they do some good work at JE. Did they quote
piston weight? I know that high quality forgings -not TRW type-
will generally be lighter, much thinner, and are stronger than
stock cast pistons. This reduction in rotating mass is worth
something in itself. Keep us posted.

Regards

-Don–
Petrarch
Los Gatos/CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

In reply to a message from Petrarch sent Sun 24 Jun 2007:

reciprocating mass, but nobody’s quibbling… :-)–
The original message included these comments:

stock cast pistons. This reduction in rotating mass is worth
something in itself. Keep us posted.


Peter Crespin 66E, 76 DD6 Coupe, 84 DD6, 85 XJS 5sp convert
Cambridge, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php