Author Topic: Mastiff A reconstruction project  (Read 1448 times)

Offline gbritnell

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Mastiff A reconstruction project
« on: August 24, 2017, 02:16:14 PM »
 First a little background to this story. I was at a local antique engine show and a fellow came up and was looking at my engines. There was conversation about model engines and eventually he asked me if I do machining for other people. I said I did and so the discussion focused on what he would like to have done. The engine in question turns out to be a Mastiff made from castings.
 For those of you who aren't familiar with the Mastiff it's an engine designed by L.C. Mason. It's one of those names that I have heard of along with E.T. Westbury. In the model engine world these fellows are respected for their knowledge of designing and building I.C. engines.

 The engine itself is a flat 4 cylinder (boxer style) with flat heads. It has a pressure oil system and the heads are made in two pieces to provide water cooling in the cavity created by the recesses in the castings. It has a spark ignition with two separate electrical units, one for the set of points, projecting out of the top front of the engine and the other for the distributor, mounted on the front engine cover.
The valve springs and adjusters are exposed between the engine block and the cylinder block. The engine can be built with dual carbs but as the author states probably using one carb would be the best way to start until the engine is up and running and tuned reasonably well.

 The owner was not the original builder and had picked the engine up for a reasonable price, but reasonable on an non running partially assembled engine can be very subjective. I gave him one of my cards and within a few days he visited my shop and dropped the engine off.

 The engine was in a plastic tub with all manner of nuts, screws, turnings and pieces laying about in the bottom of the container. The engine itself was partially disassembled and at first glance the machining didn't seem to be too bad. I told him I would take the engine apart and investigate the inner working and give him a call when I found out what needed to be done. One of the most helpful items was the original published construction booklet.
 First off the machining seemed to have been done pretty much on a lathe. You could see where the square parts and pieces had been turned and faced with not the sharpest of tools and as the cutter approached center there was a slight depression. The next thing I noticed was that there was an offset to several of the exposed lifter/valve stem/spring assemblies. Curious I thought. I removed the top crankcase cover to look in at the camshaft and saw that two of the center lifters had there diameters reduced for a short distance. The reason for this is because the builder had made the two center sets of cam lobes wrong and didn't discover the error until the cam was assembled and not wanting to undo everything and make it right opted to alter the ancillary bits and pieces to make it work.
 At this point I took the cylinder blocks off of the crankcase to have a look at the pistons. One piston had a piston ring made in the normal fashion and the end gap could be seen while the other three pistons had a discoloration where the ring should be but no discernible gap.  Another curiosity! I used my high powered magnifier to see if in fact the rings had been jammed into their grooves with no end gap and with the naked eye just couldn't be seen. Nope, there was no end gap! How on earth  did he accomplish this? I pulled out the book and turned to the piston section and discovered the construction method, not by the book but rather by the constructor's method. The pistons are supposed to be created in a fashion that I have never seen before. The body of the piston, the cylindrical part is turned from cast iron and the crown is made from a steel turning that fits closely into the inside of the body tube. Once the crown piece piece is turned it is partially milled away leaving material for the piston pin bosses. These two pieces are then silver soldered together creating a piston. What the builder did was to create the groove into the crown piece, make and install the piston ring and then solder the two piece together trapping the ring in place. The rings on these three pistons aren't split, they're just rings trapped in place.  :-\
 As I have mentioned earlier the machining was done on a lathe and the head faces had slight depressions at the centers.

 There were quite a few other issues so before proceeeding I called the owner to inform him of my findings and what if would cost in round numbers to try and fix the engine. He agreed to the work so I started making new parts and machining poorly made pieces.

  Once the new pistons and rings were made the engine with cylinder blocks was assembled. Upon turning the engine over and using the heel of my hand as a stopper the compression/vacuum of each cylinder seemed excellent. I then took the cylinder blocks back off and made a fitting to insert into the ports to check for valve sealing. Most naturally none of the valves sealed so they all needed to be lapped until a good seal was created. With the valves sealed the cylinder blocks were then reassembled and the valves adjusted. Due to the design and small confines of the valve adjusting nuts I had to make 3 wrenches to adjust the valves.

 At this point I made head gaskets and installed the refaced heads. At this point I have to mention that while doing all this work I had noticed the combustion chambers that were cast into the heads seemed rather large in proportion to the bore size (low compression ratio?) While tightening the head nuts onto the studs some of the studs turned like they were stripped in the cylinder block. I removed and measured the offending studs and found that the builder had used two different diameter rods for the studs. The original studs were to be B.A. sizes but the builder went with standard American threads, 5-40 (1/8") but some of the rod material was .124 and the other was .116. I made some new studs and hoped that there was enough thread to hold them, which there was. Thank goodness for small miracles. I then remounted the heads and tightened them down. I then turned the engine over and noticed barely any compression, on any cylinder.

 After this lengthy story here's my question for anyone who might have built one of these engines. Are there compression issued due to the large combustion chambers and if so what if any means were taken to correct the problem? I had gone through this with my own flathead V-8 engine and realized that the area extending from the cylinder bore out to and around the valve pockets created an extremely large combustion chamber and I had to make several heads until I got the C.R. to the point that the engine would run.

gbritnell
Talent unshared is talent wasted.

Offline AOG

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Re: Mastiff A reconstruction project
« Reply #1 on: August 24, 2017, 04:14:30 PM »
I haven't built one but i do have a thought. Perhaps he original builder was intending to turbo or supercharge the engine. That might explain the reduced compression ratio. Do you see any evidence of where something like that might mount?

Tony

Offline deltatango

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Re: Mastiff A reconstruction project
« Reply #2 on: August 25, 2017, 02:16:54 AM »
I haven't finished my Mastiff yet so I don't really know if it will run but I haven't heard of anyone having to change the compression ratio to make it work. From my 3D CAD model the ratio is about 3.2 : 1.

The next thing I noticed was that there was an offset to several of the exposed lifter/valve stem/spring assemblies. Curious I thought. I removed the top crankcase cover to look in at the camshaft and saw that two of the center lifters had there diameters reduced for a short distance. The reason for this is because the builder had made the two center sets of cam lobes wrong and didn't discover the error until the cam was assembled and not wanting to undo everything and make it right opted to alter the ancillary bits and pieces to make it work.

The offsets may well be correct to drawing and the reduced diameters justified.

I've tried to attach a .pdf file of a section through the engine on the valve stem centrelines but the forum seems not to like it so here is a small image:




The offsets between the #2 and #4 cylinder inlet valve stems and the tappet heads are to the original drawings. The reduction in the diameters of the tappets at the camshaft end isn't strictly necessary but the clearance to the edge of the adjacent cam is very small as drawn. I re-drew the engine from my copy of Len Mason's book because the drawings in there are very small and the reproduction poor.

Hope this helps, if you want more drawings these might get to you attached to a PM.

DT
Don't die wondering!

Offline gbritnell

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Re: Mastiff A reconstruction project
« Reply #3 on: August 25, 2017, 09:55:03 PM »
HI DT,
Thanks for the drawing. I didn't realize the engine was designed that way. I would have thought that during the design stage the bore spacing would have been increased to center the lifters.
gbritnell
Talent unshared is talent wasted.

 

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