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Hertsdriver
Nürburgring


Joined: 12 Nov 2018
Posts: 468
Location: Hertfordshire/London

2004 Porsche 997 Carrera 2

PostPosted: Mon Nov 25, 2019 4:58 pm    Post subject: Reply with quote

Alex wrote:

And to say the pitch on the crank pins has no relevance to the pitch on the bores Surprised


I didnt say that though, dont mis-quote me.
I said it had no relation to an engineering firm milling a liner.
They wouldnt have the crank, they would just have the block and the instructions. Which is why the instructions to the firm are the important part. The actual machining is simple work, I dont know why this is worth arguing over as its pretty much indisputable.
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Alex
Le Mans
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Joined: 06 Mar 2014
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Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Mon Nov 25, 2019 5:06 pm    Post subject: Reply with quote

I didn't twist your words:

Hertsdriver wrote:

When you mill a bore for a liner, you find the centre point of the current bore, then use that as the datum. The heads have got nothing to do with the bore machining, neither has the position of the crank or anything to do with how poorly made the engine was in the first place.



The pitch on the bores should be right. Some Porsche engines are, some aren't - Fact. Why would you machine them out of line when they should be in line, it's absolute crackers and possibly part reason why they need a rebuild in the 1st place.

This is where the problem lies when places sub out the machining work and they follow errors instead of doing the job properly.

Edit: and to add to what I've said - we know the bores wear more on one side than the other and go out of shape - how do you pick the centre of the bore up when it's like that?
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Hertsdriver
Nürburgring


Joined: 12 Nov 2018
Posts: 468
Location: Hertfordshire/London

2004 Porsche 997 Carrera 2

PostPosted: Mon Nov 25, 2019 5:18 pm    Post subject: Reply with quote

You are twisting things.
I was merely commenting on your insistence that machining a block to accept liners is difficult, it isn’t it’s easy as a process.
I agree that things you have mentioned and others should absolutely be part of the evaluation of where to perform the machining, but that actual machining itself is a simple task for a machine shop.
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Alex
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Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Mon Nov 25, 2019 5:30 pm    Post subject: Reply with quote

That may be the case, but once Trevor Reznik down at Renwick St Engineering is dicking about trying to pick a bore up and get's frustrated cos it's all over the place, he says f**** it, gets it somewhere near and bores it out. The machined block goes back to the engine builder, he presses a new liner in, then 12 months down the line it's up at Hartech getting done again.

I can not stress the importance of getting these blocks machined out properly (as Hartech do) to how they should be machined. This is the sole reason they do everything in house - to ensure it is done exactly to how they want it doing - the correct way.

I'm not trying to cause argument, just want people to be aware of what's involved (or not at some outfits) when rebuilding these engines and how important it is to get it right.

Off for me scram food food
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Hertsdriver
Nürburgring


Joined: 12 Nov 2018
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Location: Hertfordshire/London

2004 Porsche 997 Carrera 2

PostPosted: Mon Nov 25, 2019 5:36 pm    Post subject: Reply with quote

I don’t think Baz would call Steve Winters work at JAZ into question, Which is where this started. What you just posted I don’t disagree with, but we were talking about 2 specialists that were mentioned, not some random.
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Robert SausageTrousers
Monza


Joined: 02 Apr 2018
Posts: 235



PostPosted: Mon Nov 25, 2019 9:37 pm    Post subject: Reply with quote

In my case the work was commissioned by the previous owner of the car, the rebuild was carried out at 49k miles and I bought it at 51k miles (now on just shy of 70k). I've no idea where the block was sent, but like I said according to the invoice it was sent elsewhere for 'specialist machining'.
 
  
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bazhart
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Joined: 20 May 2009
Posts: 984
Location: Bolton Lancashire


PostPosted: Tue Nov 26, 2019 11:29 am    Post subject: Reply with quote

Now-now boys – calm down.

Alex is absolutely right on this. The fact that the original cylinders are open deck means that they distort oval but not equally both sides of the thrust line and the vertical centre line of the cylinder is no longer perpendicular to the block. They end up off centre and no longer round.

