Reuse of overheated mains caps.

[4v6]

Hello Guy, i wonder if i can call on your expertise once again.

I have been given the task of rebuilding an Audi Quattro engine by a friend.
The engine has suffered from an oiling failure, either low oil or maybe a faulty pressure relief valve on the pump, which has allowed the mains bearings to overheat to such a degree that the crank has blued at the 1st main journal and on 3 others.
It has also destroyed the big end bearings but not to the same degree as the mains and no heating of the big ends is evident.
The heat buildup has actually melted the surface layer off the bearing shell and discoloured the mains caps with oil burning on them.
The issue i have is that upon removal of the mains caps, i noticed a distinct difference in their ease of removal compared to the two mains caps that were not subjected to a bearing failure.

The question is: Is it safe to reuse these caps?
They are not slack, ie; wobbling and they do lock down but they definitely are a looser fit, such that i will worry about it.

Any advice regarding this question as ever appreciated greatly as id rather give my friend bad news now than have the crank attempt to leave the block at some point in the future.

Here are some photos for you, if you require more ill be happy to take them.

Tony.

Journal blueing, bearing material squeezed out. Cap discoloured and fractionally looser.

S8008444 (816 x 612).jpg (178.52 KiB) Viewed 3937 times

Bearing surface material squeezed out.

S8008445 (816 x 612).jpg (145.37 KiB) Viewed 3937 times

Discolouration of cap and bearing shell location.
Note the rearward one is untouched.

S8008451 (816 x 612).jpg (131.35 KiB) Viewed 3937 times

[Guy Croft]

Get a replacement crank, some new bearings and Plastigauge and see how the clearances look, plus read about checking crank bow:

http://guy-croft.com/viewtopic.php?f=4&t=928&start=30

Of course you can go down the housings with a bore gauge and measure them for out_of_round but don't expect them to be perfectly within OE limits even a 'good' used engine. You might well see some scuffing after tightening the caps (and you need to torque them) which might suggest some distortion.

G

[4v6]

Thank you Guy, the bore gauge checks are my next thing to do.

Ive already used the dial gauge to check the replacement crank for straightness and its absolutely perfect.
Ill order up a set of new shells and do the plastigauge checks once ive checked the journals for size and fitness for purpose.

Many thanks once again, Tony.

[4v6]

Just to say thank you Guy for your advice on this issue.

The main bearing cap locations were just half a thou out of round checked with the bore gauge across the full six locations so nothing appeared majorly amiss there.
New standard bearings were sourced and fitted, with the crank then being offered up.
Plastigauging them gave the result of 1.5 thou oil clearance ( max allowable is 6 thou but strangely no minimum is specified) for each journal.
After re-oiling the crank journals one by one and reinstalling a cap at a time ( rotating to check for binding) until all 6 were torqued, i then removed the caps.
Incidentally the turning torque was quite low, i was able to easily rotate the assembly with all caps torqued( no pistons fitted) by hand and between thumb and forefinger.
There was absolutely no evidence of scuffing to be seen once the caps were removed and the bearing surfaces examined.

Many thanks again, Tony.

[Guy Croft]

Sounds perfect to me, Tony, thanks for the detailed feedback and well done.

There may be tons of bearing debris in oil pump, oil lines (cooler?) thermostat etc and of course the crankcase galleries, a most diligent cleaning regime is called for. You know this, I'm sure.

GC