questions about 72 Commando

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There were three 750 heads in 73. Ones stamped RH5 had 8.9:1 cr, RH6 9.3:1, and RH7 10:1 (used on the short stroke engine). All of these have 32mm intake ports. I am assuming that my late 72 engine has one of the lower compression ratio heads, but it has absolutely no stamps. I also have a 73 RH6 head and the stamp is easy to read. I'm also going to be curious what cam I find as the 73 750's should have the standard 06-1084 cam. It may be awhile before I open up the engine as I might have a relocation in my future.
Mitch
 
The original Combat roller main bearings (bad) were Ransome and Marles 8MRJA30 with 11 rollers.
The first Superblends (good) were Ransome and Marles 6MRJA30 with 13 rollers.

Subsequent Superblends are FAG NJ306E (12 rollers), SKF NJ306E (12 rollers) RHP NUP306#TN (13 Rollers)
SKF NJ306 Superblend like 8MRJA30 had 11 rollers.

If you count more than 11 rollers it will be a superblend,
however if you count 11 rollers it will either be the original non superblend 8MRJA30 or the SKF NJ306 superblend.

I was thinking if you could remove the main crank seal you could possibly see the side of the bearing enough to count the rollers.
However at looking at my old disassembly photos it doesn't look possible.

My bearings have MRJA30 stamped on them but no 8 (bad) or 6 (great) as a prefix.
It was only by counting that it had 13 rollers that I was able to determine it was a Superblend.

Bob
 
How do i tell if they are superblends?

Superblends have barrel shaped rollers, not cylindrical or ball and they are held in place by a distinctive brass cage. They have a cousin which uses a nylon or plastic cage which are the next best thing.[/quote]


Although the rollers are barrel shaped the difference between the center of the roller and outside edge is very small and not that easy to see with the naked eye.
The Ransome and Marles 6MRJA30 for instance was 9.53mm diameter at center and 9.46mm diameter at its edge. 0.07 mm is a small difference to see in an assembled bearing.

See my previous post.
If it has more than 11 rollers you're good and if its a SKF NJ306 with 11 rollers it's also good.

Bob
 
rx7171 said:
The original Combat roller main bearings (bad) were Ransome and Marles 8MRJA30 with 11 rollers.
The first Superblends (good) were Ransome and Marles 6MRJA30 with 13 rollers.

My bearings have MRJA30 stamped on them but no 8 (bad) or 6 (great) as a prefix.
It was only by counting that it had 13 rollers that I was able to determine it was a Superblend.
Click to expand...


I've never known of any previous reference to an R&M 8MRJA30 bearing, either in the NOC Commando Service notes, factory service bulletins or made by John Hudson in his NOC twin engine strip video, only MRJA30.
 
I just opened my 750 combat engine crank case, and found p/n RHP MRJA30 no 8 or 6 as a prefix. The bearing has 13 rollers.
I guess that means i have the good/superblend bearing. The bearing is in good condition and there was no end play before splitting the case half's. I'm just going to put it back together unless someone has a reason why i should change it. The other question I have is about the rear mounted breather vent on the back of the crank case. Is it really necessary to mod or change the location of the breather if your not racing the bike I've seen a lot of threads on the subject, this bike has 10,000 miles and has never be modified.
 
stevesbsa said:
I just opened my 750 combat engine crank case, and found p/n RHP MRJA30 no 8 or 6 as a prefix. The bearing has 13 rollers.
I guess that means i have the good/superblend bearing. The bearing is in good condition and there was no end play before splitting the case half's. I'm just going to put it back together unless someone has a reason why i should change it. The other question I have is about the rear mounted breather vent on the back of the crank case. Is it really necessary to mod or change the location of the breather if your not racing the bike I've seen a lot of threads on the subject, this bike has 10,000 miles and has never be modified.
Click to expand...

It's more important to relocate the oil pickup location. If you are will to go to this length then the breather location will be just part of the process. I attained a late set of 72 cases which I did this with this winter. See here: http://www.oldbritts.com/n_c_case.html
 
Thanks ,do you really think this applies to bikes that aren't raced. Any opinion on the bearing that i found installed.
 
stevesbsa said:
Thanks ,do you really think this applies to bikes that aren't raced. Any opinion on the bearing that i found installed.
Click to expand...
Yes, particularly if you do any highway riding. For a better understanding of this read the artical from old britts.

I am not completely sure as to what you have in bearings there but if your cases are split, than serious consideration should be take as to replacing.
From what i understand from LAB, a "6" prefix is the indicator of the early superblend.
So, if you have "no" "6" than you have no superblend. Check with others about this for I am assuming to some extent.
 
Marshal,

If you remove the oil pump worm gear, the timing pinion and the dished washer you should be able to count the rollers, unless somebody installed a ball bearing on the timing side of course.

