ET: Engine (Twin) Series ‘D’ Breather

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
The spares company show an alloy type for a Comet, I think it's the same for a " C" Twin.
I think you just have to make sure more clearance and more end float.
The breather sleeve is aluminium while the spindle it is on is steel. Aluminium expands at around twice the rate of steel so if there is insufficient end float it WILL seize. Happened to me. To ensure it does not happen set it up with at least 0.015" end float. See MPH 852,page 23, point 1.
 

Robert Watson

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VOC Member
I just hate it when aluminum expands at twice the rate of --- well ---- aluminum. :);)

Never read the contents just the grammar and the logic! Once a proof reader is a habit hard to break....
 

greg brillus

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VOC Member
Cyborg, your cases were probably left on the shelf due to the bearing bores being under size, any more than 0.0015" interference and the bearings will be too tight. I would be taking your time and double checking everything. If the outer races on the timing side are too tight, the inner races will either not fit or be too tight. The small race can be honed out quite easily to fix this, but the larger inner will spell the end for that bearing, as removal will most likely destroy it. Do not over heat the cases, an actual 200 deg Celsius is too hot, the metal will start to become annealed, about 170 to 180 max. Remember to install the small outer first, then the oil pump sleeve, inner large race last. Good luck with it.
 

Pushrod Twin

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VOC Member
The breather sleeve is aluminium while the spindle it is on is steel. Aluminium expands at around twice the rate of steel so if there is insufficient end float it WILL seize. Happened to me. To ensure it does not happen set it up with at least 0.015" end float. See MPH 852,page 23, point 1.
Yes, it is the expansion of the aluminium that I am wary of. It wasnt longitudinal expansion that caused mine to bite, I believe it was simply a lack of diametral clearance in the area where it was an interference in the pinion.
I suspect that the bushes may have been press fitted to the pinion then put on the shelf for later assembly without the bore being re-sized. This would leave the bore in the pinion area a few tenths or even a thou undersize, maybe not enough for the engine assembler guy to notice, if it slides on and turns, its good, right? Probably 90% were good enough. Mine had a new spindle, .001" over size, enough to compromise the clearance when the alloy expanded.

I'll make the replacement bush from bronze, only 1/2 thou tight but loctite it, then hone the bore to finished size.
 

Cyborg

Well Known and Active Forum User
VOC Member
Cyborg, your cases were probably left on the shelf due to the bearing bores being under size, any more than 0.0015" interference and the bearings will be too tight. I would be taking your time and double checking everything. If the outer races on the timing side are too tight, the inner races will either not fit or be too tight. The small race can be honed out quite easily to fix this, but the larger inner will spell the end for that bearing, as removal will most likely destroy it. Do not over heat the cases, an actual 200 deg Celsius is too hot, the metal will start to become annealed, about 170 to 180 max. Remember to install the small outer first, then the oil pump sleeve, inner large race last. Good luck with it.

Thanks Greg. The cases are a bit of a mystery in that they were not a matched set and the mating numbers suggest that the drive side was machined prior to Harper and the timing side was done after Harper took over.
I have a mill and it is large enough to get the cases in, but then no room left to stuff anything into the quill. I formulated a plan of attack, but wanted to bounce it off the master, just to make sure I was headed in the right direction. Long story short, he took pity on me and removed the luck factor. I have machined a few precious things like Norton cam boxes, but these were a daunting task, so happy I caught him waiting for the rain to stop. Definitely above my pay grade. Mains on both sides were undersize (which strikes me as odd given the mating #’s) as were the bores for the G14 and G15. The G14,15 bores were also tapered, but there was enough meat there to straighten them out. Some of the spindle bores were undersize as well. I had 2 new ET30/3 and the smaller of the 2 is a very snug fit in the bore. The breather spindle went in with the flat up, so hopefully it’ll retain a bit of oil. One of the new E91 bearings didn’t pass muster, because it had too much runout and I was instructed to tell the vendor to stick it.
The ET92 and ET94 races were lapped to size and the decks came out to less than .001” over 10 inches, so I now have a (quote) “ beautiful set of cases”.
 
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