79 930 top end rebuild questions

[JHP]

79 930 top end rebuild- piston skirt broke/found at bottom of sump.

1. Any specific reason the skirt broke? Anything I should correct before replacing? head studs? piston squirters? or was it just an over rev situation.

2. Two of the cam lobes on 1 camshaft has worn down about .5mm. (cam squirters appeared clogged) Should I have those lobes hard-welded or just a regrind of the 1 camshaft?

3. Any issues with replacing 1 piston and cylinder with brand new mahle parts, while the other 5 are still used but within spec?

4. The heads were rebuilt about 30k miles ago, should I replace all the valve guides and seals again since they are out?

5. Do I use mahle or goetze rings and what are the min and max ring gap ?

Thanks
and any tips or tricks i should be aware of would be appreciated as well.

[Flat6pac]

Thoughts
Piston/cylinder size and weight need to be determined
The previous top end may well have included the 24 head studs.
30k on turbo heads, guides need to be looked at
The SC/ Carrera cams are a good upgrade with just an oil pump adaptor on left cam.
You really need to split the case to get all the junk out. It likes to hide behind the oil pump
It might be more cost efficient to order a full set of JE 97
Either brand of rings are good, basically factory
Right side oil lines and cooler need to be well flushed out
Of course, with all the new on the top end, you need to redo the bottom to make sure it can hold
Like I said, just thoughts
Bruce

[JHP]

thanks, I should have added more info. It is an efi build with gt2 cams.

yea, definitely gotta do some flushing. I was also wondering if there is anything I can put in the bottom end to flush out for just incase, even though I found the 2 pieces it broke in, so I feel safe to assume nothing is in there. I'd hate to open the case since that was rebuilt about 50k miles ago.

I thought about the all new Je pistons but based on my findings it seems like I would have to bore out all the cylinders and have them replated, which is not an expense im looking to have.

It has the fatory steel studs,...just was wondering if maybe the cylinders shifted and caused that skirt to break.

[Jeff Alton]

If you found debris in the motor and have plugged squirters, to me, the only option is a complete disassembly, inspection and rebuild. Like what Bruce said...

Cheers

[pkabush]

“79 930 top end rebuild- piston skirt broke/found at bottom of sump“

That’s never a good way to start a post. Sorry about your luck. Hopefully you can rebuild it stronger and better. I’m no engine builder but, given the fact that the skirt broke I don’t see how you could be comfortable disassembling and reassembling the top end without going through the bottom end.
As much as your current situation sux it would be that much more devastating to have a catastrophic failure after putting it back together again. GL with whichever route you choose

[dannobee]

Do you have a pic of the piston? Normally, only cast pistons break the skirts. Forged pistons can absorb some damage and bend before cracking.

[JHP]

Yea,thanks guys. I will have to consider the bottom end. I Just wasn't planning on any of this cause the engine was running fine. Only dropped to fix oil leaks.

The piston is the 79 model which looks a bit different compared to the later model turbos pistons. Are the 79 model pistons known to have issues?

[gsxrken]

I’ll play devils advocate. Last year I rebuilt my 930 engine, including splitting the case, and also starting from a place where I was mainly addressing leaks. If I was you, with the cam lobes also ground down and distributed throughout the engine and oil lines, I’d do the same.

HOWEVER, if you’ve been driving around for months like this without issue and just finally decided to address the leaks, theoretically you could replace the piston with a similarly weighted one, do the cam swap and squirter fix, and be back on the road no worse than you are right now. You’d just not have peace of mind that you won’t be back in there again at some point. Depends on your risk tolerance and OCD level.

[dannobee]

Mike the piston and cylinder and see what you have. Was the engine overboosted? Overheated? How about fueling, did one cylinder ever get filled up with gasoline? Did you find any other damage?

[JHP]

only other damage is the same cylinder. Some score marks around the walls. Otherwise everything else seemed normal and engine ran normal.