Timing Plug

RoadScholar · May 10, 2009

On the '75 Mk III 850s, on the timing side of the engine case, just below the points cover is a hexagonal plug that, when removed, exposes the crankshaft. If you watch the orfice whilist rotating the crank a hash mark reveals itself every 360 degrees. I assume that this is a timing mark, but the orfice has no (visible to me) corresponding mark. I could see using this with a strobiscopic light and setting the timing at some RPM by centering the mark, but I have no documentation on this feature and wouldn't want to take any unnecessary risks.

I prospect of having a timing mark right under where I intend to set up my TriSpark is very appealing.

Anyone care to offer me some counsel on this notion?

RS

79x100 · May 10, 2009

The slot in the crank cheek at 28° BTDC is only intended to be a rough guide to static timing and should be central to the opening according to the Haynes manual.

Not all Mk111 cranks have the machined slot although mine may have been a replacement item. I spent a lot of time turning my engine over looking for the slot the first time that I serviced it.

Reggie · May 11, 2009

I checked mine when I was doing the cam timing recently, and found that it was just slightly off centre in the hole when the crank was set at 28* btdc. The plate in the primary was spot on though

L.A.B. · May 11, 2009

Here's a photo showing my MkIII's timing aperture.

The 28 deg BTDC crank slot can be seen lined up inside.

There are four small notches with scribed lines on the machined face of the aperture. and I'm sure I've seen a drawing before that showed those marks, so I assume they are original case marks?

Jason Curtiss · May 11, 2009

Interesting marks around the timing hole; my '75 Mk3 doesn't have them

L.A.B. · May 11, 2009

Jason Curtiss said:
Interesting marks around the timing hole; my '75 Mk3 doesn't have them

Those marks show up quite well in the macro photo, but they are very faint.

Jason Curtiss · May 12, 2009

My MK3 doesn't have even faint marks; your marks appear to be home made.

Speaking of home made, I machined a timing plug to positively locate the crank at 28 degrees BTDC. The plug has OD threads to match the case and the end of the plug is machined down to precisely fit the slot in the flywheel cheek. I can submit a photo if anyone gives a hoot.

grandpaul · May 12, 2009

That's a great idea, Jason!

I think i'll follow suit on my 880, after verifying positive alignement with the primary side pointer. (Although it will be a one-time deal, as I'm going to be using an ARD mini-micro magneto)

Reggie · May 12, 2009

My MK3 doesn't have the timing marks either.

Do you know if your crank is exactly at 28* BTDC when the mark is exactly in the middle of the hole. Mine is slightly to the lower edge, probably about half the width off the mark off centre. At least that's where I got it at.

L.A.B. · May 12, 2009

Jason Curtiss said:
your marks appear to be home made.

I'm not exactly sure they are, as the marks are quite uniformly done, and I feel sure I've seen a drawing or a photo elsewhere that showed exactly the same markings? If only I could remember where I saw that?...My cases are not the originals however, as the originals were damaged and replaced with brand new cases at some point before I bought the bike?

Reggie said:
Do you know if your crank is exactly at 28* BTDC when the mark is exactly in the middle of the hole. Mine is slightly to the lower edge, probably about half the width off the mark off centre. At least that's where I got it at.

The slot seems just about spot on, on mine.

Reggie · May 12, 2009

LAB wrote;

The slot seems just about spot on, on mine.

That's put a bit of doubt in my mind now as to whether I had the timing wheel spot on, although I'm sure I did.

Logic would suggest that they were all machined to a very similar spec :?: and that if one was spot on, they all ought to be.

Jason Curtiss · May 12, 2009

Timing Plug

RoadScholar

79x100

Reggie

L.A.B.

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Jason Curtiss

L.A.B.

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Jason Curtiss

grandpaul

Reggie

L.A.B.

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Reggie

Jason Curtiss

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