Not Again! H/G Failure on Forged 2.5 Build
26 February 2017, 10:45 AM
#1
Not Again! H/G Failure on Forged 2.5 Build
Annoyingly after 14 months and 15k miles the headgaskets have gone on my forged 2.5 rebuilt Hawk Sti engine 
. The gaskets fitted were RCM 0.78mm with the 11mm RCM studs, and ACL race bearings + Malhe forged pistons.
I can't afford to send it back to the shop so I've started the repair myself. As a starter for 10 I have a few questions:
- any other things to look for that might have caused the gaskets to go again?
- with the acl bearing set, is it likely the bottom end is ok and does not need any work?
- what determines required head gasket thickness, is thicker better/worse, or is to do with head skimming?
- any other useful tips for stripping it down!?
Cheers!
. The gaskets fitted were RCM 0.78mm with the 11mm RCM studs, and ACL race bearings + Malhe forged pistons.I can't afford to send it back to the shop so I've started the repair myself. As a starter for 10 I have a few questions:
- any other things to look for that might have caused the gaskets to go again?
- with the acl bearing set, is it likely the bottom end is ok and does not need any work?
- what determines required head gasket thickness, is thicker better/worse, or is to do with head skimming?
- any other useful tips for stripping it down!?
Cheers!
26 February 2017, 11:09 AM
#3
no track but lots and lots of tuning 'pulls' and I mean lots, in the last few months leading up to Christmas. Overheating actually only started last week (Thursday), but contamination of the coolant has been happening for a while, though I was hoping it was something else (naively), as I might have added a small amount of the wrong type of coolant at some point.
26 February 2017, 11:15 AM
#5
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Wow that must be doing your head in? 
I don't think I've spotted a H/G failure thread after a forged rebuild.
Where the heads skimmed and did you have any block work done, e.g pinned or cdb?
I don't think I've spotted a H/G failure thread after a forged rebuild.
Where the heads skimmed and did you have any block work done, e.g pinned or cdb?
26 February 2017, 11:51 AM
#6
no track but lots and lots of tuning 'pulls' and I mean lots, in the last few months leading up to Christmas. Overheating actually only started last week (Thursday), but contamination of the coolant has been happening for a while, though I was hoping it was something else (naively), as I might have added a small amount of the wrong type of coolant at some point.
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26 February 2017, 12:48 PM
#8
Aye through my tuning exploits I've hit mid 20s psi and been runnin at about 22psi with an mbc for the last 2 months.
As for power not sure exaclty as it a my own tune on the road, but i reckon about 350 ish. Im using the std vf43 turbo which from what lots of people have said shouldn't lift the heads.
The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
As for power not sure exaclty as it a my own tune on the road, but i reckon about 350 ish. Im using the std vf43 turbo which from what lots of people have said shouldn't lift the heads.
The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
26 February 2017, 01:05 PM
#9
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The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
my forged build should be ready in a couple of weeks. My ultimate goal would be 400 maybe 420 with an SC42 or similar. My spec is CDB and 14/12 rcm ultimate studs, please tell me I've gone in the right direction
26 February 2017, 01:20 PM
#10
Aye through my tuning exploits I've hit mid 20s psi and been runnin at about 22psi with an mbc for the last 2 months.
As for power not sure exaclty as it a my own tune on the road, but i reckon about 350 ish. Im using the std vf43 turbo which from what lots of people have said shouldn't lift the heads.
The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
As for power not sure exaclty as it a my own tune on the road, but i reckon about 350 ish. Im using the std vf43 turbo which from what lots of people have said shouldn't lift the heads.
The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
26 February 2017, 02:29 PM
#13
Aye through my tuning exploits I've hit mid 20s psi and been runnin at about 22psi with an mbc for the last 2 months.
As for power not sure exaclty as it a my own tune on the road, but i reckon about 350 ish. Im using the std vf43 turbo which from what lots of people have said shouldn't lift the heads.
The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
As for power not sure exaclty as it a my own tune on the road, but i reckon about 350 ish. Im using the std vf43 turbo which from what lots of people have said shouldn't lift the heads.
