924Board.org

[emoore924]

I'm thinking about building up a short block to have on a shelf that I could use for a turbo -OR- an n/a. I have a whole bunch of turbo and n/a stuff laying around that I can use.

Unfortunately, it is a mixed bag. For example I have n/a cranks and turbo rods so I'd have to combine things to build up 1 complete assembly.

I was wondering what parts are interchangeable between the turbo and the n/a and which ones are unique to one or the other so I can pick and choose my assembly parts and build something I could use for either.

Do-able? If yes, what goes with what...?

For example, Pistons: unique to n/a or turbo.

But what about:
Block?
Crank?
Con rods?
Wrist pins?
Mains?
Rod bearings?
Pan?
Oil pump?

Anything else I should know about?

Thanks!!

[WEASEL149]

Main difference is pistons. Other parts can interchange.

I have an almost new NA bottom end that I will build up as a spare turbo short block.
The NA pistons seem to take more abuse. They also have a larger top ring land and larger compression rings.

All I will do is deck the block some to get the CR acceptable when using the turbo head.
The rods i'm having made will go in this block.

If I was to use it with NA head the CR will be just over 10:1 so still fine and will make more power with higher CR.
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1979 UK 932

[emoore924]

So, are you saying that if I deck the block appropriately that I could use a pure n/a setup for the bottom end to build out a turbo -- n/a pistons, rods, crank, all of it??

I'm thinking then that I could assemble a n/a spec short block. Then if I need to use it for a turbo setup, I could deck the block and stick a turbo head on it...? Or do I have to deck the block before I put it all together?

Hmmmm...

[WEASEL149]

[ideola]

All of the parts are interchangeable. The only differences I can think of are:
- Pistons
- Oil pickup mounting arrangement
- Block breather adapter
- Oil filter console on the 931, which attaches to the block with a banjo bolt, rather than the simple threaded adapter used on the NA block. Incidentally, I am looking for the latter, so if you're wanting a 931 oil filter console arrangement, maybe we could work a trade, as I have multiple spares of the latter, but none of the NA oil filter dealios.

I don't think there is any difference in how the oil pickup actually functions, but the 931 and the 81-onward NA had a multi-piece bracket rather than a welded-on bracket.

The block breathers are also different, but I don't know why. It's something of a moot point, however, because both parts are NLA. Whatever you do, don't throw away your old ones!

If you're using an NA crank, make sure you can get the NA-style pilot bearing out. The 931 uses a different setup (press fit roller bearing in the flywheel rather than needle bearings pressed into the end of the crank), and you'll need to get the NA pilot bearing out of the end of the crank, which can be a bear of a job.

One last thing to note: the 931 oil pan has the port for the turbo oil drain line, which the NA oil pan will lack. So if you want a turbo engine using OEM configuration, it would be best to have the 931 oil pan.

Also, don't forget, the flywheel / starter / clutch / bellhousing is all different on the 931, so you won't be able to (easily) use any NA stuff there.
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erstwhile owner of just about every 924 variant ever made

[emoore924]

[MikeJinCO]

I'm building what appears to be at this point a NA based on the 931 motor similar to Ideloa's. I didn't plan a copy, it just sort of evolved there. I just checked and the 931 crank has a hole in the end that looks like a pilot bearing would fit. I saw the drain back on the pan and thought wonderful, now I can but a catch tank on the blow by and drain in back to the pan for a carbed version. The large drain back fitting in the pan looks to be about 26mm with the same thread pitch as the FI injeciton port fittings so something should be around. I found th 22mm FI plugs on line from someplace in Canada-cheap so maybe time to digging again. There is another oil line in the back of the pan for-I have no idea what, but looks to be a fairly normal size. So making the pan work should be fairly doable.

From what I have read the problems with 931 pistons were with rings and piston ring lands breaking. Remember turbocharging was really pretty new back then, I worked for TRW pistons rings then and the best material we had was nodular iron with various coatings, they have undoubtedly changed by now to better materials particularly for turbocharged apps. I wouldn't waste money on stock aftermarket rings quality is typically so bad I'd buy Total Seal and their cost appears to be half that of getting both new pistons with rings and pins from JE or Diamond.
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Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[leadfoot]

there are two extra ports on the turbo oil pan,
turbo breather return port (side)
might be able to modify with a weld on AN bung and change the bottom fitting on the breather line...
purlator return (rear, above oil drain hole)
this can be drilled and tapped easily enough...
dipstick looks to be in the same position behind the steering wheel... or maybe that's just my car?
Stu
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[MikeJinCO]

I looked at my 931 crank and it has a hole in the end for a pilot bearing. Never having seen the 924 pilot bearing I can't verify that it is the correct size. I measured the fitting into the turbo breather port(side) and it was 25.8mm so I googled 26mm pipe plugs and found a 26mm x 1.5 Drain plug, and Audi PN 059-103-193 which is sold by Sherco-Auto PN DP8020, for about $15. I did not verify the thread pitch, The back port appears to be a 10mm thread or close.

So closing up the extra ports in the pan shouldn't be a great problem. I'm building a carbed motor so will make a drain back catch tank like the 931 has.
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Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[leadfoot]

Here's what I measured on the pan... all 3 holes are 1.5 pitch
PCV return m14
turbo breather return 30mm
oil drain 26mm and here : http://fumoto.com.au/sizes.php
Stu
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...