750 Crankcase Deck Height?

[TJBaker57]

The 2S cam has a higher lift than the stock cam. When installing an Atlas cylinder on my 1972 Combat, the 2S camshaft lobes hit the bottom of the tappet bores, just as you were describing. It was a simple matter to remove a small amount of material from the bottom of the tappet bores. It does not need to be done precisely. I may have used my mill, but it easily be done with a hand held grinder.

You may want to add a couple more possibilities as to why your engine displays contact marks.
E). Assuming the original cases, cylinder, and cam, inadequate clearances were machined at the factory.
F). The cases, cylinder, or cam were changed later and the tappet bores were not machined.

Meanwhile, back in the real world, it probably doesn't matter. Get out the grinder. It's a 5 minute job.

Agreed, a simple fix.. Already done months ago. I am just one of those that like to get to the bottom of such things.

FWIW, I put like 25000 miles on this bike in 3 years time back in the late 70s. I only discovered this issue while inspecting the parts recently.

I could find no documentation or discussion on this even being a possibility back in August when I first discovered the issue.

I relieved the interference in mid August using the original factory 2S cam as a guide to how much to remove. Used hand held rotary tools. I have no lathes, mills or any sort of such machinery. Hell, I don't have so much as a compressor or drill press!

In late September when I acquired a replacement 2S Hepolite branded cam I again had clearance issues. Hand to remove more material.

All discussed on my "ressurection" thread.

At this point I have no doubts whatsoever that cases, barrels, etc are original factory and this engine left the factory with this manufacturing defect and another I will talk about later in a separate posting.

The reason for this separate posting was that it had been asked if the cases had been 'decked'. This is the only engine I have seen so I have not the experience to tell by looking at it. I sought a definitive means of determining if a set of cases had been decked or not.

 

[TJBaker57]

on the base of the cylinders it should say Birco and there should be a string of numbers - what are the numbers.

24237

 

[L.A.B.]

24237

Normal marking for a 750 Commando barrel.

 

[mdt-son]

TJBaker57,

As deck height is a dependent variable, the deck can't form a reference plane, so my question remains, where and how do you set a plane of reference? If the bottom of the crankcase had been machined, it would have made a perfect plane of reference. Unfortunately it is not.

After perusing the drawing for a while, I found a way how to define a plane of reference. It goes right through the main bearings and is defined as follows: Using the fixing points of the ignition points housing, the center of which is found by triangulation of R80/2 (mm!), vector Center-to-boltpos@11o'clock is rotated 30 degrees CW, thereby a normal to the chosen plane of reference can be found. A downward vector 3.765" along said normal defines a point in the plane. Now the plane of reference can be drawn, at least for the timing side crankcase half. Looking at the number of dimensions using the horizontal line, it's rather obvious that this *has to be* the plane of reference. How to determine the plane for 850Mk2 and Mk3 engines I don't know yet.

Taking another component into the equation is not a good way how to determine crankcase deck height, I think.

Crankcase deck height needs to be fixed relative to this plane of reference. For this model of cases and with reference to the drawing detail, height appears to be slightly less than 4.562", and 4.5603" is not a practical checking figure. My bet is that nominal height is/was 4.560" and that it was stated in another GA type of drawing. You won't go wrong by using this figure.

This definition applies to the timing side case half only. It can't be applied to the drive side case half, which means the crankcase deck has to be machined in a combined operation, and it probably was, either clamped together or side by side in a fixture.

- Knut

 

[marshg246]

After perusing the drawing for a while, I found a way how to define a plane of reference. It goes right through the main bearings and is defined as follows: Using the fixing points of the ignition points housing, the center of which is found by triangulation of R80/2 (mm!), vector Center-to-boltpos@11o'clock is rotated 30 degrees CW, thereby a normal to the chosen plane of reference can be found. A downward vector 3.765" along said normal defines a point in the plane. Now the plane of reference can be drawn, at least for the timing side crankcase half. Looking at the number of dimensions using the horizontal line, it's rather obvious that this *has to be* the plane of reference. How to determine the plane for 850Mk2 and Mk3 engines I don't know yet.

Taking another component into the equation is not a good way how to determine crankcase deck height, I think.

Crankcase deck height needs to be fixed relative to this plane of reference. For this model of cases and with reference to the drawing detail, height appears to be slightly less than 4.562", and 4.5603" is not a practical checking figure. My bet is that nominal height is/was 4.560" and that it was stated in another GA type of drawing. You won't go wrong by using this figure.

This definition applies to the timing side case half only. It can't be applied to the drive side case half, which means the crankcase deck has to be machined in a combined operation, and it probably was, either clamped together or side by side in a fixture.

- Knut

It's not decked. May not be as scientific as your way but his cam is relative to the case/cylinder matting surface as is mine. This could not be the case if someone decked the cases. If I remember right, I started the whole decking issue thinking out loud but when thinking more about it, that would cause so many issues that it is a fools errand and I should have never asked!

The root problem has been found in that his tunnels were simply extending down further than normal based on my known Commando cylinders. @L.A.B. verified the casting number, still don't know if Atlas and Commando machining was the same so still don't know if he somehow ended up with Atlas cylinders like @Stephen Hill had.

In any case, shortening his tunnels resolves the issue.

 

[jseng1]

They are not exactly acurately made. Mating up unmatched case halves usually reveals that the decks are out of plane with each other and the heights are not exact. Same with the cylinder height. You can use unmatched cases - for instance if you want to run a magneto on an 850 drive side for an 850 - but you'll have to level the deck and hopefully correct the desaxe at the same time.

 

[mdt-son]

They are not exactly acurately made. Mating up unmatched case halves usually reveals that the decks are out of plane with each other and the heights are not exact. Same with the cylinder height.

Thanks Jim,
I hope varying deck heights is a result of a design change, not a way how to accommodate barrels not meeting the design specification. True, milling alloy is easier than milling cast iron, but it would be an inefficient and shortsighted way of conducting QA.

- Knut