Porsche 911 UK Enthusiasts Online Community Discussion Forum GB

Welcome to the @Porsche911UK website. Register a free account today to become a member! Sign up is quick and easy, then you can view, participate in topics and posts across the site that covers all things Porsche.

Already registered and looking to recovery your account, select 'login in' and then the 'forget your password' option.

3.4 cylinder head crack

Armagreggon85

Silverstone
Joined
4 Jan 2017
Messages
104
Hi all

I've not posted for a while, as per title I've had some issues to attend to with oil in my coolant. Combined with having a baby 6 months ago I've only just gotten around to taking the engine out of my 3.4 for the 2nd time in 12 months. Initial diagnosis is a crack in the cylinder head on bank 4-5-6. This appears to be between one of the core plugs and and the oil gallery underneath the cam follower housing.

Obviously a new head is out of the question due to cost. A 2nd hand head could be an option but I'm worried about having a similar issue so keen to know more about a welded repair. Has anyone had their 996 cylinder head welded? Can anyone recommend anyone who does this? The car will take a beating on track so it needs to be reliable.

Secondly, what's the consensus on vacuum coolant filling? When filling my engine with coolant, when I last put the engine back in, I followed the workshop procedure in the Bentley manual. Raise the rear of the car, fill coolant, run the engine, top up as level drops etc etc. This is the same procedure I've used a number of times on my car and, like all the previous times, I had zero issues, no airlocks and the car bled fine. However 100 miles after installation there was oil in my coolant. It has been suggested this could be due to not vacuum filling the coolant. Interested to hear opinions....

:dont know:
 
I think repair will be prohibitively expensive. Replacement heads are not all that much, but worth crack testing. 4-6 heads are much more common than 1-3 heads due to the 1-3 heads cracking more often. I have a good M96.21 3.2l head you can have for £300, this is the same as the 3.4l head apart from a couple of drillings for exhaust brackets which can easily be done. This is from an engine I stripped due to a crack in the other head, which is much more common.

If you do buy a head, bear in mind that the cam cover, head, and two bearing caps are matched and must not be swapped. A head is useless without them. Check bearings in the head, they can wear as they are much softer than the camshafts.

I would always vacuum fill, it is designed to stop airlocks. Cracking can be caused by overheating, airlocks will not help with that.

Good luck.

MC
 
When we first came across this problem many years ago now we sectioned a head to discover an area where the cross section of aluminium became thinner than the surrounding material and it was obvious that the expansion and contraction of continual heat cool cycles in which the inner cylinder head area would heat and expand long before the coolant allowed the exterior to - would fatigue the material resulting in a crack just there.

The crack was usually seen across the spark plug hole threaded area (or near to it) exactly where this thinner cross-section was and so we knew that it would take time (or numbers of journeys cold to hot to cold again) to influence the lifespan and also that the faster rate of heating (and expansion) would shorten the lifespan.

Later models changes that section and the problem went away.

Some clever aluminium welders will cut that section out and re-weld it but in so doing introduce more stresses that may or may not be a permanent repair but will probably last several years of normal use.

Mileage therefore is not so much an influence as the number of times a head has been used from cold to hot (i.e. number of journeys) and how quickly or fast the engine was driven from cold.

A car used daily for short journeys to work and back might have low mileages but a lot of thermal cycles whereas one used occasionally for long journeys might have higher mileage but far less thermal cycles and be less prone to head cracks.

Without knowing enough about the history of a particular car or engine it is a gamble using a used on although a lowish mileage Boxster head would probably be a better risk because the smaller engine generates less heat and takes longer to warm up (which is good) and they tended to be driven less aggressively on average.

I think there is a picture somewhere in our old buyers guide on our web site showing the cross section and the thin area - but don't have time to find it today.

Baz
 
Sorry - I couldn't find the picture of the section in that very old (and largely out of date) part of our guide - so will have a look at work when I get in later and try and post it.

Baz
 
Thanks for the interesting information Baz, it does seem to be s very common area for cracks. It wood be great to see the section if you manage to find the image.

MC - I wasnt aware that the cylinder head had to be matched with the cam cover and bearing caps, what's the reason for this? It's a very kind offer which I might take you up on. I'll weigh up my options and let you know.
 
Armagreggon85 said:
Thanks for the interesting information Baz, it does seem to be s very common area for cracks. It wood be great to see the section if you manage to find the image.

