850 case with 750 sn?

(I'm learning the details as fast as I can, so my sincerest apologies for my naïveté.)

I posted elsewhere that I recently bought a frame and matching case thinking it was a 73 850. Then when I learned more about the serial number ranges, I came to the conclusion that this was a 74 850F frame titled as a 73 750. Now I am confused. Does the case serial number in the pic look authentic?

I sent the case up to Matt thinking it was a 750 case based on the serial number, but when I got the case back (beautiful job btw) and fit the 850 barrels I have, they slid in like butter. I don't think that they should right? Then I found an older thread comparing 750 v 850 cases, and I'm pretty sure this is an 850 case. So what gives on the serial number? Did someone smooth out the case number and punch their own to match the title? I thought 850s were in the 3xx range not the 2xx range.





Thanks in advance.

I'm stumped too; never seen that before. I'll be interested in the answer ... perhaps L.A.B. will chime in ???

Right behind me I have 220738 they are 750 combat breather cases
Yours are 850 timing chest with breather tube
They are likely machine shop "re-purposed"
The SN surface is NOT normally sand blast finish but very smooth and will polish up quite nice up if desired
http://atlanticgreen.com/engcases.htm

Could also be a set of replacement cases. Dealers could get new, non numbered cases for rebuilds back then. Stamp any number you like on them.

B

OK sounds like the numbers being re-punched is common. I had Matt at CNW 'tumble' the case which likely changed the finish on the sn area. All good. Thanks for the validation.

Am almost certainly making this a 750. I have a rebuilt 750 head so will be easier finding good 750 cylinders than a good rebuilt 850 head.

I can’t tell from the two photos. Does the case have a large sump filter like an 850 or small drain plug and breather on the lower timing case? Numbers are definitely 750 MkV from ‘73. I owned 220627 in the past.

Will 750 Cylinders go on 850 Cases? I thought there was a difference?

possm said:

Will 750 Cylinders go on 850 Cases?

Click to expand...

Yes, apparently so.
850 heads don't fit 750 barrels and vice versa (without modification).

Interbak said:

Could also be a set of replacement cases. Dealers could get new, non numbered cases for rebuilds back then. Stamp any number you like on them.

Click to expand...

Bought as "a frame and matching case". The "74 850" frame which, I believe, had a matching 220189 (73 750) plate suggesting the 'matching' of frame and engine cases was somewhat contrived rather than a straightforward case replacement.

illf8ed said:

I can’t tell from the two photos.

Click to expand...

unquestionably 850
1. There is evidence of the 850 tube breather on the rear of the timing chest near daytona 500 decal
2. case mouth opening has straight sides between forward 1st and 2nd hole 750 are curved.

possm said:

Will 750 Cylinders go on 850 Cases? I thought there was a difference?

Click to expand...

3, yes, however 850 case has helicoils in the side 3/8 holes used by the long allen bolts which are screwed in and out.
750 short studs go in 850 just fine. 750 barrels in a 850 case is fine,
850 barrels in 750 cases need a bit of clearance made.
IMO 200000 cases undesireable for 850 engine...20M3S better!

As to the frame? Is there a part# stamped on the front left tank mount ear.
see here:http://atlanticgreen.com/commandoframes.htm

Thanks to both L.A.B. and dynodave. I appreciate your notes and agree on the contrivance.

To explain myself, I bought what was advertised as (and I thought was) a 73 850 with matching frame/case. (It was not very expensive.)

After the purchase, I was looking up serial numbers to start buying parts on AN and found that 220xxx was a 750, not an 850. I assumed then it was a 750 and the advert was a typo. I posted on YouTube and Les pointed out to me that, this was an 850 frame and that my plate rivets looked funky. (Les had pointed me to the Atlantic green site and frame number info, which says 850F.)

I had already shipped out my case to CNW so assumed that when it got back I'd be looking at a 750 case. I don't have 750 barrels, but have some cheap 850 barrels and they slid right in. Which is why I was confused and needed confirmation. Now I know.

I know. It's a cluster but hey that's how things go for me so I'm used to it. It's a Norton and I want to put it back together regardless. Time to carry on and make a cool bike.

On the 750 barrels, I saw that RGM sells 'modified' 750 barrels that use the 850-style through bolts? I assume I can use those cylinders with JSM's 'stretchy' bolts to prevent the bolts from pulling out. My plan is to get a balance/welded crank with all the other fancy JSM bits. I'm realistic though that this will be a several years effort.

And now the hunt for parts begins!

Conrad said:

On the 750 barrels, I saw that RGM sells 'modified' 750 barrels that use the 850-style through bolts?

Click to expand...

The '850'-style through-bolt type is the one generally available for 750 models now.
https://andover-norton.co.uk/en/shop-details/15667/cylinder-barrel-750-06-0396-

As you already know, vendors' descriptions aren't always that accurate and not all Commando parts from different production periods are compatible, for instance, your 850 frame needs a pair of 'ANG'-marked yokes/trees as steering geometry differs between 750 and 850 frames, many 850 Mk.3 parts don't fit earlier models, etc. so if you are not sure if a particular part or parts is/are suitable then don't hesitate to ask the forum.

I recommend you start a thread in the 'Rebuilds' section that you can update or I can move this thread to the Rebuilds section if and when you're ready?

dynodave said:

unquestionably 850
1. There is evidence of the 850 tube breather on the rear of the timing chest near daytona 500 decal
2. case mouth opening has straight sides between forward 1st and 2nd hole 750 are curved.


3, yes, however 850 case has helicoils in the side 3/8 holes used by the long allen bolts which are screwed in and out.
750 short studs go in 850 just fine. 750 barrels in a 850 case is fine,
850 barrels in 750 cases need a bit of clearance made.
IMO 200000 cases undesireable for 850 engine...20M3S better!

As to the frame? Is there a part# stamped on the front left tank mount ear.
see here:http://atlanticgreen.com/commandoframes.htm

Click to expand...

Hi Dave
Yes I see what you mean item 2 case mouth openning. This photo is of my ‘72 750 drive side paired to an 850 timing side showing what you say. I’m not planning to use these, just left over. Too bad the original post cases have been re-stamped.

For your reference, here is a case stamp 53 past yours. Not sure how much elapsed time that would represent.

NickZ said:

For your reference, here is a case stamp 53 past yours. Not sure how much elapsed time that would represent.
View attachment 17350

Click to expand...

In reality Conrad seeing the real thing doesn't mean much. He has a 300000 series case despite the repurposed #.
All 200000 cases are the same with only 2 milestones.
1. The early ones with the sharp cutter stress risers, up to 206 or 208
2. The next milestone is when they stopped the prep machining for the first E-start ( begun in 20M3S)
Of course the 200000 series had std tune, combat tune and two more head options at the end of this series.

Conrad,
I'm late to reading this thread. How is your project progressing? I had another thought. Use those 850 cases and an 850 Barrel & Head. Get a short stroke crankshaft and make yourself a really bada$$ 750 screamer!

Looks like one of those "hand jobs" that have carefully produced tiny pock-marks and faux factory finish, then stamped as they pleased.

NickZ said:

For your reference, here is a case stamp 53 past yours. Not sure how much elapsed time that would represent.

Click to expand...

Most likely within one workday. Best I can tell form the few records I've seen they were doing around 50 a day at that time.

To LAB's post, 850 style barrel on a 750.

marshg246 said:

Most likely within one workday. Best I can tell form the few records I've seen they were doing around 50 a day at that time.

Click to expand...

Given that, I would expect the number punching to be pretty close to identical. Yet the digits on Conrad's stamp appear to have more space between them.