Mastiff An engine reconstruction project

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Jul 16, 2007
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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.

I guess a little measuring and math will give a static compression ratio. On the model A Ford I have the original CR was 4.22:1 so my guess is anything higher than 4:1 should give a running engine.

I think I would try to get my hands on the original drawings to be sure the builder cut enough off the casting to begin with.

Try measuring the distance from the block to the valve face at full open and compare that to the depth of the chamber in the valve area and see if more can be cut off the head.

All that failing I guess the only option would be to make a piston that would fill as much of the chamber as you can.

Keep in mind that the more you cut off the head the more you pinch off the area that transfers fuel from the valve area to the piston area. Flat heads are a pain but they sure are COOL!
Had a look at masons building mastiff and the combustion chamber depth is 1/16 on the piston end and 3/16 on the valve end with a transfer ramp between the two and this appears to be divided up into thirds , 1/3 valve pocket , 1/3 transfer ramp and 1/3 over the piston .
You will have to cc the head to find out what the comp ratio is with any real accuracy .
Had a look at masons building mastiff and the combustion chamber depth is 1/16 on the piston end and 3/16 on the valve end with a transfer ramp between the two and this appears to be divided up into thirds , 1/3 valve pocket , 1/3 transfer ramp and 1/3 over the piston .
You will have to cc the head to find out what the comp ratio is with any real accuracy .

Maybe you could send a photo copy of the head drawings so George could put the micrometer to the piece to verify the head was machined properly?
George, I built the Mastiff from solid block and made the heads to the plan. I have good compression. I did use cast iron rings for while but changed to viton "o"rings on a recent rebuild and love them. you might tyr the viton and see if you get more compression.

Did you say you got the flat head V-8 running? I must have missed the video. :eek::eek:

I don't know if he would be able to read them after scanning or how well they will scan , the drawings are so small i need a magnifying glass to make out the dimensions ( my eyesight is still pretty good and i don't wear or need glasses ) and i would say they have been resized for the reprint of the book .
If needed i don't mind doing a hand sketch if he needs some basic dimensions here and there but i don't have time to re draw an exact copy , for that i would suggest trying to find a copy of the book .
I have spent more time reading through the build book than I care to. If it were my engine I would work on it until the problems were resolved but at this point I'm probably working for $5 per hour.
Yes I could cc the heads etc but I was just curious how the engine runs with the stock cast heads.
As was stated the drawings in the book are so tiny and light that in some cases it's hard to make out what is needed. As for the cast heads the combustion chambers are as cast with no exact dimensions to work to.
The book does call for lapped Pistons with one ring and being as I made the replacement pistons from aluminum I allowed.0005 with one iron ring.
I have never purposely built an I.C. engine with a 4:1 compression ratio so I don't know how that would feel when being turned over.
Thanks much for taking the time to read my lengthy thread and offering helpful comments.
Trust me i know how finicky flathead engines can be ! We run a c class junior dragster that is powered by something similar to a 5hp briggs engine running on alcohol , you want compression but also need airflow and with a small bore you are really up against it !
I hope to build a mastiff one day but reading those damned small drawings is going to be a killer !
I would say that the comp ratio will also depend on the fuel you are running , you want something that goes bang pretty easily with such a low comp ratio .
A fellow on the other forum is scratch building a Mastiff and has modeled all the parts in a CAD program. He said that as drawn the compression ratio is about 3.5:1. I have heard 5 or 6:1 working but I don't know about getting into the 3 or 4's.