CrookedRacer

I'm rebuilding my engine after spinning the #2 bearing.

Let's talk about the bearings! Then we can talk rods and pistons and rings, oh my!

Anyway, the #2 bearings are gone.





So here are the bearings I just took out:

#1 bottom



#1 top



#3 bottom



#3 top



#4 bottom



#4 top

CrookedRacer

Tomorrow I will take some pics of the pistons and the rods, particularly the big ends, comparing # 2 to the others.

The number 2 rod is discolored and is definitely going to get extra scrutiny.

Van

So, what exactly is your question?

I suggest you replace the bearings.

Seriously, like I said, I think you need to have the big-end of the rods resized - any engine machine shop can do that. Also, what's the condition of the crank? If the #2 rod got hot, so did the crank journal...

But, the reality is: What's your budget? And how reliable do you want this engine to be? Everything is a compromise.

CrookedRacer

Thanks Van! I really look forward to the wisdom of you and everyone on the site.

So I hadn't really gotten around to my question, but this is my first ever rebuild, so you can safely assume it's "What do I do now?"

The GOAL:

The car currently has a junkyard engine that I threw in just so I could complete my provisional races.

My goal for this engine is to gain experience with rebuilding an engine, have fun doing it, and to put it back in the car for the 2017 race season including only the most cost-effective best practices for engine longevity. NASA PTE class.

1) oil pan baffle
2) external oil cooler
3) junkyard crankshaft that's not galled at all (cross-drilled if it makes sense to do so - I know there are many opinions out there)
4) new bearings, belts, sprockets, and seals all around
5) valve job w/ new guides (there was at least one somewhat wiggly valve when I changed all the valve springs earlier this year)
6) replace the OEM rubber core clutch with SPEC stage 1, lightweight flywheel and pressure plate.

I suppose someone with any rebuild experience at all would have known there would certainly be substantial machine work involved in the bottom end of an engine that's spun a bearing. I don't know what's commonly done in that area, and the stupid car shows all make it look too easy. So I wasn't counting on extensive machining costs, or worrying about bearing clearances. What would I do if they're out of spec anyway?

I'm learning a lot.

I haven't gotten the crankshaft yet. But I'm hopeful I'll find a used one in excellent shape that won't require much work, if any. And I can get a used connecting rod that hasn't been beaten up for $25. That would be a far better starting point than my fried one, I think.

My BUDGET:

My budget will expand to what makes sense to do as a DIY racer. I'm enjoying this so far, and if it takes outsourcing a few things to do it right, I'm willing to do that. But we're talking maybe $2500 - $3000 total including a lot of timing pulleys and idlers that wouldn't normally need replacement.

Belts, Bearings, seals, idlers, rings, etc.: $750
Used oil cooler console: $140
External oil cooler, hose, connections: $250
Zims reconditioned water pump $120
Valve job + new guides + installation + .004 deck shave $530
Rennbay header stud kit $45
Used crankshaft: $150*
Used connecting rod $25*
Oil Pan Baffle Kit (Lindsey) $80*
Baffle installation $100*

--------
$2150 (so far, and not including tools and consumables)

Also necessary, but not technically part of the engine rebuild:

flywheel, clutch, pressure plate, bearing $1100*

*not yet purchased

My PLAN:

Get the crankshaft, get a replacement #2 connecting rod, assess the bearings, journals, clearances, and pistons themselves (pictures to come), and then ask more questions.

Van

How are the cylinder bores in the block?

If they're not perfect, you have 2 courses of action: use as-is, or have some machine work done. If you're doing the former, than you might not want to spend the money to get the rods in spec... Why, you ask? because the engine will have other issues that'll probably rear its head before the next set of rod bearings goes.

Or, if you go with the latter, then I would suggest new oversized pistons (which will have tighter ring groove tolerances and help make more power). And if you go through all that, then why not have your rods machined and balanced so they're contributing their very best? But, of course, you can easily spend a few more thousand on all of this.

