E: Engine Crankcase deck height in relation to main bearings

[chark_mandler]

Final preparation to comet crankcases before building. One check I always used to do on my racing engines was to check the crankcase deck height against a mandrel through the main bearings. For a number of engines (not Vins) one side could be .007” higher than the other side and sometimes worse.

I lack a big enough face plate or depth on the mill to check and flycut the face if out so would need to send out for the work.
Has anyone experience of how accurately the deck was machined on Vins in relation to the mains?
Thanks, Mark.

 

[Bill Thomas]

Be very careful, If you look inside the mouth, There is not a lot of alloy case,
Above the camshaft spindle, It's a weak spot from standard !,
But if you take much off the top face, The spindle will be hard to hold it's self,
I would not think it would be out, More like the alloy muff might be not true ?,
Also if you have to take the cylinder studs out, One of them may jam on the rear follower spindle,
You may have to take the follower spindle out first ?.
Cheers Bill.

 

[Comet Rider]

Hi Mark,
On my Comet cases I had get machined off about 15-20 thou, as the deck was out of true.
If you can find someone with a big CNC mill with the digital edge finder it's not too bad to do.

My local machinist trued the cases then machined the deck. No problems in about 15k miles

 

[Martyn Goodwin]

Not sure about deck heights though I have seen reports that the 2 decks in a few motors were NOT machined at 50 degrees, remedial work was required to restore the correct angle

 

[chark_mandler]

Yes the cam is very close. Attempted to measure again on plate glass but getting a variation on repeat, as near as I can measure it the height varies the the mandrel through the mains by between .004" to .007". I feel an accurate setup and a minimal skim would be ok for the cam fixture and would create an engine with less friction.

 

[passenger0_0]

Unless you have access to the proper measuring equipment I'd hold off any machining as you must get better repeatability than near 50% error.

 

[oexing]

Engine cases were numbered as sets so should be allright - basically. So I´d be very suspicious about detected mismatches. Possibly the dowels and bores were mangled and the real cause for visible flaws, so first do careful measurements before machining. Not an easy thing to just get reliable measurements.

Vic

 

[chark_mandler]

The cases are matched and I know the history of them. The top surface is totally flat where the two halves meet. I'm aware that my measuring equipment does not give me the space to measure with repeatable accurate results but they are near enough to show that the top is out by a small amount - as I've found on different engines for racing when I had the equipment to measure and machine accurately. If I do get them done it will be by a company that can do the job properly.
My overall query was if other people have measured the deck in relation to the main bearings and what the finding were. I feel its attention to detail like this that really helps create a good engine.

 

[Michael Vane-Hunt]

John McDougal decked my Comet cases, I don't rember how much they were out but John was not happy with them. John also used to check that the crank was square to the gearbox main shaft on the twin engines he built. He said they were seldom parallel to each other.

 

[Bill Thomas]

The cases are matched and I know the history of them. The top surface is totally flat where the two halves meet. I'm aware that my measuring equipment does not give me the space to measure with repeatable accurate results but they are near enough to show that the top is out by a small amount - as I've found on different engines for racing when I had the equipment to measure and machine accurately. If I do get them done it will be by a company that can do the job properly.
My overall query was if other people have measured the deck in relation to the main bearings and what the finding were. I feel its attention to detail like this that really helps create a good engine.

It's also another good reason to give the piston a bit more clearance.