Three Bearing Crank Engine - Centre Bearing Question

[Jamie]

Once I have finished the head and block on my engine, I shall start on the crankcase and crankshaft.

Everything which I have read about the centre bearing seems to suggest that the middle of the crankshaft deflects downwards, towards the head, when the centre bearing is tightened. I understand that this is because the centre casting in the crankcase is lower than the other two bearings. For this reason, shims are sometimes fitted to raise the bearing.

Are the any cases where the centre casting is higher, causing the crankshaft to deflect upwards and, if so, how can you detect this when test-assembling the crankshaft and bearings?

Regards,

Jamie.

[Chris KC]

Jamie,

I have never conducted a survey as I don't have enough engines nor time and energy to do it.

What I can say is that on engines in my possession, the centre bearing is (was) too high in relation to the front and rear mains.

Also, that the mounting structure conceived by Austin allows this to vary depending on how the nuts / studs are done up.

The proper engineering solution to this is to

a. divorce the C/M main fixings from the block studs.

b. make up a new centre main upper housing with undersize bore.

c. skim the joint face of the lower housing

d. then get it lined bored with the block bolted down in place


It's quite neatly explained in A7 mag 1985D

  1985D.pdf (Size: 67.24 KB / Downloads: 68)

I don't consider it strictly necessary to make a new lower housing, but without a new upper line boring will achieve nothing (at best).

An engine prepared in such a manner can use standard shell bearings without shims and fiddles and can be reliable as long as your shaft is sound to begin with.

[Robert Leigh]

You could set up a dial gauge aginst the crankshaft to see how much it moves as you tighten the centre main nuts, and also when you are test fitting the block.
Robert Leigh

[Jamie]

Thank you, both for the suggestions.

Jamie.

[squeak]

Jamie does your use of the  term 'up' mean towards the head, and 'down' mean towards the sump ?  We need to be on the same page.
Have you seen Dave Armstrong's Speedex page description of a fix in this area?  Difficult to find this page, Jon Edgar may know , I couldn't find it just now.

---

https://web.archive.org/web/201301070156...ge2152.htm

maybe persistance pays off

[Tony Press]

The Speedex website has been mentioned a few times recently - I understand the A7CA is working on recovering it.

[Bob Culver]

Can anyone explain? Does the whole roof of the crankcase bow up? Does it all alter again when the block is bolted? Are Seven crankcases noted for misaligned cam tunnels etc? The bearing housing does not seem to be very heavily clamped? Does the housing just fret its way up? Alloy crankases do seem prone to distort . The split crankcases of Javelin cars are seldom in alignment.
VWs with no counterweights fret the centre assembly.
Austins had much to answer for. Retention of the frail steering. And the 3 bearing engine at a time when the merits of rigidity were well understood.

[Jamie]

SqueaK: thank you for the link, which I shall read. In my post, "up" is away from the head and "down" is towards it: as far as I can ascertain, most crankshafts move downwards, towards the head when the middle cap is tightened. I am assuming that the engine is upside down. Doing this, I can see how to measure the deflection with a DTI or depth gauge. My question is how can you tell whether the crankshaft is being push upwards, away from the head, if the centre casting is inaccurately cast, but in the opposite direction? Then, tightening the centre cap will not deflect the crankshaft, yet it could still be slightly misaligned.

Jamie.

[Speedex750]

Hi Jamie et al,
The Speedex site is in the process of being transferred onto the A7CA site, it's all there, I'm just waiting for it to be uploaded.

Your experience matches mine, these crankcases are invariably distorted with the centre housing being too high, nearer the head. You can check this with a DTI on assembly and fit shims to overcome it. I haven't worked out why this is, there's no visible evidence of distortion or movement. I've found that different stud tensions do not make a difference to the housing position. Another factor is that the bearing housings are not all the same and you may not have a matched pair, matched into your crankcase. If as you muse that the bearing housing is too low, towards the sump, you'll have a lot of difficulty getting the shell in, though I've never found one like this. I find working on these a satisfying engineering challenge, but maybe there's other things in life..............

Dave

[Chris KC]

I think there's more than one issue at play here. I can't say whether it's distortion or whether they were bored out of line in the first place, but the C/M is often not quite in line with front and rear to begin with. There's also distortion of the thin crankcase upper deck as it is clamped up tight to the cast iron block - this may take various forms depending on the state of flatness/ cleanliness of both. The original A7 studs are a tug o'war fight between block and bearing and will clamp differently depending on which is done up first. Dave may well be right that the individual components (c/m housings but also potentially front and rear bearing housings) may mess things up if chopped and changed. Plus of course not all cranks are straight...