MC - I wasnt aware that the cylinder head had to be matched with the cam cover and bearing caps, what's the reason for this? It's a very kind offer which I might take you up on. I'll weigh up my options and let you know.

The cam-cover has some of the bearing caps cast into it and are line bored as one complete assembly when produced so there will be some slight deviation using another cam-cover. They have serial numbers etched into the heads and covers so you know you have a matching set.
 
Armagreggon85 said:
Thanks for the interesting information Baz, it does seem to be s very common area for cracks. It wood be great to see the section if you manage to find the image.

MC - I wasnt aware that the cylinder head had to be matched with the cam cover and bearing caps, what's the reason for this? It's a very kind offer which I might take you up on. I'll weigh up my options and let you know.

As Sports and Classic said. There are 8 bearing caps, 6 are in the head cover, 2 are individual caps (the two next to the chain) All are bolted to the head then it is line bored during production.

MC
 
Can i just say from experience .. if one of the numbers on the cam cover or head when buying a second hand unit is not there then reject it ... im reluctant to say .. ground out .

There are also x2 camshaft holders ( bearing caps ) that also have this number on them so a complete " kit " is req .

Baz ... interesting .. i had wondered ..

My own experience of repairs to a cracked head .. not something i would tend to advise .
 
I noticed a receipt for a previous cracked head repair at 76K 2014 to my late 996.1 after purchase. The invoice in the folder from Northway Porsche is for £3300, to repair crack in cylinder head bank 1-3. The sub con £460 Machine shop included. This price also included clutch kit £321, pas pipe £128 ac regas and head gasket £98 to list just a few. It's done 30k since. With Northway recently completing a bunch of work from service through to changing coolant tank (£90 Porsche item), AOS, RMS etc. Anyway on 800mile round trip to a LeMans it showed a few moderns the repairs working just fine. :eek:ld:
 
Finally found the pictures.

The second shows a typical crack near the spark plug area.

The first shows the section where the bottom is where the heat is generated from combustion and the top is where the cracks occur.

You will see that the section is much thinner at the top so when the lower area heats up first it stretches the top (which is still cold) but that eventually expands as well but never quite as much as the centre combustion area.

The repeated stress cycles eventually metal fatigue the thin area of the aluminium.

Baz
 

Attachments

  • cracked_head_1_301.jpg
    cracked_head_1_301.jpg
    378.3 KB · Views: 3,927
  • cracked_head_section_210.jpg
    cracked_head_section_210.jpg
    38.5 KB · Views: 3,927
All really interesting information thanks. And thanks for digging out the photos baz.

What was your negative experience DeMort? I might have a lead on a welder who's highly recommended so tempted to give it a go. I'll keep you updated how I get on
 
I would be amazed, and very encouraged, if you could get it welded and machined for less than the price of a secondhand good head. The prices I have seen quoted in the US were around $1k.

MC
 
bazhart said:
Without knowing enough about the history of a particular car or engine it is a gamble using a used on although a lowish mileage Boxster head would probably be a better risk because the smaller engine generates less heat and takes longer to warm up (which is good) and they tended to be driven less aggressively on average.

Baz

Sorry for dragging up an old thread, but I thought it was better than starting a new one.

I've got a 3.4 M96 with the cracked 1-3 issue. Are there any implications to using a 3.2 Boxster head or is it a straight swap other than adding the muffler support bracket holes?

Thanks
 
AFAIK the chamber volume is smaller in the 3.2l engine by around 3cc to give the same compression ratio as the 3.4l

MC
 
I had seen that 3cc elsewhere and wondered how crucial it was. Realised I should have worked this out already - see below, sorry about the formatting.

It would lift the compression ratio to 11.8. Baz was suggesting in his post that a 3.2 head would be a better bet but I wasn't sure if that was as is or with the combustion chamber enlarged to that of the 3.4 head.

Thanks



Bore Stroke CR Swept Volume Crown Volume
mm mm cc cc
3.4 96 78 11.3 564.6 54.8
3.2 93 78 11.1 529.8 52.5
3.4/3.2 96 78 11.8 564.6 52.5[/code]
 

New Threads

Forum statistics

Threads
124,854
Messages
1,449,246
Members
50,063
Latest member
Stanley Goodspeed
Back
Top