Help with head clearance on a CMB .21

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DaveHalverson

Well-Known Member
Joined
Mar 9, 2015
Messages
223
Help! I need to know a good head clearance for my older CMB red head .21 motor. Without shims, the clearance is approximately .022-.024. At this clearance and 60% BAM fuel, I am blowing plugs on every run. I experimented with a .008 shim and it ran great without blowing a single plug. .008 seems too much though but I need to know where it should be. Thanks in advance for any advice... below is a photo of the measurement taken from the motor on a gauge.

Dave

CAM00409.jpg
 
If I read your post right, without shims the head clearance is 0.022"? That doesn't seem right at all. A good clearance, like Julian said would be 0.008".

It could be a number of things but you might want to take a look inside to see why the clearance is so high.

The reason your blowing plugs is because you probably have to lean the needle out a lot to make decent power. Drop the head and you'll be able to run richer, save plugs, and make more hp.

Brian
 
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If I read your post right, without shims the head clearance is 0.022"? That doesn't seem right at all. A good clearance, like Julian said would be 0.008".

It could be a number of things but you might want to take a look inside to see why the clearance is so high.

The reason your blowing plugs is because you probably have to lean the needle out a lot to make decent power. Drop the head and you'll be able to run richer, save plugs, and make more hp.

Brian
Unless I am doing something wrong with the measurement (I tried twice and got the same result) the total head clearance from TDC to head is approximately .024 without a shim. I included a photo of the measurement. The motor is used and older but it does run pretty good. It has a NovaRossi 3.5 carb on it with a remote needle. I put it in an older Seaducer B Mono that once was a race boat and setup pretty well. While I could, I got a reading of 37.1mph on a GPS. It has issues with the strut and shaft tube that will be addressed over winter but for now, I need to race 2 more times before snow flies and would like to at least finish a race without blowing plugs.
 
I would imagine .008 would be conservative for a healthy 21 motor. I've heard from some of the faster guys that they run them as low as .005-.006, but most of them run the higher nitro also 65%+

If you're confident that you are measuring this correctly, and that you don't have excessive wear/slop in some of the motor components, e.g. rod, crank pin, wrist pin, needle bearing, etc. You will probably have to resort to turning down the shoulder of your head button to get the desired squish clearance. You may want to ask some of the guys that run this specific model 21 what the optimal volume and shape should be for your head while your at it, as this also plays a key part in your total cr.

Or if you don't feel confident and could swing it, just send it off to someone like Steve Wood that has the proper tools, knowledge and experience., So for a reasonable fee they'll set it up for you. http://woodrcboatproducts.com

Good luck!
 
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Thank you all for your info. I guess this motor is just too worn. I guess I will be looking for a replacement piston and rod. If anybody knows of some lightly used in good shape parts that are reasonable, send them my way....

Dave
 
So I take it you found a worn out rod and piston?
Not really. The piston rides tight on the rod. I'm thinking somebody did some work on it not knowing what they were doing. It has a bigger NovaRossi carb on it which has a bigger throat. I do know it's not right and the guy I bought it from is willing to take it back, full refund. I'm debating this now thinking I might keep it and rebuild it.
 
Assuming that the measurement is correct, I wonder if it may have a sleeve shim (under the lip of the liner, to raise the timing), or perhaps the liner is not seated properly. Otherwise, some sort of issue with the components I guess, but sounds too much to be wear (to me at least). At that level. I think wear would be very noticeable.
 
Help! I need to know a good head clearance for my older CMB red head .21 motor. Without shims, the clearance is approximately .022-.024. At this clearance and 60% BAM fuel, I am blowing plugs on every run. I experimented with a .008 shim and it ran great without blowing a single plug. .008 seems too much though but I need to know where it should be. Thanks in advance for any advice... below is a photo of the measurement taken from the motor on a gauge.

Dave
Dave I use 0.030 lead solder, then you can accually measure the squish & where taper starts & how much.
 
dont panic.

ever since the father died cmb has been all over the map with their engines.

i have 1 or 2 where the clearance is ridiculously large with no shims,in fact i read that a few guys are running the latest or second to latest engine with no shims.

if you dont have a lathe contact steve wood and he should be able to sell you a head button that will get to the right clearance.

wow dude if u get down to .005-.008 ur gonna crap ur pants lololol
 
Help! I need to know a good head clearance for my older CMB red head .21 motor. Without shims, the clearance is approximately .022-.024. At this clearance and 60% BAM fuel, I am blowing plugs on every run. I experimented with a .008 shim and it ran great without blowing a single plug. .008 seems too much though but I need to know where it should be. Thanks in advance for any advice... below is a photo of the measurement taken from the motor on a gauge.

Dave
If it ran GREAT without blowing a single plug with one .008" shim, why not just put the shim in and race it! Then you can send it to Steve Wood or other engine guru over the winter to have it checked out. If it blows a plug on every run, without a shim, but works great with a shim, It sounds like your probably getting detonation and fouling the plug. You may also try putting in 4 .002" shims and taking out one at a time until it starts blowing plugs again, then putting the .002" shim back in. I ran .008" in my .21 novarossi without issues, but I had to run my CMB a little higher to get it to stop detonating and blowing plugs every run at any needle setting. There is no perfect number that works in every 21 engine as there are so many factors that come into play. Just test and go with what works!
 
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The key to measuring the deck clearance on a dome piston with a dish is to find "0" when the piston hits the or touches the head button.

The turbo head you need an adaptor to screw into the plug hole and then screw the depth gauge into the adaptor.

I run .006 on my dd 21
 
Dave,

Make sure you don't have a liner shim under the shoulder of the sleeve, then call Stu Barr and order a new head button. Somebody would have had to have cut the squishband area of the head, so turning the shoulder back on it won't fix the concert of problems you've got going on. You can call one of several guys on here and have them look it over if you're not sure what you're looking for. Dave Roach or Steve Wood should be able to figure it out for you.

Thanks. Brad.

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