Crank taper angles

[5portsrock]

Hi,

Does anyone have any info regarding what taper angles were on

997 cranks (thin tail)

and the later (normal) cranks?

Thanks alot

Should add im trying to modfiy an ultralight flywheel to fit a friends tuned 997

[R&R]

Before you start, are you sure you have a thin tail 997 crank? What is the diameter of the parallel section before the taper?

[5portsrock]

Before you start, are you sure you have a thin tail 997 crank? What is the diameter of the parallel section before the taper?

Yes

[R&R]

Well that's interesting, I had pretty much convinced myself they weren't put into production with a thin crank. I've had oil fed large tail primary gears, blocked oilway large tail cranks, and undrilled large tail cranks. The parts book lists only cranks compatible with a 22g109 primary bush, and doesn't list any of the thin tail 850 primary gears. That's why I questioned it, and was heading down the avenue that the crank may have been modified.....which ironically would have been the second such modification I had seen in a fortnight!

997 engine parts are such rareties, it is difficult to find information. I had to make a backing washer for the primary gear of one crankshaft because the groove for the C-washer was further down the crank, and then had to machine the clutch housing to allow the room for the later type deep clutch release bearing to fit with the coil Spring clutch.

I suggest measuring exactly what you have there, and making your flywheel fit that, but I would say it's going to require some (crank out) checking to make sure it will all work together.

It sounds like an interesting project, I'd love to see how you adapt the flywheel.

[Spider]

The thin Tail 997 Crank had a P/N 12A298.

The later thick tails were P/N 12A375.

[R&R]

Hi Spider,

I've been here before with this, and the 298 is listed twice in the parts book, before and after production of the 375. The initial 60 high compression engines are listed with a 298 crank with a DU bush 22A344 primary gear, do you think this is thin tail?

[Spider]

R&R, now you've got me scratching' my head on this one. I've just had a quick look through a few listings I have and it's not consistent, one has it one way and another the other way. I am inclined to think though that the 298 is the predecessor to the 375.

[R&R]

My theory is that the first 60 of production 997s for the home market were dispatched with a DU primary gear and undrilled crank(298). I am assuming that DU is a self lubricating material. This was a test to see if it would work, meanwhile the general production continued with the oil fed crank (375) until stocks were exhausted and the self lubricating primary gears were proved a success, once this had been determined the undrilled crank was reintroduced. It would explain why the parts book asks you to contact BMC Service technical if you wanted to replace the primary gear on any of the first sixty hc engines. They were looking to examine failures perhaps? Both the 375 and the 298 were large tail cranks as far as I can tell.

[ianh1968]

Does anyone have any info regarding what taper angles were on
997 cranks (thin tail)
and the later (normal) cranks?

Put the crank in a lathe and use a dial gauge at 90 degrees...
Wind the tool-post (or whatever), say, 1" along the taper and
note the dial gauge measurements at the start and end.

Use trigonometry to work out the angle:

EXAMPLE
If the difference on the dial gauge is 0.1", then
EXAMPLE
Tan θ = Opposite / Adjacent
Tan θ = 0.1 / 1
Tan θ = 0.1
Angle = 5.71 degrees
EXAMPLE

You could obviously use a metric lathe as well, the math(s) would be
exactly the same. Whichever is used, a sensible "long" dimension
should be used. The crank should also be rotated to get a reasonably
accurate/average dial gauge reading taking into consideration any
divits/damage on the taper.

Do the procedure a few times with different "long" side measurements
as a cross check - They should all come out the same!

This is the "Technical" section and no-one has answered the question
regarding the diameters of these thin/thick tails...

Ian

[R&R]

Nice trig there Ian!

Admittedly the thread has got diverted from the Original poster's question, but it's still relevant, all I'm trying to say is that I'm pretty confident there was never a production thin tail 997 (i'm happy to be proved wrong mind), so the crank he has is possibly a special, therefore perhaps not the most accurately machined, or in fact the best starting point for his plans, and there is no point machining a perfect taper to suit a thin tail crank based on info from another crank. I'm sure he is an excellent machinist to even consider this, but there is no point in machining something "perfectly wrong", plus there is all of the knock on effects of where the diaphragm clutch sits in the housing, and where the flywheel is in relation to the primary gear. As an engineering exercise it's quite interesting, but as a practical proposition in a performance car, it's risky and probably a bit of a waste of time as only a couple of years ago, boxed new cranks were quite easily still available.

