Backplate problems

[IslipMinor]

I would weld a segment on BOTH sides and let it cool in the ambient air until it is warm to touch - welding on one side only will almost certainly result in distiortion. Pre and post-heating is only required for cast iron, to reduce the risk of cracking; it will do no harm with mild steel, but is not necessary.

With a long crack, start at the 'open' end first to close it up, and maybe after that leave a gap of 2", weld for 2" both sides and then come back a fill in the gaps once the first group of 2" welds have been completed. if you do get any distortion in flatness after the weld has cooled, try welding a 2" run on the opposite side to the 'dish' and let it cool.

Good luck!

[bmcecosse]

The 948 backplates are thinner - just pressed steel, with a formed edge. The 1098 backplates are thicker - flat steel plate - certainly not cast! I think someone (the infamous Owen Burton??) used to produce 'special' backplates supposedly for the 1275 conversion - with the oil pump location mistakenly 'moved' to accommodate the oil pump better? You may have one of these backplates - the one that doesn't fit very well! If you look at a 1275 Marina backplate - it has a larger oil pump cover - but it's still on the same centre as the original Minor covers - because the oil pumps have never moved! Buy a stick welder - usually ~ £40, less if secondhand, no gas to buy and the rods are v cheap for a pack of various sizes from Machine Mart. Handy for many jobs 'around the house' - well - that's what I told SWMBO anyway!

[Alec]

Hello BMCE,

it looks like I'm outvoted on the cast iron, but why would a steel backplate crack? Steel, even the alleged 'poor quality' mild steel is quite malleable and doesn't readily crack in such an application?

I agree with the stick welder advice, a very useful tool, I have no problem welding much thicker steel with one and at only about 60 to 70 amps. Multiple passes admittedly for critical joints.

Alec

[billlobban]

Well --- its welded - for good or for bad.
Reasonably easy but needed the MIG on full whack. Followed most of the advice above and doesnt appear distorted. Suppose Ill find out when I bolt it back on.
Thanks you lot - much appreciated.

[bmcecosse]

Dunno why they crack - they just seem to be brittle, and if the bolts are all done up snugly there really shouldn't be any forces on the backplate - it's just a means of attaching the gearbox and the starter. In the past - I only ever put in half the bolts - and only the top starter bolt (for quick changes!) - and never had any cracking problems....... So - is the plate still at Gas Regulo 9 Bill ??

[billlobban]

Thought of asking you to stick it in the firebox for a while

[bmcecosse]

It would MELT in there!

[les]

Well I must admit to thinking the plate is cast. I say this because of the way it cuts with a jigsaw and the fact that it has a casting number recessed in a rectangular 'box' then machined over!

[billlobban]

Les - a jigsaw? why?

[bmcecosse]

I assume to cut a larger hole to clear the 1275 main bearing - I used angle grinder! I have noticed the 'mark' - I assumed the plate had been stamped out from flat stock and the 'mark' embossed at the same time..........

[les]

Les - a jigsaw? why?

I have modified an original to fit to Ital block, found a jigsaw made the final filing easy.

[bmcecosse]

My angle grinder sorted it in two minutes!

[les]

My angle grinder sorted it in two minutes!

What's the hurry!!

[katy]

Bill, what did you discerne from welding it? Steel? Cast iron? I would suspect that if you welded it w/out a problem that it's steel.

[billlobban]

To be honest I dont know that my welding was all that great. Ive never welded anything that thick before and the result left a lot to be desired.
I have an old spare engine which has a broken backplate which I was going to use to practice on but I just went ahead and welded the cracked one.Looking closely at the broken one it has a very 'cast like' crystaline structure on its broken edge. So I'm not convinced either way I'm afraid

[bmcecosse]

Buy a stick welder Bill - you know it makes sense..........

[Alec]

Hello Bill,

if both faces of the plate are smooth and even it is steel and can be welded with a MIG, you do need to vee the crack on both sides and the weld from both sides as said in earlier posts. If there are any hollows on one or both sides it is cast.

A cheap stick welder is a good tool and I fear the MIG is overrated particularly for car repair often used with CO2. I do concede that it is the cheapest way but it does have a few limitations.

Alec