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Engine mount failure

Mike Meakin

I got this e-mail from Mike as a follow up to the forum thread on this topic.


Hi David,

For me, "cracked lampbar" is fairly topical. Duncan, Dave Parr , Tom and me have experienced lampbar failure (of an early frame). Phil has taken remedial action by ensuring that frames now go out with a heavier duty lamp bar, with additional bracing.

There is a number of detailed photos showing the typical damage: Silverfish is probably the greatest casualty - the lampbar suffered complete fracture next to the offside engine hanger.

We have all opted for the "replace the complete lampbar" option, adding diagonal strip braces and corner gusset plates rather than patching up the original, thinner-wall bar. I'm waiting for my new lampbar to arrive.

Attached are images of my lampbar cracks (the crack started on the underside next to the weld bead and has crept round back and front, almost to the halfway) and the 3mm mild steel braces and corner gussets. The other three will no doubt be happy to send you their photos - I have copies in my archive.

I may well take advantage of the necessary partial dismantle to allow space for welding the new lampbar and go the extra mile in fitting my reconditioned gearbox (with Dyane CW& P).

regards

Mike

My comments for what they are worth

Hi Mike,

Thanks for the pics and words - I'll put it in.

I would have expected cracks to develop in places with excess tensile strains but the only way the bar gets these forces is by torque from the engine. The BMW and Guzzi engines aren't that powerful - say twice the 2CV - which suggests the original design (if it was designed!) had very little margin. Maybe it's the cyclic torque from the power strokes of the engine that's causing fatigue - this would show up on high mileage cars.

Regards

David

And Mike's response

Hi David,

Thanks for the reply.

Duncan's, Tom's and my car (s) were all fairly low miles (under 10K miles) and failures were underside cracks adjacent to the join with the front upright. Dave's Silverfish (pic by Dave attached) was a total bar failure - apparently attributed to the two big block Guzzi mounts being independent. Dave has manufactured a tubular brace between the two mounts.

From what you say about the tensile strains, obviously I'll be setting in the corner gusset plates between the underside of the lampbar and the inside face of the front uprights to counter up and down strain, but are the diagonal braces (from side rail to lamp bar) therefore necessary? Phil fits them to all frames now.

My BMW 1 litre engine generates 76lbf.ft compared to the 2CV's 28; the big block 850 Guzzi around 60+ (depending on model/state of tune - a HUGE hike in torque and given the complete lack of shock-absorbtion in the drivetrain the torque effect must be quite significant.

I've included two of Dave's photos - please confirm that he's happy to have them used; like Duncan, he's on holiday presently.

regards

Mike


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