Author Topic: MK Front bearing manufacture  (Read 658 times)

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Offline tck

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MK Front bearing manufacture
« on: 04 Nov 2020 at 18:43 »
It does look as if the front bearing for the Mk (26057) is not available, with some trepidation I am considering making one. I have done my reading of the forum I will make the one piece bush of phosphor bronze to protrude .010 thou into timing case make a small spacer to replace 37936 oil retainer cut a 030" thou deep internal grove in the bush  to match the crank oil hole and most importantly have .003" thou clearance in the bush to crank dimensions. the only thing I am not sure about is the OD of the bush how much interference should I allow? should I heat the crankcase half before inserting the fridge cold bush? (I assume loctight as well). finally would a camshaft bush need the same interference?

Offline eddie

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Re: MK Front bearing manufacture
« Reply #1 on: 04 Nov 2020 at 19:52 »
Tim,
       The reason the Club doesn't stock front main bearing bushes is that most engines either have undersize crankshafts or oversize bearing housings in the crankcases, or both, so bushes usually have to be custom made.
   Firstly, I would make the front main bearing bush the same overall length as the front journal on the crankshaft - this will mean that it will project into the timing chest by the same amount as the thickness of the flange on the front half of the old 2 piece bush. Also, your crankshaft should have an oil groove around the front journal, so there is no point in machining another one in the bush. The early one piece bush just had a single hole to line up with the oil gallery in the front crankcase half - it is important to get that positioned correctly relative to the cutout in the flange that fits over the locating pin. 1 to 2 thou interference should be fine on the bush, but DON'T use Loctite - it does not discriminate - it will just as easily lock the crankshaft into the bush as the bush into the crankcase.  Camshaft bushes only need 1 to 1 thou interference, but don't make the bores too tight - they are only splash fed, so the oil needs enough clearance to get into the bush and do it's work - something as trivial as a tight camshaft bush can make a motor noticeably sluggish!

  Regards,
                 Eddie.

Offline tck

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Re: MK Front bearing manufacture
« Reply #2 on: 05 Nov 2020 at 08:03 »
Thankyou Eddie
That clarifies the final worries I had about the dimensions and fitting I can see in my searches you have comprehensibly covered the fitting of the crankshaft and the 5 thou play in its assembly to the crankcase.
I shall now get some phosphor bronze and some spare cutting bits for my lathe tools I know PB can be very tough

Again many thanks
Tim

Offline Hamwic

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Re: MK Front bearing manufacture
« Reply #3 on: 07 Nov 2020 at 09:29 »
In addition to Eddie's information, I had a bearing made some years ago in the tool room at work, and beautiful job it was. It cost me several packets of cigarettes! However, the crankcase housing bore wasn't circular, so after fitting the bush, quite a bit of scraping was required to get a nice fit on the crank. You might want to make a couple of bushes, one a little undersize on the bore in case you have to do something similar!

Offline tck

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Re: MK Front bearing manufacture
« Reply #4 on: 07 Nov 2020 at 09:35 »
Yes its not an easy item to manufacture I have a decent sized lathe and correctly shaped and angled tools but an expert who had machined PB for a living told me  if you are looking for  a 3 thou internal clearance you cannot cut with 1 or 2 thou deep cuts you must be prepared to make your last cut at 10 thou deep and pray!

Offline eddie

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Re: MK Front bearing manufacture
« Reply #5 on: 07 Nov 2020 at 10:20 »
With bearing materials like PB, depth of cut is not the problem - it's the feed rate that needs to be coarse enough to keep the tool cutting - as soon as it starts rubbing, it will polish the surface and make size control more difficult.

  Eddie.