Hi guys. I've been an engine reconditioner since 1985. I have machined 100,s if not 1000,s of holden 6 blocks. And maybe 15 to 20 JP and NP blocks. There seems to be no differences in the cast iron construction of the blocks. They all machine up the same. I can say this because other car manufactures use different blends of cast iron and they machine up differently..As far as differences in the blocks. They are all cast the same. Clock and date markings can move a bit from date code to date code. No internal extra ribs. 72 and 73 jp blocks not all but some had valve reliefs in the top of the bores.I think the Bathurst cars got the valve reliefs..What else can I say. That's just what i've noticed over the years.. I have pulled down HDT holden 6 engines in the past and two of them from 1973 had round exhaust ports.and other weird stuff.. Phil
what makes a jp block different
#27
_duggan208_
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Posted 04 May 2016 - 12:03 AM
Surely someone with a JP and a good pair of eyes can compare blocks to see if there are any differences, but why would Holden bother to spend extra money on stronger block castings. Did any race XU1s break blocks to pressure Holden into strengthening the block?
Regards
#28
_repco_
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Posted 05 May 2016 - 08:58 PM
My friend from the 73 holden holden dealer team has told me that they destroyed 100,s of engines.Mainly piston out the side. and cams being pulled out of the engine due to helical cut cam gears and to much valve spring pressure. Phil
#29
_Bomber Watson_
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Posted 05 May 2016 - 10:18 PM
At the end of the day were turning them a lot harder and making a lot more hp now days out of generic QL blocks and the like, so I highly doubt the strengh of the factory casting would have had any relevance whatsoever to what HDT was doing back in the day.....
#30
_duggan208_
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Posted 06 May 2016 - 12:03 AM
Yes, that's what I reckon Bomber.
regards
#31
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Posted 06 May 2016 - 08:13 AM
I've been talking to Dave Mclean about this exact thing, will be having more discussions later when he has time. He assures me you can pick certain XU1 blocks instantly when the boring machinery hits them. Between Dave and Tatey they have probably built more XU1 engines than everyone else combined. Dave still regularly rebuilds them.
Edited by yel327, 06 May 2016 - 08:14 AM.
#32
_repco_
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Posted 08 May 2016 - 02:20 PM
Back in 1989 repco macine shops in Sydney did the metallurgy on 186, 202 3.3 jp and np blocks. I am trying to find the report on this. and will post asap.. There is no difference in the blends of cast iron used. No higher nickel content. Spoke to Ian about this years ago and he says the cost would of been to grate to do a limited number of blocks. Sorry boys... Phil
#33
_Skapinad_
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Posted 08 May 2016 - 04:52 PM
#34
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Posted 08 May 2016 - 05:09 PM
Back in 1989 repco macine shops in Sydney did the metallurgy on 186, 202 3.3 jp and np blocks. I am trying to find the report on this. and will post asap.. There is no difference in the blends of cast iron used. No higher nickel content. Spoke to Ian about this years ago and he says the cost would of been to grate to do a limited number of blocks. Sorry boys... Phil
Thing is it isn't all JP blocks. I'm not talking the special ones built for race teams for 1973, as Dave told me all about these too last week. Just certain production engines.
#35
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Posted 17 May 2016 - 02:21 PM
These special race blocks that you are talking about. Do they have an engine number????????? I have a block that has this as an engine number ...HDT...... Three centre pop marks then HDT then 6 centre pop marks. Ive had this engine since 1987 Are the race engines that you speak of like mine?????? Im still looking for the metallurgy report and will post a pic of the engine number when I work out how to do it. Phil The block also has the three ribs and valve reliefs... the head has round exhaust ports 308 valves that are canted...
Edited by repco, 17 May 2016 - 02:26 PM.
#36
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Posted 17 May 2016 - 02:56 PM
These special race blocks that you are talking about. Do they have an engine number????????? I have a block that has this as an engine number ...HDT...... Three centre pop marks then HDT then 6 centre pop marks. Ive had this engine since 1987 Are the race engines that you speak of like mine?????? Im still looking for the metallurgy report and will post a pic of the engine number when I work out how to do it. Phil The block also has the three ribs and valve reliefs... the head has round exhaust ports 308 valves that are canted...
I don't know. Will ask Dave. There was some casting trickery that went into them but I can't say more until (or if) I get approval from Dave to make it public. But put it this way - these were not production engines. They were specially cast for race duty and cast outside of GMH's normal foundry with inbuilt tweaks to achieve certain outcomes one of which you've I think hinted at with the canted valve.
#37
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Posted 17 May 2016 - 03:15 PM
I think ive just worked it out.. I just knocked a welch plug out of my HDT engine???? and it seems to have a siamese bore ..ill be buggerd. Never have I seen this on a holden six.. Has any one els??????
#38
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Posted 17 May 2016 - 04:38 PM
He did mention far thicker bores, but there was a reason for it which I need permission to tell.
#39
_repco_
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Posted 17 May 2016 - 05:13 PM
Siamese bores give the block heaps more rigidity. All the bores are cast as one inside. I will pull it apart and have a good look..
Edited by repco, 17 May 2016 - 05:15 PM.
#40
_Skapinad_
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Posted 17 May 2016 - 06:04 PM
Gotta be a dual cast !
#42
_repco_
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Posted 17 May 2016 - 06:36 PM
No cast date. the block is very smooth, like its been polished. Can Dave please contact me..
#43
_Skapinad_
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Posted 17 May 2016 - 07:29 PM
^^^ rivet counter is out
Who, me ?
Lol
#44
_repco_
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Posted 19 May 2016 - 05:27 PM
Thank you NA-PWR for that insightful read. I will now do my own research on the engines that I have.. could the dual cast date blocks be????????? . One cast date for the sand mould and the other cast date for the actual pouring of the cast iron to make the block.????????
#46
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Posted 19 May 2016 - 06:40 PM
Thank you NA-PWR for that insightful read. I will now do my own research on the engines that I have.. could the dual cast date blocks be????????? . One cast date for the sand mould and the other cast date for the actual pouring of the cast iron to make the block.????????
Oh no, don't you start on the dual cast date BS. 
Edited by S pack, 19 May 2016 - 06:44 PM.
#47
_repco_
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Posted 19 May 2016 - 06:46 PM
No. Not going to start anything. But may get rid of the BS. With actual fact.
#48
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Posted 19 May 2016 - 06:50 PM
No worries Phil,
very helpful members on here like Dave (S Pack) and a few others,
and will be great to see some pictures of the different casts etc.
#49
_GGG_
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Posted 21 May 2016 - 12:26 AM
It's entirely possible they could have made a temporary alteration to the production water jacket, core box (tooling) and knocked out a few experimental blocks with siamese bores.
You would wonder if heat build up in the bores would be a problem though. Considering you create a thick section between bores and alter the water jacket.
#50
_repco_
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Posted 21 May 2016 - 09:05 AM
Siamese blocks are grate.. Most v8 blocks used in race cars will be Siamese. There heaps stronger than a standard block. Heat soak happens in all engines so a thicker casting has no relevance to temp.
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