Alex is also right about the alignment of the bore that was originally bored in the raw casting. All castings contract differently and the first datum is important to the outcome. We find that relative to the centre of the crankshaft journal – the bores can be all over the place – so when we machine out the old liner – some are in different places to others.

It took some time to jig the job accurately enough to centre the new liners (which in our case should more properly be called new cylinders) and the result is more accurate than the originals.

However I also agree with Hertsedriver that most people would imagine that it is a simple job – it just really isn’t in this case - which I will try and explain to keep the peace.

One interesting difference is that we completely machine away the old cast in alloy (with its reinforced Lokasil cast in - inner core) and fit an entire new cylinder. This is an important distinction because the old free-standing top of the original liner tube has already proved it is not strong or stiff enough not to distort under the pressure from the piston forces. Boring it out makes the remaining material thinner and therefore even less stiff and then fitting a liner results in the outer alloy being less able to resist the forces from the piston through the liner than it did before and we have found that over a period of time the top of the new liners can work lose and the liner can move or drop slightly.

It is a difficult problem to fix because if you try and leave the alloy thicker (by making a thin liner) the thin liner cannot impart enough interference fit to remain tight and if you make the liner thicker the outer alloy is even thinner as well and even less able to restrain movement.

Really any liner fitted to one of these blocks would be much better if it was also located at the top with the outside of the block (as we do) forming a top hat to convert the outcome to a closed deck that supports it against further movement.

However despite replacing a very large number of liners fitted elsewhere - we have not seen one in which the top actually fits into a precision machined location (although some look like they do).

Machining the top recess is also not easy because it cannot be done on all cylinders with a rotating tool and has to be interpolated whith a CNC machine (and ours Hertsedriver WAS A BRAND NEW AND VERY EXPENSIVE MACHINE THAT DOES JUST THAT WITH GREAT PRECISION).

Then the liner would have to start out as a much larger diameter and thicker tube and have more machined off it to cover both diameters.

Also the difference in expansion coefficient between dissimilar materials means that either a thick ferrous liner forces the thinner alloy exterior outwards on cooling (so the interference fit reduces) or a thinner ferrous liner is not strong enough to do that and is crushed inwards (reducing the interference fit) either way it is not ideal and when the block expands and contracts those interference fits vary.

The difference in expansion coefficients means also that hot and cold clearances have to be bigger for ferrous liners.

Our alloy liners fit precisely top, middle and bottom (how is our secret not yet correctly described by others - thank you!), convert to a closed deck, expand and contract with the block, suit the original designed criteria for the expansion and contraction of the piston, have the best material for cylinder sliding surfaces (Nikasil) and are technically the same as the GT3 and turbo variants that despite being more powerful have no reliability problems – and it is not surprising that they are superior.

Whereas a subcontracted machine shop has other priorities and work (and machines would need setting up every time they did a Porsche block) ours are permanently set-up with expensive tooling (that we can justify by amortising the costs over continual high numbers) and the repeatability is therefore better – and no delays in undertaking that specific work (except our workload).

Due to the efficiency of our internal process our prices are little different to ferrous liners anyway, we also offer a way to support existing liners, various discounts if the full 6 liners are replaced and as Alex rightly pointed out - additional cooling adjustments we found beneficial.

It is true Hertsedriver - to say that if we decided to fit a ferrous liner we could do it. We would first need to source a large enough ferrous liner to fit to the top machined recess but do know how to do a good job of it (but not like anything we have seen provided elsewhere so far) so we don’t dispute that someone else also might know what to do - but they would still have to secure the top hat properly, have the problem of differential expansion, lower thermal conductivity, greater wear rates (or shorter life span) and the interference fit would probably work lose over time (reducing thermal conductivity still further).

Ferrous liners are a good compromise in closed deck blocks and smaller diameter pistons when the pistons, engine and cooling design was originally meant for it (where the differential expansion rates are reduced proportionally) and can work in closed deck applications for racing (where coolant rates are sufficient and the engine clearances always run flat out and no one is bothered about the additional clearances that normal road driving in traffic would apply or the clearances lasting for 100K) but we do not think they are as suitable as our provision in these engines or the application – which is why the industry spent so many years perfecting alloy cylinder blocks – in the first place when ferrous liners would be cheaper.

