OT - plating query

Surely the crankcase was also ali and therefore just as vulnerable? as were numerous other parts in the engine.

AFAIK you can electroplate almost any metal onto another metal as long as the solutions and the base metals are compatible, although dissimilar metal corrosion must be a factor with metals too far away in AW.

For the sort of engine life they were looking at, making the plugs of plain ali would have been just as effective, just chuck a new set in on overhaul.

There's a lot of detail in the RRHT book on the Merlin 100 (which you also have?)

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

Were the the plugs more to do with reducing windage losses within the engine than corrosion protection? I don't know it the ends of the borings are exposed. The need to have good crankshaft aerodynamics has been recognised in performance circles for many years. For details of the plating, look up zincate. There's a number of hits such as this one:

John

Big bored holes in journals is old hat for Rolls-Royce - certainly the vee12 Phantom II cranks had the same design.

I always took it that it was done to save weight in the reciprocating mass chain and offered another 'machined surfaces' opportunity.

Regards,

Kim Siddorn.

Sludge traps ?

Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk

The crank doesn't reciprocate. Is the Merlin a sufficiently long stroke for centrifugal forces on the big end to be significant? In WW11, metal was scarce. Perhaps it was just an excersise in ecconomy. Seems daft to then intoduce more machining operations and components as a result.

John

Erm, the crank rotates, but the big end *journal* does add to the reciprocating mass that includes piston, conn rod etc.

Not a sludge trap, Peter, as there is an oil drill way at TDC (as it were) .

Regards,

Kim Siddorn.

Does the oil drilling intersect with this bore? Is it part of the lubrication system? If so, that would explain the plugs to contain the oil pressure. Do I remeber correctly that the Merlin runs 2 conrods per journal? If so, that would explain the need for an oil gallery in the crankpin. The 'normal' technique of a single drilling from main bearing to big end would only feed one bearing. On the subject of recip/rotating masses, surely any part of the crank only experiences rotating motion. The con rods vary between rotary and reciprocating motion with the split varying with distance from the big end. Obviously the piston only experiences reciprocating motion unless something has gone very wrong. I would have to think about whether the big end of the conrod sees anything other than purely rotary. My head's hurting now though so that's one for later.

John

Not for the reactive metals, like aluminium and titanium, that form oxide films. They're fussy - you have to find something that will displace the oxide film chemically and leave an electroless flash coat, then you need a plating process that can either strip or reliably cover the flash coat -- without exposing th ebase metal in the process, so that it re-forms the oxide film. It used to be that nickel was the only thing that would go onto aluminium, and only via using a zinc compound flash. Later it was developed as a copper plate onto aluminium, using the same zinc flash. Copper is also the only thing that goes onto titanium.

I suspect Kim's nickel plating is because it's the only thing they could get to stick there. Besides which, consumption of strategic metals for plating is minor, compared to alloying with them.

Merlins (& other Rolls-Royce aero engines too) have an interesting rod design. The crank has six throws and huge big end journals. These support a forked rod that with a bearing cap that traps a separate subsidiary cap that has a bearing surface on the OD as well as the ID. This is centrally located and a thin rod of wide diameter fits over that OD. Both these bearings are fed with oil carried in the crank in the usual way

Having read that twice, I'm not certain I'd understand it if I'd not seen it, so I've found a drawing of a Crecy rod which are sufficiently similar for explanation. See

Combat Merlins were rated at 2,800RPM with 3,000RPM on hand for emergencies, but this meant an engine change as they tended to crack the main bearing housings if over revved & the bottom ends had to go back to Derby for crack testing. What always amazes me is the level of organisation required to operate these cutting edge devices in the depths of such a conflict.

Sorry, I'll just get my anorak ...........

I remember from the days (my God, over forty years ago!) when I looked into balancing cranks that all non-pure rotating masses must be considered in the balance issue. It is hazy now, but I recall literally weighing the piston and crank in two accurate scales to assess the changing factor of balance from BDC to TDC. I didn't actually do the balancing work, but arriving at the figures saved time that I'd have had to pay for.

The result was a MUCH smoother Triumph twin & well worth the cost.

Regards,

Kim Siddorn.

Should it be of assistance, I have uploaded an image of a pair of Merlin pistons etc. to this page:

Also included are a picture of Peter Grieve's Merlin XX together with a short video clip of it running.

Interesting, the Crecy rod has a small bearing running over a large bearing that runs on the crank. Any idea what materials they used? The usual wisdom seems to be to run a soft bearing on a hard shaft, so was the large bearing a composite of soft inner materials and hard outer?

Mark

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The Merlin rod setup used steel backed lead bronze shells in a one size fits all. These would then be fitted to the rods, bolted to a jig with lining bushes and reamed out with a tool like a shell mill reamer to the size needed. The arbor that carried these was about 2" diameter and was turned with a large, about 4 foot long, wooden tap wrench with metal insert where the arbor fitted. Can't remember if the max size was 40 or 60 thou oversize but it made your bloody arms ache after you had done a set at full oversize if the reamer was getting a bit blunt.

Don't ask...............

.

-- Regards,

John Stevenson Nottingham, England.

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ISTR the early ford flatheads used a single pair of bearing shells for two rods with lining inside and outside. 'course, they were side-byside. sammm

Thats right, the big ends on the 1930's V8s were "fully floating". Not sure what the verdict was, but they don't seem to have caught on generally.

I think they were fine for relatively low bearing loading applications, but R-R had lots of trouble with the Merlin big ends before finding a bearing material solution. Allison seems to have got through with their design without as much trouble, albeit at lower power initially.

Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk