I recently purchased a MVVS 2 cc diesel. I just
about have it broken in, but now the contra
piston is loose enough to back off compression
while running on the bench. It starts fine and runs
ok. my tach is showing 12k on a MA 8-4.
Obviously I can't use it on a plane with the
comp. screw backing out. Any ideas?
Ken

http://www.rcuniverse.com/forum/m_2687406/tm.htm Check post #7 here.

Kray on my MVVS 1,5 the compression screw does not have enough threads left on top to do this. If you get a metric allen screw of sufficent length wull work fine a piece of brass flat stock 1/8 " thick would be fine for the locking lever the reason I double the thickness on mine at the end was because I only had 1/16th of course make sure you do not run the contra down to far that it will strike the piston this can be checked by just turning the engine over slowly by hand and when you get interferance back if off to clear you can adjust the length by simply using a small washer under the bar and cutting the screw so it not too long alo you can finish the screw
end to remove any sharp edges on its bottom maybe round it a little with a dremel. So far my MVVS 1.5 has not had this problem\
my MVVS 61 has a davis head so no issue there my orginal 61 diesel head Francis Pierson has If he sees any issues sure he will do the same or similar Not really sure about this but I think Davis suggestd the "hole idea" for the stock heads, not an issue in his martin

I purchased an MVVS 2 cc diesel with the R/C carb for 8 years or so. I traded for an airplane about two years ago. A good engine and I should have held on to it. I never had that problem with it. When I did have that problem back in the 1950's and 1960's on 2.5 cc diesels it was solved by balancing the prop and if that did not work go to a different diameter and/or pitch prop to get in a different rpm range. If it is vibration related this may work.

Thanks for the inputs. I had thought about the
locking lever idea, having seen it on other diesels.
I'm new to the diesel scene and feeling my way
around still. I seem to remember reading some-
where that it was possible for the contra piston
to move up and down with the regular piston if
it was loose enough. I know there is some slight
leakage around the contra as I can see oily
residue around the compression screw after 3 0r
4 runs. Don't know if this should be of concern or
not. I don't think vibration is a problem. Prop is
balanced as well as I can do it on magnetic balancer.
I may call Carlson next week and see if I can get
another contra piston to try. Hate to disassemble
this engine because it is really user friendly. A plus
for a beginner diesel man such as I.
Ken

Has anyone tried blue Lok-Tite on a contra screw? It allows movement after the seal has hardened. Not sure about the temperature properties.

George

Your contra is most likely OK some little bit of oil around the compression screw is not a problem usually after a while a lillte soot will build up around the compression screw and it no longer leaks. A couple of mine did this (irvine comes to mind) and it eventally stopped
just a trace now and yes a little gets past the contra in all engines this is normal. Some if us have actually drlled a small hole in the top of the head to relieve any pressure buildup and the chance of ignition of the charge in the space between to top of the contra and the bottom of the head this is not an issue in smalll diesels. martin

Hmmm...Interesting point. I had assumed (shudder) that any blow-by at the contra was just oil and not combustible. My idea for the Lok-Tite was to prevent the screw adjustment from wandering, not necessarily for seal.

Perhaps the locking lever or nut would be better.

George

I have been using Diesel motors ( S.T. / Olly T / PAW ETA etc ) for over 30 years and have a tip.

Find a nut that fits the comp screw thread, then find a course type "star washer" that is a neat fit.
Put the star washer between the head and the nut, pinch the nut down just enough to slightly compress the star washer and also give the correct amount of stiffness to the comp screw.

This allows the comp screw to be adjusted but the stiffness stops it backing off.
I have motors that have been setup like this for years without having to mess with the nut tension at all, once set.

p.s. - surely there is no likelihood of stuff in the gap above the contra piston igniting !! - at least I have never heard of it.

Ghost `123 I have never had it happen but with time I can see enough oil getting up there to not allowing the contra to go up all the wayup
one of my paws does get oil past the contra but it runs fine and have never drilled the head I did remove the comprresion screw after a bit and poured out some oil after months of running martin

Ghost `123 I have never had it happen but with time I can see enough oil getting up there to not allowing the contra to go up all the wayup
one of my paws does get oil past the contra but it runs fine and have never drilled the head I did remove the comprresion screw after a bit and poured out some oil after months of running martin

Thanks again for all the info, guys. I talked to
Carlson, and he suggested I use a dremel
tool with a cutoff wheel and make a small
cut across the top of the cylinder to allow
any pressure build up to escape. I did that
and now the compression screw stays where
I put it. Apparently there was enough pressure
between contra piston and head to reduce the
pressure against the screw and allow it to back off.
The engine still starts and runs fine.
Ken

A slit, intresting how big? I would think a hole drilled maybe a 1/16th would be plenty big for this purpose from what I read on the site maybe a year ago this what a couple of the guys did, My MVVS 61 had th proplem the DAvis head being thicker, having more thread area for the screw and a lockin set screw to set tension solved the proplem without locking bars etc. the davis head for for irvine for solved the dropping contra pistion on the Irvine 40 diesel also martin