.21 CMB RS Outboard Crankshaft Problems

[kesso2118]

I love the performance I get from the CMB RS on my HTB290 but unfortunately I now have a "track record" of twice having the PTO Drive Pin on the crank break off, then take out the bottom of the piston and damage the rod, this makes for a very expensive re-build :angry: :angry:

Has this happened to anyone else or am I the only person in the world with this problem??????

When I first encountered this problem early last year, I swapped over to a MAC while I was waiting on the parts to rebuild the CMB. The MAC ran fast for 18 months until it needed the bearings replaced.

I have been running 50% Nitro with 18% Oil (10% Klotz & 8% Castor) in both engines and I had no problem with the MAC.

Anyone got any ideas???

Thanks,

James

 

[HTV Boats]

Same here 2 motors and I have replaced two cranks that broke off at the drive pin. Only one piston got taken out. Must be a batch of cranks that were not wvery good. Expensive power to say the least. Hindsight I would buy Nova Rossi or just plain OS's and leave em stock and have fun finishing heats.

Mic

 

[Grimracer]

make sure the pin slot is loose. VERY LOOSE...

Grim

 

[kesso2118]

make sure the pin slot is loose. VERY LOOSE...
Grim

Thanks Grim.

When we rebuilt after the first crank broke, we opened up the PTO Hole to ensure it wasn't binding, however, I will compare the size of the hole on the CMB with my MAC to see if there is any difference.

Regards,

James

 

[Tim_Duggan]

When we rebuilt after the first crank broke, we opened up the PTO Hole to ensure it wasn't binding, however, I will compare the size of the hole on the CMB with my MAC to see if there is any difference.
Regards,

James

The MAC uses a slotted PTO, where as James' RS21 has a hole. See pic of MAC O/B PTO

 

[BB Mods's]

1 more thing to look at is the amount of play when the motor and PTO are bolted up, between the drive rotor, and the crank shaft.

Also.. I want a broken RS crank

 

[Ken Retallick]

James,

if it has a hole as Tim says, i would be making it into a slot. What is most likely happening is the crank flexes, bearings distort, coupled with wear, and the little drive pin takes all the load from the engine, snaping it off.

It needs to only drive the PTO shaft, not be supported by it.

Ken

 

[kesso2118]

Thanks Tim & Ken, I'll look into that, sounds like we might be onto something.

Regards,

James

 

[Tim_Duggan]

The crankshaft to PTO outer race was checked when the engine was rebuilt. I fitted the PTO assembly to the case without the P&L and could fit a 0.30mm feeler guage between the crankpin and the pto plate. I then dismantled - removed the pto plate and refitted the PTO housing and checked the clearance to the PTO outer race from the end of the pin and there was about the same amount of clearance, if not more. Sadly, this hadn't been the case prior to the rebuild as there was evidence that the crankpin had been rubbing on the PTO outer race.

Regarding the PTO hole diameter and the pin diameter, in initial inspection the clearance was about 6 thou! We opened this up considerably, but in hindsight I should have slotted it like Ken suggested and re-balanced it accordingly. At the time I thought it was very similar to the drum rotor drive on a valvola, so it should be OK.

The thing that did concern me at the time (and it's something I couldn't do anything about) was that the PTO pin had no noticable radius on it - which creates a shear point. Guess where it failed?

 

[Kris Flynn]

I wouldn't be looking at the engine for the problem, I would be asking the guy who built the engine to cough up

 

[Tim_Duggan]

Hows your Rossi going smart guy? Yep James, it's all my fault for making that expensive Italian "race" engine with fundamental engineering flaws........ :wacko:

 

[TUNLBOTR]

The crankshaft to PTO outer race was checked when the engine was rebuilt. I fitted the PTO assembly to the case without the P&L and could fit a 0.30mm feeler guage between the crankpin and the pto plate. I then dismantled - removed the pto plate and refitted the PTO housing and checked the clearance to the PTO outer race from the end of the pin and there was about the same amount of clearance, if not more. Sadly, this hadn't been the case prior to the rebuild as there was evidence that the crankpin had been rubbing on the PTO outer race.
Regarding the PTO hole diameter and the pin diameter, in initial inspection the clearance was about 6 thou! We opened this up considerably, but in hindsight I should have slotted it like Ken suggested and re-balanced it accordingly. At the time I thought it was very similar to the drum rotor drive on a valvola, so it should be OK.