I guess you need to look at it this way: Junkyard engines are $500 or so. "Freshening up" an engine costs about $2k. And a new build to factory specs (or better) can easily exceed $5k. But, you can get 10 junkyard engines for that price... There's no "right" answer, different choices just fit different people better.

CrookedRacer

The cylinder bores are all perfect except for #2 which has a couple minor lines that can only just be detected with a fingernail. It's not as bad as many I've seen on this forum (which some folks said they'd run as is).

I'll take and post pics tonight. The side of the piston itself must be softer than the bore, since I can see significantly more scratches on the sides of it. Looks like tiny pieces of bearing must have made their way in between the piston and the walls.

Thanks for the post - it's encouraging to know there's no "wrong" answer! ;-)

Van

The block is "alusil" which is an aluminum silicon alloy. After the bores are machines, an acid honing process is used to etch away the aluminum, leaving the harder silicon behind. So, yes, the pistons are softer.

333pg333

Might it not be worth getting the bearing mods done by Michael Mount while you're at this point?
https://rennlist.com/forums/944-turb...aring-fix.html

CrookedRacer

I actually did get in touch with Michael Mount today. He's retired, but he still does some things. I think I'm going ahead with the rod mods because I really don't want to risk having too small a clearance for proper oiling.

Doesn't matter how cool the oil is kept, or whether it's always available at the pickup... if it can't squeeze into the bearing, or if oil pressure is lost because the clearances are too great, the oil is not going to do its job.

Thanks!

333pg333

If he will do the bearing mods then I'd definitely do this. Small price for a good option.

CrookedRacer

Pics of the rods and main bearings. Looks like pieces of #2 bearing made their way throughout the engine?










I will let Michael Mount assess the rods... he may want to start with a fresh set. The crankshaft is toast, so I'll source a fresh one one way or another.

The bearings are surprisingly ugly. So I will have to be fastidious about cleaning this block thoroughly.

Dave W.

It's common for the #5 main bearing to have more wear than the others since it has to support the clutch and flywheel. The center main/thrust bearing look clean, no sign of dirt going through them. The other main bearings look normal with just a few specks of dust embedded.

SamGrant951

I have 2 good 2.5 n/a cranks - of course shipping may cost a bit.

odonnell

I have a full set of Rarst sinter-forged rods from a low-mile '82/83 NA engine if you want them for cheap. They're beefier than the cast rods, but it's unknown how they stack up to 951 rods. I installed rod bearings into them (zero miles) and have plasti-gaged them against the crank (from the same NA motor) and they were perfect. I have the numbers written down somewhere, but I remember them being on the tighter end of the spectrum. I can include said bearings - there are a few small superficial "wear" marks on them from fit-up and torquing to the crank.

Dougs951S has a few good cranks too, as of the last time I was in his garage.

harveyf

I'll chime in. I have done 2 rebuilds along the lines of what you are contemplating. There are details on my website (www.newhillgarage.com).

The first rebuild was after I killed the #2 bearing on a junkyard motor. I rebuilt the engine pretty much as you have described, minus the oil cooler. I have put 200 hard DE hours on it since then in the last 4 years with nary a problem. Those hours include several 1 hour plus "enduros" during Chin Happy Hours in summer weather. Your results may differ but I think the original Porsche design is more sound than many would give it credit for. See my article on Porsche 944 Oiling System Explained. Oh, make sure your cooling system is top notch. New radiator. New hoses. New overflow tank.

Make sure you understand what NASA PTE allows in the way of engine mods. For instance, in NASA 944spec, I think the answer is "none". But if the Michael Mount rod bearing mod is allowed, I would vote for that. Otherwise, I'm not sure if many of the other mods that folks focus on (cross drilled crank, oil pickup baffle) really address the problem in any significant way.

You might want to review the forum at NASA 944spec. There is some very good real world experience reflected there regarding 944 engine reliability.