The engine was built with rcm high tensile head studs and was claimed to be good to 420hp (rods as the weak link) however if you read their website the high tensile ones are only good to 350hp, so if these were the ones fitted that might be it.
Imo the stock headbolts are stronger, the extreme can cope with more or arp2000 and cosworth etc but them 1635 in not fully convinced tbh.
With the extra tourque your 2.5 does and mi e with 400+ at times with 2 bar.
There's no other weak link in mine.
So going 14mm next and overspeccing
26 February 2017, 03:52 PM
#15
Given that I'm not looking for more than 400hp and want to keep the 11mm stud size, what's the best option, rcm ultimate, cosworth or other? From a quick look the rcm ultimate ones appear to have the highest strength, maybe?
26 February 2017, 04:06 PM
#16
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just minor head work for the 14mm if needed, but I doubt I will go that far. Anyway I set myself a budget on the build of 3.5k and it actually came in at 3.6k 
should be well safe upto 450bhp I would think?
26 February 2017, 04:19 PM
#18
I have a knock sensor permanently fitted to the block and use det cans and logger when doing tuning work and a logger for most of the normal driving. I've spent a lot of time logging my daily commute and other drives to smooth out my timing map. Also been using a mbc to adjust to various boost levels to hit different rpms and load cases to test the robustness of the map.
Got a wideband to and spent ages sorting the maf scaling.
The car was running bob on aside from it contaminating the coolant and now overheating!
Got a wideband to and spent ages sorting the maf scaling.
The car was running bob on aside from it contaminating the coolant and now overheating!
26 February 2017, 05:23 PM
#20
Its no good guessing until its stripped out, it may well not be the head gaskets, a split liner could be an issue here also.
Were the heads & block faces skimmed on the rebuild ? what sort of finish of the skimming achieved ? were the head studs, if new, were run in on initial torqueing up ?
You also mentioned adding the wrong type of coolant, the Cosworth / RCM gaskets are very funny on the coolant you can run with them, it causes the outer finish of the gasket to pickle up, & detach from the steel layer.
As for gasket thickness, it will require the engine to cc'd to establish the compression ratio, as a rule, with std compression Mahle pistons fitted, block & heads skimmed etc, a 1.1mm gasket is the usual fitment to maintain std OE comp ratio.
Were the heads & block faces skimmed on the rebuild ? what sort of finish of the skimming achieved ? were the head studs, if new, were run in on initial torqueing up ?
You also mentioned adding the wrong type of coolant, the Cosworth / RCM gaskets are very funny on the coolant you can run with them, it causes the outer finish of the gasket to pickle up, & detach from the steel layer.
As for gasket thickness, it will require the engine to cc'd to establish the compression ratio, as a rule, with std compression Mahle pistons fitted, block & heads skimmed etc, a 1.1mm gasket is the usual fitment to maintain std OE comp ratio.
26 February 2017, 06:41 PM
#21
Its no good guessing until its stripped out, it may well not be the head gaskets, a split liner could be an issue here also.
Were the heads & block faces skimmed on the rebuild ? what sort of finish of the skimming achieved ? were the head studs, if new, were run in on initial torqueing up ?
You also mentioned adding the wrong type of coolant, the Cosworth / RCM gaskets are very funny on the coolant you can run with them, it causes the outer finish of the gasket to pickle up, & detach from the steel layer.
As for gasket thickness, it will require the engine to cc'd to establish the compression ratio, as a rule, with std compression Mahle pistons fitted, block & heads skimmed etc, a 1.1mm gasket is the usual fitment to maintain std OE comp ratio.
Were the heads & block faces skimmed on the rebuild ? what sort of finish of the skimming achieved ? were the head studs, if new, were run in on initial torqueing up ?
You also mentioned adding the wrong type of coolant, the Cosworth / RCM gaskets are very funny on the coolant you can run with them, it causes the outer finish of the gasket to pickle up, & detach from the steel layer.
As for gasket thickness, it will require the engine to cc'd to establish the compression ratio, as a rule, with std compression Mahle pistons fitted, block & heads skimmed etc, a 1.1mm gasket is the usual fitment to maintain std OE comp ratio.
For the coolant I'm not really sure because the bottle isn't very forethcoming to whats in it, it was about 200 ml I added about 6 months ago. But that's an interesting point that I guess you can see the evidence of?
