The symptoms of a 3 bearing crank failure?

[Terrytuned]

Conrod check this is a two bearing crank but the check is the same for any crank.    Terry.

[fatcatvera]

My check for c/m fit with the new -50thou shells to suit the newly machined -50thou c/m journal was to assemble the crank into the case using the c/m bearing only. This allows for the correct installation and clamping strategy of the c/m bearing. I did find that when tightening 'normally' (no torque figures yet, sorry) I achieved a clearance of 2thou (using plastigauge) and a freely rotating crank. If I then tighten further (again no torque figures handy) the c/m housing would then start to become oval - wider and less tall - causing the crank to tighten in the bearing.

I've had some guidance from Chris KC regarding the amount of crush of the c/m shells. I'll hopefully measure this tonight. The shells supplied to me are clearly not originally designed for the 3bearing A7 engine and most liklely are for a 3bearing Reliant sidevalve engine - the tang for one half is on the incorrect side and incorrectly positioned oil feed holes. It seems the c/m housing on a Reliant engine could be ever-so-slightly larger than the A7 engine leading to incorrect crush and this could be why I am seeing the ovality occuring.

Will report back...

[Speedex750]

Hi Mr f,
Sounds like you have Reliant shells (and maybe 600/750cc OHV shells), the other difference being the oil groove being straight round the shell rather than at an angle. After drilling holes where you want them you can either trim the tag off and reform one in the 'Austin' position or you can make a new slot in the bearing housing to suit the Reliant tag - a small cutting wheel in a Dremel with a steady hand will do this. I believe the major dimensions of the Austin and Reliant shells are the same, but I'm open to being corrected. Your 2thou clearance sounds about right, do you know the housing did deform/spread sideways nipping vertically - from Plastigauge checks? I've experienced the same effect and wondered whether the increasing tension increases the nip of the shells and swells them at the join nipping the journal horizontally? I use 35ft.lb and wouldn't want to go less than 30 on threads this size?
Dave

[Chris KC]

Reliant side valve shells were designed for a housing diameter 1.8940~1.8945".
I have attempted to measure three different A7 housings and come up with figures ranging from 1.875 to 1.893".

A marginally smaller housing dia would have the effect of increasing 'nip', which may acceptably be 0.003-0.007".
Anything above 0.008" will cause local deformation at the parting line, less than 0.002" and the shells will spin in the housing, with or without tabs.
Nip (or 'crush') is measured by doing up both bolts then removing one, and trying feeler gauges in the gap which should open up between the two housing parts.

[Zetomagneto]

My check for c/m fit with the new -50thou shells to suit the newly machined -50thou c/m journal was to assemble the crank into the case using the c/m bearing only. This allows for the correct installation and clamping strategy of the c/m bearing. I did find that when tightening 'normally' (no torque figures yet, sorry) I achieved a clearance of 2thou (using plastigauge) and a freely rotating crank. If I then tighten further (again no torque figures handy) the c/m housing would then start to become oval - wider and less tall - causing the crank to tighten in the bearing.

I've had some guidance from Chris KC regarding the amount of crush of the c/m shells. I'll hopefully measure this tonight. The shells supplied to me are clearly not originally designed for the 3bearing A7 engine and most liklely are for a 3bearing Reliant sidevalve engine - the tang for one half is on the incorrect side and incorrectly positioned oil feed holes. It seems the c/m housing on a Reliant engine could be ever-so-slightly larger than the A7 engine leading to incorrect crush and this could be why I am seeing the ovality occuring.

Will report back...

Why not just get the proper shells from Jamie at Sevenworkshop

[Chris KC]

My check for c/m fit with the new -50thou shells to suit the newly machined -50thou c/m journal was to assemble the crank into the case using the c/m bearing only. This allows for the correct installation and clamping strategy of the c/m bearing. I did find that when tightening 'normally' (no torque figures yet, sorry) I achieved a clearance of 2thou (using plastigauge) and a freely rotating crank. If I then tighten further (again no torque figures handy) the c/m housing would then start to become oval - wider and less tall - causing the crank to tighten in the bearing.

I've had some guidance from Chris KC regarding the amount of crush of the c/m shells. I'll hopefully measure this tonight. The shells supplied to me are clearly not originally designed for the 3bearing A7 engine and most liklely are for a 3bearing Reliant sidevalve engine - the tang for one half is on the incorrect side and incorrectly positioned oil feed holes. It seems the c/m housing on a Reliant engine could be ever-so-slightly larger than the A7 engine leading to incorrect crush and this could be why I am seeing the ovality occuring.

Will report back...

Why not just get the proper shells from Jamie at Sevenworkshop

As far as I'm aware Zeto our cherished suppliers are only able to offer NOS Reliant. There's nothing wrong with Reliant shells, but they should be installed with proper consideration for fit, not just bunged in hopefully. Anyone aspiring to make the C/M last should seriously consider line boring, in which case the housing can be sized to fit perfectly.

[fatcatvera]

The c/m shell was indeed supplied by Jamie at The Seven Workshop; it did require modification to suit the A7 engine installation as described in my earlier post. Having now measured the crush by tightening both c/m fixings and then releasing one of them the crush on the shells was measured at 4thou, which is a good figure.

Regarding c/m fixings: tightening to 30lbft is the figure that results in distortion of the housing causing tightening of the bearing on the crank. A figure of 25lbft did not distort the housing to any significant extent so this is the figure I have used. 

Tappets now modified with a 2" radius, tappet guides modified accordingly - anything I can do to squeeze even a fraction more performance out of the little engine can only be a good thing when you're being hunted down by modern cars.

Engine is assembled and ready do go back in the car - hopefully in time for Christmas!

Ray

[Chris KC]

Well done Ray, sounds like a good job to me.

Have you got a high compression (e.g. 1937 type) head on it? That's probably the single easiest win in roadgoing performance.

[fatcatvera]

Yep, the engine has a HC head fitted.

Another significant factor on performance is ignition timing. Bear in mind the engine will never consume less than 95RON fuel now and in the 20's/30's it was perhaps 60RON.

The spec for ignition advance is static timing set to points opening at TDC for the HC 3brg engine; and the distributor provides a whopping 8degrees of advance....so what's considered safe/optimised ignition timing for this engine based on modern fuel usage? I've heard mention of over advancement leading to crank failures - which isn't something I want to happen (again!).

A Bosch 009 - which I understand has been a fairly standard mod in the recent past - has about 24degrees of advance available (but no vac advance function either).

[Chris KC]

The standard advance setting is fairly conservative. 24 deg at full advance (>4500 rpm let's say) is OK on a strong, soundly built engine (n.b. I'm talking about engine speed and angle not distributor).

The old way was to set timing then slog up a hill in top gear; if the engine pinked you would back it off a bit. 'Unfortunately' pinking is a thing of the past and doesn't happen with modern fuels. An engine which is over advanced does sound noticeably harsh though (to a trained ear) - if in doubt play safe.

Those who use electronic ignition units say they are a little harder to set up but run much better - the amount of slop in an old DK4 leads to wide scatter in timing. If you stick with the original try to make sure it's in peak condition - new bushes and advance springs are available as are drive gears.