twin mac 84 problems

[Timbo Whalen]

I found a old thread from 2018 that said you use cmb 80hr piston and liners in a 84 is this still the case or does anyone have experience with this?

For all intents and purposes, CMB and MAC are cut from the same cloth, and a lot of the internals are interchangeable. The MAC 84 may have a bit more stroke than 80 HR.

Both engines were built by CMB. The CMB acronym stands for Camillo & Mauro Braghieri - father and son.
MAC is the acronym for Mauro, Andy, and Camillo, as Andy Brown essentially enhanced the CMB architecture to specs he favored.

I just rebuilt both a CMB 67HR for a customer and a MAC 67 for resale.
Visually, the c/shaft, con rod, and piston appeared virtually the same.

It's too bad the CMDi website is gone, otherwise you could've compared the MAC specs against the CMB HR specs over at the Hobby Supplies of Australia site.

Someone here is going to be able to give you a definitive answer, eventually. IF the MAC does have a longer stroke than the HR there MAY be differences in port heights and piston deck heights (from wrist pin C/L).

If it comes down to it, we do have an 80 HR torn down in a container that we'd gladly provide dimensions for a comparative analysis.

 

[Drew St. Amant]

I have some spare 84 liners that are no good I will take some measurements and send them to you fingers crossed they will work

 

[Timbo Whalen]

I have some spare 84 liners that are no good I will take some measurements and send them to you fingers crossed they will work

Ok, top and bottom of sleeve ports to underside of top flange....also, OD's of sleeve and top flange.

I'm sure Andy would know off the top of his head, but seems he's been MIA for a bit.

 

[Drew St. Amant]

The od of the 84 is 29.78mm from flange to top of exhaust port is 13.54mm flange to bottom of exhaust port is 23.29mm from the flange to the top of the side transfer ports is 19.14mm from flange to the first top of the side transfer ports is 25.18mm from the flange to the very bottom of that same transfer port is 34.7mm from the flange to the top of the front transfer port is 19.65mm from the flange to the bottom of the front transfer port is 27.14 the over length from flange to bottom of sleeve is 42.39mm and the width of the flange is 2.95mm below I have pictures of all the measurements I just stated in case it helps

 

[Drew St. Amant]

These are some old sleeves that used to be in the motors for this twin the ones currently in it are Identical with the exception that they have a slightly different exhaust port shape the the top and bottom heights are the same if I remember correctly

 

[Timbo Whalen]

The od of the 84 is 29.78mm from flange to top of exhaust port is 13.54mm flange to bottom of exhaust port is 23.29mm from the flange to the top of the side transfer ports is 19.14mm from flange to the first top of the side transfer ports is 25.18mm from the flange to the very bottom of that same transfer port is 34.7mm from the flange to the top of the front transfer port is 19.65mm from the flange to the bottom of the front transfer port is 27.14 the over length from flange to bottom of sleeve is 42.39mm and the width of the flange is 2.95mm below I have pictures of all the measurements I just stated in case it helps

Thanks....will try to get to it this afternoon.

 

[Jeff Lutz]

I found a old thread from 2018 that said you use cmb 80hr piston and liners in a 84 is this still the case or does anyone have experience with this?

They are different timing and port sizes. It can be done but is a decent amount of work. Jeff Lutz

 

[ALLAN YUEN]

Just Need to add liner shims and will look pretty close timing Good Luck !

 

[Jeff Lutz]

I Would disregard Allans post. I just did a pair for someone. The Timing is higher on the HR sleeves than the Mac. You would not want to shim it at all. The boost port is wider and requires case mods. The transfers are smaller and need modding if you want them as big as the Mac.

 

[Mike Hughes]

The main issue with the 84 is the case. There aint many of them left. When they bored them for the larger sleeve is weaken them. You would be running and it wouild pop a jug. After the third time with my twin and I was basically going thru that crap pile to build a motor for the twin, I gave up. Sold it all and went CMB.
Mike

 

[Drew St. Amant]

Mike where your motors highly modified we haven't ran this twin a ton but definitely more than 3 times just a little weird we seem to have such a different experience knock on wood I guess

 

[Mike Hughes]

Bone stock. The MAC 84 is just the 67 case bored out to accept the larger sleeve. This the issue. Not enough meat left. Besides the 3 I pop. I have seen several other do the same thing. This a known issue. My motors were all used when I picked them up. I dont know how long they were run before i got them. I ran my twin for quite a few yrs with the 84s. So those motors I would say had around 50-75 runs a piece on them from me. Good luck. I got out while I could get some decent money for them at the time. I think around 750-800. Reinvested it into my 80s.
Mike

 

[Drew St. Amant]

Oh I see you meant after the third motor not that they would blow in 3 runs that makes more sense

 

[Jeff Lutz]

Andy had the cases beefed up on the last few batches. I’ll have to round up photos to show you but they are very different. None broke after the changes..

 

[Mike Hughes]

Jeff, Never had any of those motors. Shame Those were some nice running 84s.
Mike

 

[Timbo Whalen]

I Would disregard Allans post. I just did a pair for someone. The Timing is higher on the HR sleeves than the Mac. You would not want to shim it at all. The boost port is wider and requires case mods. The transfers are smaller and need modding if you want them as big as the Mac.

Andy actually went in there and widened the transfers on the 67/84 cases he modded. We've seen it, and just sold one of the world record 67's.

What gets me is, the HR series engines never had these case failures on the 13cc engines, to the best of my knowledge, and CMB made the MAC's.

Yeah, sounds like maybe there weren't any educated, experienced engineers working on these projects with a grasp on structures and metallurgy.

Drew, I wouldn't waste your time entertaining mixing the HR and MAC parts. If the boat means a lot to you just buy a pair of 67/80 HR engines and be done with it....makes no sense making a silk purse out of a sow's ear. Jus' sayin'.....or puttin' lipstick and Chanel #5 on a pig.

 

[Drew St. Amant]

That's kinda what I was gathering I'm not sedimentle to the motors just figured if the liners would work might be cheaper than getting all new motors but sound like new ones will be the ticket once these give up the ghost

 

[Timbo Whalen]

That's kinda what I was gathering I'm not sedimentle to the motors just figured if the liners would work might be cheaper than getting all new motors but sound like new ones will be the ticket once these give up the ghost

Yes, my findings are similar to Jeff's on port dimensions. Port mapping and design uses time/area analytics in designing 2 stroke engines, and port window area and timing are relative. I think AB found out through his testing that the MAC transfers needed to be wider, similar to CMB's HR series arrangement.

If I may throw in an opinion, we'd advise a matched pair of late model 80 HR's. We had a matched pair, slightly modded and enhanced with honed sleeves, but we sold one of them.

I checked the dimensions on the remaining 80 like we said, but came to the same conclusion Jeff did.
The liner shim scenario someone mentioned isn't a solution, and only illustrates the individual knows nothing about port time/area theories, and the relevance.

If you're interested, we may know where there's a pair of matched 80 HR 'red head' engines that are pristine...don't know if they have the OE cast rotor backplates or have been upgraded to the billet pieces.
Some people seem to think the cast backplates are better.

 

[XtremeRcBoats]

Interested to see the pics. Still have some mac84's also in a twin (Probably the old ones).

 

[Jeff Lutz]

Here is a couple of pics. Look at the material behind the flywheel area above the engine bolt. There is a step in the first version that Andy designed. Then, he had them make the case straight (bigger OD) basically on the last version. These changes were not advertised. But this change solved the problem. Jeff Lutz