Naive question possibly, but what is "cc'd" compression checked?
26 February 2017, 07:48 PM
#22
I was thinking this to, especially if the head studs are lower than expected spec.
Given that I'm not looking for more than 400hp and want to keep the 11mm stud size, what's the best option, rcm ultimate, cosworth or other? From a quick look the rcm ultimate ones appear to have the highest strength, maybe?
Given that I'm not looking for more than 400hp and want to keep the 11mm stud size, what's the best option, rcm ultimate, cosworth or other? From a quick look the rcm ultimate ones appear to have the highest strength, maybe?
And as above. On a 2.5 the liners can crack as their pretty thin and block surface needs skimming on the 2.5s from what I've read.
Personally I think I chose the wrong studs and paid the price for it but the seller was t going to admit that as I was questioning it with them but the usual they've ran loads with them, but if rcm only rate them to 350
Anyway, learn and move on I suppose.
27 February 2017, 07:00 AM
#23
Thanks for the advice. As for exactly what was done at the rebuild I will need to ask the guy who built it for the specifics.
For the coolant I'm not really sure because the bottle isn't very forethcoming to whats in it, it was about 200 ml I added about 6 months ago. But that's an interesting point that I guess you can see the evidence of?
Naive question possibly, but what is "cc'd" compression checked?
For the coolant I'm not really sure because the bottle isn't very forethcoming to whats in it, it was about 200 ml I added about 6 months ago. But that's an interesting point that I guess you can see the evidence of?
Naive question possibly, but what is "cc'd" compression checked?
It's measuring your piston tdc volume, head combustion chamber volume, head gasket volume, with a burette to establish its static compression ratio.
02 March 2017, 02:40 PM
#25
To continue this thread... the engine is out and I'm considering the scope of work required. It's common practice to replace the main bearings when the H/G go first time as we know, but what is actual mechanism of failure that means the bearings need to be changed, is it because of oil contamination?
Generally would I be looking to replace the ACL uprated ones as a result of this second gasket failure?
Obviously that massively changes the scope (and cost) of the work required!
Cheers
Generally would I be looking to replace the ACL uprated ones as a result of this second gasket failure?
Obviously that massively changes the scope (and cost) of the work required!
Cheers
02 March 2017, 03:32 PM
#26
You really need to strip out to the bare block & get the faces skimmed, heads also, you need to start on a complete fresh level surface, otherwise they will fail again, quite quickly.
You can also asses the bearings, as a rule the Subaru 2.5 gasket issue rarely see's coolant in the oil, the biggest issue with bearing failure after a head gasket issue is usually down to the mileage when the failure occurs, the b/end bearings by this time are worn, stripping & refitting the heads will lead to contaminate the oil system with debris, from various sources, from cleaning up the fire faces, cam cover silicon residue etc, its just enough to tip the bearings over the edge, usually 4 to 6 weeks down the line.
You can also asses the bearings, as a rule the Subaru 2.5 gasket issue rarely see's coolant in the oil, the biggest issue with bearing failure after a head gasket issue is usually down to the mileage when the failure occurs, the b/end bearings by this time are worn, stripping & refitting the heads will lead to contaminate the oil system with debris, from various sources, from cleaning up the fire faces, cam cover silicon residue etc, its just enough to tip the bearings over the edge, usually 4 to 6 weeks down the line.
03 March 2017, 12:49 PM
#29
Can't comment on the torque condition of mine as the work was done about 15 months ago by a shop.
As for RCM bolts, from my understanding they used to sell one spec of high tensile head stud, but now they sell 2 specs, 'high tensile' and 'extreme', rated 350hp and 450hp respectively. That makes me wonder if they had a problem with old single spec range (i.e. it was sold as rated as higher than it could cope with), so down graded it and brought in the extreme ones also.......
As for RCM bolts, from my understanding they used to sell one spec of high tensile head stud, but now they sell 2 specs, 'high tensile' and 'extreme', rated 350hp and 450hp respectively. That makes me wonder if they had a problem with old single spec range (i.e. it was sold as rated as higher than it could cope with), so down graded it and brought in the extreme ones also.......