Bert
 
stevesbsa said:
I just opened my 750 combat engine crank case, and found p/n RHP MRJA30 no 8 or 6 as a prefix. The bearing has 13 rollers.
I guess that means i have the good/superblend bearing.
Click to expand...


If there is no etched "6" prefix-then it is not safe to assume they are "Superblends".

The etched 6 on this Superblend can just be seen in this photo from a previous thread: commando-resto-t3691.html?hilit=mkv
questions about 72 Commando


Of course if the engine was to be treated gently then there's no knowing how long they may continue to give service, but to re-use them in my opinion would be a false economy and they really should be changed to the higher spec. FAG NJ306E.M1 C3 bearings.



stevesbsa said:
Is it really necessary to mod or change the location of the breather if your not racing the bike I've seen a lot of threads on the subject, this bike has 10,000 miles and has never be modified.
Click to expand...


It's not absolutely necessary if the engine is not going to be kept at high RPM (7,000+) for long periods.
 
If there is no etched "6" prefix-then it is not safe to assume they are "Superblends".

The etched 6 on this Superblend can just be seen in this photo from a previous thread: commando-resto-t3691.html?hilit=mkv


I can just make out the number 6 prefix. pretty much worn away. I took a picture of the bearing but can't make out the #6. After reading all the info on the scavenge system I'll probably send the case to OLDBRITT for their Mod. So I Guess I'll buy that Fag brg NJ306E.M1C3 brg. I assuming that both the drive and timing side use the same P/N bearing?
 
If the cases are open, then modify the pick up! It isn't that big of a job. I would think you could do it with a Dremel tool if you don't want to take it to the machine shop.

Better now than wishing you had done it after the bike is all back together. If for no other reason it will help with resale.

Russ
 
Trying to remove the crank bearing outer race from the case half, heated the housing with a good grainger heat gun for about 20 min then slammed the housing on the work bench a few times, bearing would not budge. sprayed with kroil reheated still wouldn't budge. Any Ideas

Engine s/n 204162 with 10,000 mile has the 6 MRJA30 bearing , the bike hasn't been run since 1984. The previous owner keep everything lubed. When i checked the end play there was none. All the other blogs that i have read said .010 to .015 any thoughts on this. Not sure if this was original or someone changed the bearing everything on the bike is original. Looks like the cylinder was removed based on the blue rtv around the cylinder studs.
 
stevesbsa said:
Trying to remove the crank bearing outer race from the case half, heated the housing with a good grainger heat gun for about 20 min then slammed the housing on the work bench a few times, bearing would not budge. sprayed with kroil reheated still wouldn't budge. Any Ideas
Click to expand...
Let it get cold again. Next, concentrate the at the bearing journal area only keeping the heat away from the bearing race as much as possible. Another thing is that a heat gun will make it harder to keep the heat where you need it rather than propane or better yet a Mappgas torch would do. It shouldn't take 20 minute. Heating both the case and race defeats the purpose.
 
Well i finally got it,broke down and told the wife i need the oven, 30 min. at 300 degrees came out after slamming on the work bench, first try. Not really sure why I'm changing it, the bearing looks good,its the new style 6 MRJ30 and has the shims in the timing case of the outer race. I thought that is something new where original had the shims on the crank. The engine hasn't been touched for at least 27 years.
 
If not plain jane flat rollers bearing the it bombed prior or someone just upgraded as preventative. Likely they are still fine. If takes a non lubed dry bearing rolled and rolled and rolled slowly to line up all the tumblers to feel it like pasting over crust, then smooth rolling and rolling slowly til an instant of grit grain sense. If non of that me and hillbilly friend stick em back in and forgetabout it. Worse then for these bearings flat or barrel is chugging lugging, thank goodness real Combats discourage that, even though the lug just fine thankyou.

I found plain flat rollers in my 1st Combat now called Ms Peel. It had area history of being the bad bike around and was test rig in shown room to sell brand new ones. It rarely was lugged and lucky not to be over rev'd, and the taboo bearing were just about pristine, sent off to be measured just to verify my exam.
 
So if i Check the bearing and if feels fine stick it back together and go. This is the good bearing 6 MRJ30 with 13 rollers right ? I hate changing parts that are still good. It's a proven bearing to this point. A new bearing isn't proven until you have time on it. unless it is a new design that is proven to be better. Just because I'm there isn't a good reason to change it. There was no end play on this bearing. There was no load on the bearings when turning crank. should i reinstall the bearing with no end play. thanks for all the advice.
 
True they are proven, But be very certain that they are perfectly clean now that they have been exposed to dirt and dust. Also look at the outer race very closely and be sure you dont see any wear or pitting. It sounds as if you really want to reuse them, If the cases are heated enough they do come out fairly easy. I tend to change bearings without really any reason other than I'm already in there, Good luck.
 
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