750 engine in an 850 block?

motorson · May 20, 2014

Can I put together my Combat engine in a set of 850 cases? What would be the specific problems? Are the crankshafts the same length? I already checked and the cylinder fits on.

What about mixing case halves? It was always a big no-no on Hondas in the '70s but now they say they aren't even machined as a set any more. What about Nortons? Did they line bore the cases as a set?

Dan.

Rohan · May 20, 2014

motorson said:
I already checked and the cylinder fits on.
.

Do the 4 studs that need to be there for the 750, where the 850 through-bolts are, line up ?

Don't know about the rest of it - we will be interested to see.
They always say the cases were line bored together, so whether unmatched cases were any good was always a crap shoot.
You lose a lot of power if they are way out of line, or so they say....

hanslymoto · May 20, 2014

Yes, you can build a 750 on 850 cases (well, I don't know about mkIII cases, but pre-mkIII cases are fine). When I rebuilt my '73 Combat I used a set of '73 850 cases.

Rohan · May 20, 2014

Its been discussed here recently they didn't make Combats in 1973 ?
1972 only. And not even a whole year of them.

So was it a Combat, or a 73.
That could mean different cases....

I'm sure I've seen it mentioned can't just bolt 850 cylinders to 750 cases, or everyone would be doing it.
That must mean the stud pattern, or cylinder base flanges/machining are different.

hanslymoto · May 21, 2014

It's a '73 750 that came with a combat head. And the 750 top end works fine on the 850 cases with no mods.

I don't know if the same is true for 850 barrels on 750 cases...

-Hans

hanslymoto · May 21, 2014

Found this quote from lcrken in another thread (750-850-heads-t16156.html)

750 and 850 cylinders will fit either 750 or 850 cases. You do need to screw in the 750 base studs on the sides to fit 750 cylinders, and remove them to fit the 850 cylinders. It's also a really good idea when fitting 850 cylinders to 750 cases to helicoil or use inserts in the holes on the sides where the through bolts screw in.

-Hans

hanslymoto · May 21, 2014

Whoops, I missed the next post in that thread (from kraakevik) -- 750 cases do need to be modified to take 850 barrels:

. . . not to mention that the 750 cases will need machining to accept the bigger 850 barrels where the "liners" protrude below the crankcase openings.

-Hans

baz · May 21, 2014

yes all the combat parts will fit the 850 cases its a good upgrade

ashman · May 21, 2014

In 1979 a friend of mine did mount 850 barrels onto his 750 cases, he had to grind the cases to fit the cylinders, but it did fit and some other mods, I was there when he was grinding the cases, I throught he was mad at the time and he was but he did it, it went well but he sold the bike a few months later and brought a near new 850 for the right price.

Ashley

750 engine in an 850 block?

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