The thing that did concern me at the time (and it's something I couldn't do anything about) was that the PTO pin had no noticable radius on it - which creates a shear point. Guess where it failed?

BINGO ! The lack of radius is your main concern, we went through this years ago on NR crankshafts, but we were breaking at the top of the crank, under the Front (little) bearing. Was there any shinyness at the fracture point?

For some reason CMB over the years have also shown design flaws in their PTO assemblies. A lot of racers will remember people giving up on making the 21 TT o/b work, not being able to keep a needle, and the reason was PTO design. They tried to "double stack" the bearings on the assembly, Which would slop out the housing below the bearings, causing blow by. At the time I machined down a K&B PTO, and problem was solved.

Andy did the PTO right on the MAC, and that was how I corrected the issue on past CMB motors I have.

And Rod Geraghty does the finest PTO work I have ever seen, bar NONE.........

Randy

 

[kesso2118]

I wouldn't be looking at the engine for the problem, I would be asking the guy who built the engine to cough up

Kris,

The rebuild was a great job, the engine was smooth as silk when we finished.

The change in performance of the boat when I did a direct swap one day between the MAC and the CMB was amazing. It had tremendous acceleration and terrific top end, I just wish I could have kept it going long enough to run it at next weekend's Winter Spectacular.

 

[Kris Flynn]

Sorry, of course Tim does a good job.

 

[Mike Larson]

this will be theory to add to the mix.... i've done the nova heads, the cmb heads, whatever... an approach to having freeplay between the lower ( any lower) and the powerhead is subject to question. thinking along inboard lines also, leaving the gap between the drive dog and the strut for expansion/contraction of the flex, mitigating pressure on the small front bearing(inboard).....

My outboard thinking was, as the prop pushes the flex rises up, in the lower, hence on the PTO(it may be in the bearing races for sure, pressed, but floating) talking thousandths to make it to the crankpin bottom, it's more than likely enough to cause heat at the pin juncture, and subsequent failure.......

Fred Howe( lawless) actually made spacers(lower to head) for me years ago, because I had an oddball nova head, that once getting it right, i havent even seen pin wear, and the motor runs to this day..... cutting the flex to a more understandable length would work too..... they are all different to some degree.....

Until somebody comes out with a propshaft assembly that has bearings or better to no wear times, the in and out slop will allow the flex to push into the head more and more with wear, and in my thinking wear the pto to the point of failure eventually.......

disassemble and look for the swirl on the pto top.... cause for concern easily......

lotsa good thinking on the other posts..... thought i'd try to get in..... Mike

 

[DonSoper]

Hello everyone. This is my first post. I have been lurking a lot. I used to race boats about 18 years ago. The bug has bitten me again!

Anyway, I have a new CMB .21RS outboard on it's way to me. I am not happy to hear about the problem with the crank pin/PTO. Does anyone know if this issue has been resolved? Thanks.

 

[Tim_Duggan]

Hello everyone. This is my first post. I have been lurking a lot. I used to race boats about 18 years ago. The bug has bitten me again!
Anyway, I have a new CMB .21RS outboard on it's way to me. I am not happy to hear about the problem with the crank pin/PTO. Does anyone know if this issue has been resolved? Thanks.

Welcome to IW Don!

It's a VERY expensive repair if the crank pin breaks off, so it would be wise to have it checked out to avoid the hassles the other guys on here have had. Do it before it see's any fuel. Maybe check with whoever you bought it from if there is any implications on dismantling prior to use.

If you are prepared to check and maybe do some surgery on the engine yourself - slot or open up the hole on the PTO that the crankpin sits in. it does not have to be a precision fit. Sloppy is good. There should be a lot of clearance.

If you aren't confident about doing it yourself, send it to an engine guy and have him do it for you. Every CMB I've looked at had things that a competent engine builder can improve on.

Tim

 

[DonSoper]

Thanks for the welcome & the reply. I will probably just open up the hole a bit. I didn't know if that was all that was needed. Thanks again.

 

[Tim_Duggan]

Open it up a lot or slot it completely. You should be able to rock the engine back and forth near BDC and hear the click as the pin takes up the play to the PTO.

 

[kesso2118]

Good luck with the CMB, I sure did like the way the engine ran, I just wished it would stay in one piece for longer!

As we speak, there is good man down here in Oz who is building some replacement pistons & rods for me and modifying the PTO to open up the drive hole into a slot.

Regards,

James