Finally – most engine rebuilding machine shops do not have the right CNC machinery to machine out the blocks as we do and many would find it difficult to obtain accurate enough shoulder depths to prevent the liner slipping – however there will be some that can.

Many specialists use our services but of course engines can be rebuilt with original new crankcases and run as Porsche designed with Lokasil bores and if there are any providers that have perfected fitting ferrous liners – we would not see them for rebuild anyway.

So – boys - there is no need to all fall out over who does what and why – ferrous liners should be cheaper – our Nikasil alloy cylinders are better – our in-house quality control cannot be bettered and our reputation speaks for itself - your choice!


Baz
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Alex
Le Mans
Le Mans


Joined: 06 Mar 2014
Posts: 17119
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Tue Nov 26, 2019 11:35 am    Post subject: Reply with quote

Thank you Salut
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Robert SausageTrousers
Monza


Joined: 02 Apr 2018
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PostPosted: Tue Nov 26, 2019 1:10 pm    Post subject: Reply with quote

lol, perhaps I should find out who did the machining then!
 
  
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Phil 997
Le Mans
Le Mans


Joined: 05 Dec 2015
Posts: 15550
Location: Bournemouth,Dorset

2009 Porsche 997 Carrera 4S

PostPosted: Tue Nov 26, 2019 1:42 pm    Post subject: Reply with quote

Alex wrote:
They do various machining mods to improve oil / water flow for better temperature control of the engine.


Ah I thought thats what Hartech had told me that they improved the flow of Oil and Water around the engine as part of their rebuild , I knew there were a number of points that made me think that Hartech rebuilds go further than many if not all other rebuilders and it was more than their own liners that made them the best of the best. I also after reading this understand the machining process much better as well and Baz points about having dedicated machines set up with the correct tooling makes so much more sense now ,even though they had spent ages showing me around the site some of what they told me went over my head Grin it is a fascinating place to walk around as there are different areas and workshops all dedicated to just a single process in the whole build. Thumb I know I was left feeling that a Hartech engine was better in many ways than an original Porsche factory engine Thumb Thumb
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bazhart
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Joined: 20 May 2009
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Location: Bolton Lancashire


PostPosted: Tue Nov 26, 2019 3:09 pm    Post subject: Reply with quote

I guess it would help clarify things if people provided the information so we all know what they are talking about - but anyway - if your car is running OK (which I hope it is wherever it was done) - what's the problem?

Baz
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Robert SausageTrousers
Monza


Joined: 02 Apr 2018
Posts: 235



PostPosted: Wed Nov 27, 2019 10:34 am    Post subject: Reply with quote

bazhart wrote:
I guess it would help clarify things if people provided the information so we all know what they are talking about


Absolutely, I couldn't agree more...


Quote:
but anyway - if your car is running OK (which I hope it is wherever it was done) - what's the problem?

Baz


There's no problem at all, and yes the car's running great thanks (can't help thinking I'm tempting fate every time I type that!), just having read the discussion between the two gents and your informed response, it would be interesting to know who it was that did the machining work on my block.
 
  
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robrac
Newbie


Joined: 18 Nov 2019
Posts: 4



PostPosted: Thu Dec 05, 2019 1:17 am    Post subject: Hello New to forum and going paranoid. Reply with quote

update,,,,, cut a long story short it wasnt a crack in the no 6 piston. It was a casting/machining mark.

The Crankcase pressure..... I am thinking a little ovality was the culprit.
Almost no oil useage other than that I would expect, almost no oil smoke,

Even with that in mind, I have decided to bite the bullet and to bits it will come.
I am keen to get the ims bearing upgraded despite it being the larger type and so why not.

Your words of advice,,,, Thumb worship thumbsup
 
  
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Alex
Le Mans
Le Mans


Joined: 06 Mar 2014
Posts: 17119
Location: The Ribble Valley, Lancashire

2000 Porsche 996 Carrera 4

PostPosted: Thu Dec 05, 2019 7:57 am    Post subject: Reply with quote

What upgrade is there over the larger type? It's the best one out there isn't it? Dont know
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