[Spider]

From one of the parts books I have;-

"12A375 CRANKSHAFT, bare, oil-fed bush type, 1 3/8" tail diameter 1 997cc 1961-62. Stamped no. 12A375.
12A298 CRANKSHAFT, bare, Deva bush type, 1 1/2" tail diameter 1 997cc 1962-63. Stamped no. 12A298."

I've seen and worked (a bloody long time ago now!) on 997 Thin Wet Tails and 997 Thin Dry Tails, which I think was converted from a wet tail. I've never seen a thick Tail 997 Crank, though I know they were produced.

[ianh1968]

Nice trig there Ian!

Thanks! Just trying to prove that my schooling did not totally
go to waste...

<SNIP>so the crank he has is possibly a special,
therefore perhaps not the most accurately machined,<SNIP>

Really? There's a good chance that a skilled toolmaker would be able
to put a taper on a shaft equally as accurately, probably more so,
than what would end up on a productionised standard item.

From one of the parts books I have;-
"12A375 CRANKSHAFT, bare, oil-fed bush type, 1 3/8" tail diameter 1 997cc 1961-62. Stamped no. 12A375.
12A298 CRANKSHAFT, bare, Deva bush type, 1 1/2" tail diameter 1 997cc 1962-63. Stamped no. 12A298."
I've seen and worked (a bloody long time ago now!) on 997 Thin Wet Tails and 997 Thin Dry Tails,
which I think was converted from a wet tail. I've never seen a thick Tail 997 Crank, though I know they were produced.

... so for the benefit of the ignorant amongst us...

Would it be correct to say that:
1) The "Thin" tails were (1.3/8") and are or were originally, oil fed?
2) The "Thick" tails were (1.1/2") and were Deva bushed?

Were the available permutations the same as was produced
for the 850 engines built at this time?

Were there other size/configurations for small bore
cranks which are not either 1&3/8" or 1&1/2" and listed here?

Ian

[R&R]

Interesting! But the evidence I have seen contradicts that, I built a really early 997 a few years ago, and got really into finding what i thought were the right bits for it. One purchase was a complete engine, and box. It was standard bore and I had no reason to suspect it had been unduly messed with, although in the 40 years between it been made and it landing in the back of my van, strange things could have happened. It had the 204 block which differs in having the early thin crank thrust washers, and what turned out to be a 375 crank which according to the parts book was predominantly fitted to these early blocks. I suspected oil fed clutch and thin tail, so put it in the back of the workshop while I acquired the rest of the stuff I wanted for the car..... Thinking on, I went to great lengths to find a NOS thin tail deva bush primary gear, along with NOS +20 pistons etc.etc. In pursuit of this stuff I also acquired a NOS 298 crank new in a box.

I came to strip the engine, and found a large tail crank with the oil holes blocked, and the 1058 type primary fitted, which came as a surprise because as we know " Early 997's are thin tail" right? If I could, I wanted to use the 997 thin tail crank in this car, so I went to the box with the other crank, and it turned out to be a thick tail too, but undrilled. The parts books I have support this, as does the large tail oil fed primary gear I came across a year or so later. I still have a doubt in my mind, and the thin tail one may exist, but there are no primary gear parts (thrust washers, gears, bushes, or c-clips) shared between the 850, and the 997 according to the parts book.

Typical of these situations, and the reason I was slightly reserved in coming forward with this is that I have pictures of the block, head, and the early remote type 363 gearbox, but none of the cranks.

Sorry for the boring thread Hi-jack 5ports, but if you want to build a stonking 997, I was tempted when I owned mine to build an engine using an S crank into a 10cc midget block to make things a little stronger. Never tried it though!

[ianh1968]

Interesting! But the evidence I have seen contradicts that,<SNIP>

What contradicts what, exactly?
I've asked for some formal definition of "Thin" vs "Thick", the sizes, the types and what engines they apply to...

I am still none the wiser...

I'll start a new thread for this one!
viewtopic.php?f=3&t=14085

Ian