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Two types of Big Bore ports?
https://www.ausmini.com/forums/viewtopic.php?f=2&t=65515		 Page 1 of 2
Author:  VulcanBB18 [ Tue Sep 07, 2010 12:07 pm ]
Post subject:  Two types of Big Bore ports?
I've recently acquired a 12G1316 head (smog 12G940) and noticed something I've seen before - the exhaust ports on this head seem really...good!

Usually on most small valve 940s, the exhaust guide boss is really intrusive into the port, like this :

Image

But the head I've recently purchased has nice open exhaust ports like this :

Image

The head appears stock otherwise. I've seen this once before, where I had 2 big-bore head castings, one had the intrusive boss, the other not.

Anyone seen this before? Or know what's going on? I always thought the factory had one mould for the big-bore head ports... maybe not?

cheers

Jacob
Author:  drmini in aust [ Tue Sep 07, 2010 12:23 pm ]
Post subject: 
All 3 of my 12G1316s had these restricted ports. But I've seen old original `70s 12G940 heads with and without em.
I usually knock the old guides out, remove most of the boss then press new guides in.
Author:  GT mowog [ Tue Sep 07, 2010 1:07 pm ]
Post subject: 
I have seen about 4 different types of stock exhaust ports in the '12G940' castings over the years, not small changes but fairly dramatic.
Author:  VulcanBB18 [ Tue Sep 07, 2010 1:35 pm ]
Post subject: 
Thanks for the responses, guys.

Just comparing the "unrestricted" to the "restricted" types, I would assume there would be quite a difference in airflow, as like GT says, the differences in port shape/guide boss are pretty dramatic.

Has anyone ever tried the different types on a flowbench and/or engine to measure the differences?

It seems strange to me that this isn't commonly mentioned - surely it would make sense to go for the type with the least restricted exhaust port as a basis for a performance engine?

I am thinking of using the 1316 on my upcoming 998 build and figured I would need to do a fair chunk of work on the exhaust port, but now it looks like it's half done...

cheers

Jacob
Author:  drmini in aust [ Tue Sep 07, 2010 7:14 pm ]
Post subject: 
The 1316 castings have more metal around the exhaust ports and it's easier to get good flowing port shapes (according to Vizard).
I took the exhaust boss restrictions out in a 1316 head for my Moke, with a good carbide burr in the die grinder it only took an hour or so's work.
Don't try and do it with the guides in there- either knock them out, or up out of the way.
Author:  VulcanBB18 [ Wed Sep 08, 2010 3:42 pm ]
Post subject: 
Yeah I'm aware of Viz's musings on the 1316.

What seems strange to me is that no-one has done any testing of the different types of big-bore head "out of the box". Looks like a chunk of performance to be lost or gained there!

Yeah I wouldn't try on work on the ports with guides in there. Do you use a press to get them out of the way or just a drift?

cheers

Jacob
Author:  TheMiniMan [ Wed Sep 08, 2010 5:13 pm ]
Post subject: 
I use an air Hammer tool with a modified chisel-drift bit in the end, works a treat :-)
Author:  sgc [ Wed Sep 08, 2010 5:24 pm ]
Post subject: 
TheMiniMan wrote:
I use an air Hammer tool with a modified chisel-drift bit in the end, works a treat :-)


An air hammer!? Pfft, Matt.. you're getting soft! :lol:
Author:  Phat Kat [ Wed Sep 08, 2010 5:37 pm ]
Post subject: 
You can punch them out with a punch and hammer..... but I preffer to use a press.

Just remember to re install them at the right height. The top of the guides should be proud of the spring seat by 19/32 (15.08mm). I ground down a nut to the right height that I slide over the guide, it acts as a stop so that I can only push the guides down to correct height. Works a treat :)
Author:  drmini in aust [ Wed Sep 08, 2010 7:45 pm ]
Post subject: 
I use a custom made stepped punch and a BFH. Trust me, I'm a fitter. :P
To fit them the right height I have a sleeve I put over the guide.
Author:  justminis [ Wed Sep 08, 2010 7:58 pm ]
Post subject: 
I also use a custom made stepped punch with sleeve/s of the required heights, but use the press.
Author:  GT mowog [ Wed Sep 08, 2010 8:25 pm ]
Post subject: 
justminis wrote:
........ but use the press.


+1. I don't like hitting things unless there is no other way. Now, where's the wife........ :lol:

<EDIT> Besides, the pain from a hammer comes and goes, but with a press you can make it last as long as you like :wink:
Author:  Phat Kat [ Wed Sep 08, 2010 8:36 pm ]
Post subject: 
drmini in aust wrote:
I use a custom made stepped punch and a BFH. Trust me, I'm a fitter. :P
To fit them the right height I have a sleeve I put over the guide.


hahahaha

You fitters and your bloody hammers :)
Author:  drmini in aust [ Wed Sep 08, 2010 8:40 pm ]
Post subject: 
Phat Kat wrote:
drmini in aust wrote:
I use a custom made stepped punch and a BFH. Trust me, I'm a fitter. :P
To fit them the right height I have a sleeve I put over the guide.


hahahaha

You fitters and your bloody hammers :)

Mate I spent the 1st 12 weeks of my apprenticeship filing and chipping steel. I have the callouses to prove it.
Did teach me how to file flat though. :wink:
Author:  VulcanBB18 [ Wed Sep 08, 2010 9:42 pm ]
Post subject: 
Ok so I can go the BFH or the press route.

Got access to a press at the preserved railway (read:steam train) I do work at (crazy volunteer), so that sounds best.

Anyone got a guide reamer I can borrow? :wink:

cheers

Jacob
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