The Knowledge - Velocette Owners Club

#4153 A friend of mine had his iron VMT barrel nikasil plated from worn .040 back to standard! He went
through Dave Smith. It seemed to work fine.
L #4177 Dose anyone know what the inlet bore for a 49 mac ( iron ) should be? I have a head that
I think is for a mac but the carbs inlet seems small by comparison to the head I have on my mac now,
although it may be nothing because the head that is currently on my mac has been changed a lot.
Someone put in bigger valves ( from god knows what ), and has ported it nicely but it is not preforming
well and I suspect that it was set up for the kind of driving that I do not do. looking thru the parts book
many items are shared between the mac and mov and I want to make sure that I do have a mac head
and not a mov head so any help on what the differences between the two would be helpful. just trying
to save myself a lot work if the spare head is not for this bike.
#4204 Mov 7/8", MAC 15/16"
L #4375 I recently bought a 1936 rigid mac which had not been standing for too long after getting
it running and 200m of trouble free riding it seized up while riding up the wendover bypass I left it a
while to cool down and it freed itself, got it home and took the head and barrel off it had only minor
scores on the skirt of the piston , I thought I had found the cause of the problem when I noticed one of
the holes on the skirt of the barrel was blocked after unblocking and reassembling and a new
concentric carbs I took it for a spin yesterday and after 10 miles it did it again! it seems like a heat
seizure but I don’t know why any ideas?
#4376 The simple answer is that the piston got too big for the bore. The real question is, Why? The
variables are cold piston clearance established by whomever bored the cylinder, and the amount of
heat buildup in the piston during use as the piston will expand in tandem with it's temperature. The
place to start is to measure the piston clearance and compare to specification (which I do not know). If
within spec, you are simply running too hot and the piston is expanding with the heat until seizure.
Excess piston heat is generated most often by lean carburetor settings, although incorrect spark timing
or high compression ratio can be the culprit. Unfortunately, the maker's recommendation for carburetor
settings cannot be relied on today due to changes in fuel chemistry, so you are looking at a sometimes
long and agonizing education as you learn how to work with the beast. A useful and easy assist is a
cylinder head temperature gauge, using a sender that is a ring held under the spark plug. These can be
bought from www.westach.com and many other suppliers.
#4382 Hello all, I would be very much interested to hear what other listeners have to say about this new
engine seizure problem. Over the past 5 years I have been working restoring a 1956 Ariel VH Red
Hunter- 500 single. This bike has an aluminum head, aluminum piston, and cast iron cylinder. The first
engine rebuild I used a 7.5-1 compression ratio piston, per the piston manufacturers suggestion I bored
it to 4 thousands clearance. During the shakedown ride, about 20 miles from home, going about 45
mph the thing seized, locking up the rear wheel. Ill tell you, a full power slide at 45 mph, in traffic, on a
bike that you have spent two years, and many thousands of dollars restoring, is not something I would
recommend to anyone faint of heart! After getting to the side of the road, and waiting to stop shaking,
amazingly it started again and seemed to be running fine. Off down the road I went again, about a mile
or two and it seized again! Two full speed power slides in one afternoon was enough for me. The bike
seemed to run fine after that. After putting about 100 miles on it, I crashed it head on with a car in an
intersection. It was the classic car making a left hander in front of me as I entered an intersection. Of
course the whole front end was mangled. Not to mention my own person. The bike sat for nearly a
year. When I started to fix it again, I decided to have a look at the cylinder to see what had become of
the seizing issue. The engine thankfully was not damaged in the crash. The piston and cylinder wall
had light score marks just as Dave described. I made the decision to fit a new 9.0-1 piston this time; I
thought it could use a little more power. This was a different manufacturer of piston than the first rebuild
I did, but the recommendation was still 4 thousands clearance. After a very long rebuild period, and
much searching for parts, painting, etc., etc., the bike was finally finished. I was very careful with the
break in period; never more than half throttle, never more than 50 mph, etc. After about 500 miles of
this kid glove treatment, I took the bike on its first real club ride. Riding up Wolf Creek Pass here near
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