Silver Bullet in Utah

[quinette7]

I've begun building Bob Shores' Silver Bullet from castings I bought 15 years ago.



My kit didn't include the engine stands, but a friend made castings using the ones from his kit. Thanks, Birk!



I decided to start with the gears, since I had never made any before, after about 30 years in this hobby. A friend offered to let me borrow his brand new gear form milling cutters. What a treat! Thanks, Richard! But I was nervous every minute that they were in my possession so I got right to work on the 64 tooth gear.



I made enough for three gears, and then parted them off.





I then did the same with the 32 tooth gear. And returned the cutters ASAP.

It looked like the cylinder head was one of the more complex parts, so I started it next.

I squared up the block, and drilled the through holes for the coolant and cam shaft.



drilled and reamed the holes for the valve guides,



and milled the combustion chambers with a .750" end mill.



The bore diameter is .755", so I had to enlarge it a tiny bit with a boring head.



Next, drilling and tapping the spark plug holes. The distance from the cylinder head centerline to the spark plug center is a little ambiguous on the drawing due to the 80 degree angle of the spark plug and which surface it intersects, but I figured it wasn't too critical if it stayed within the combustion chamber.





Then to flip the part over and carve out the camshaft support posts. The drawings SB-14 Rev 1 and SB-16 Rev 1 give contradictory dimensions for the width and location of these posts. Adding the .891" spacing from the far side and a width of .470" comes to 1.361", and not the 1.375" width of the cylinder head. This .014" difference is not too significant in itself, but it caused me some consternation since the drawings show the valve guide holes to be tangent to the face of this step. If that interpretation is true, the post width can't be more than .4275" due to the valve hole locations. I searched this site to see how others interpreted the drawings. I've posted below two photos by danthompson58 and Mosey show different approaches to this issue. Thanks, guys, for your help!





Cutting that step tangent to the valve guide holes comes pretty close to the cooling hole, and risks damage when one end of the hole is tapped and the other is plugged. Would not doing so interfere with the valve action? I chose to err on the side of not removing enough metal rather than removing too much. I can remove more later, if needed.



Bob

 

[stevehuckss396]

Great start. Looking forward to more updates.

 

[mnay]

I am watching from across the valley too.
Mike

 

[michael-au]

Nice to see another silver bullet being built
Nice work

You could mill more out between the posts if you want to

here is a picture of the head I made for my bullet, its a bit had to see but it is milled in more between the posts

 

[quinette7]

michael-au -

Thanks for your suggestion about milling between the posts. I have been out of town on a 5-day canoe trip, and I had to look at your photo several times until I got my brain back in gear and actually saw what you were referring to. It looks like a nice subtle touch. I also like how you added the grooving to the top of the head.

Bob

 

[quinette7]

I made some more progress on the Silver Bullet today. I followed the suggestion from michael-au and milled out more between the posts to allow clearance around the valve guides.



And milled the coolant slots.



Then shaped the posts and milled the shallow counterbore for the sparkplugs.



I don't know yet how far the sparkplug should extend into the combustion chamber, so I've left the deeper sparkplug counterbore for later.

I have two more questions where the drawings are ambiguous.

The drawings call out the locations of the intake and exhaust ports, but they don't call out the required angles to properly intersect the valve guides and avoid hitting the sparkplug holes. How did you other guys figure out the angles to drill the holes?

What's the proper angle of contact between the valves and valve seats?

Bob

 

[michael-au]

I will try and get time tomorrow and do a mockup with pictures of a peewee head

I didn't drill the ports until the valve cages were installed, easier to do once, rather than try and setup again to drill the cages

Good job on the head

Michael

 

[michael-au]

HI
This how I drilled the ports out

Install the valve cages first so they will be drilled at the same time

I set the head up by putting a small drill in the chuck and lining it up with the entrance of the port and the middle of the valve cage, do this by eye, there is no measurement or angle to go by

Its easy to set it up this way, put the head in the vice don't tighten it up yet then adjust the angle by eye until you can see it pointing from the port to the valve cage, when you are sure that you have the angle right then tighten the vice

First use a small end mill to make a flat spot in the port and then centre drill and then drill a small hole all the way through to the valve cage and the drill to the final size


View attachment ImageUploadedByModel Engines1443749035.464766.jpg

View attachment ImageUploadedByModel Engines1443749062.210036.jpg

I hope this helps

Michael

 

[quinette7]

Michael -

Thanks for your time to answer my question and take photos. It will be a few days before I can get back in the shop, however.

Bob

 

[quinette7]

I've made what I hope is progress with the Silver Bullet block and oil pan. First I'll show the photos, and then ask a question for those who have already built this engine.




Question - The Drawing SB-3 dated (4/15/00) shows the block's cooling water port placed on the side of the engine, between the two cylinders. But photos and videos of engines built by others show the port at the back of the block. Had there been a drawing revision, or was this a personal choice?

Issues about the drawings, or my interpretation of them

The rest of this post is about drawing issues that puzzle me, and which I've included for the interest of those who actually have the drawing set and and not yet built their engine. My drawing set appears to be older than some, so perhaps newer drawing revisions were corrected.

First, unless I've totally blundered, on Drawing SB-16, the labels for both ends of the coolant ports should be reversed. That is, "Tap 10-32 Front End" should be on the right side, and "Plug with tapered aluminum rod" should be on the left side. Luckily, I had decided to tap both sides, rather than driving in a plug, so I can switch them if I blundered.

More puzzling was that Drawing SB-3 gives the distance between coolant holes in the block as 2.342", with each hole equidistant from its respective cylinder hole. Drawing SB-16 contradicts both of these call-outs. It places one of these holes at 1.170" from the centerline and the other one as 1.116" from the centerline, giving a total distance as 2.286". I'm guessing that the 1.116" dimension is a typo, and perhaps should be 1.166" instead, that makes the distance between them 2.336" - still not right. The Head section of this same drawing gives both of the matching holes in the head at 1.166" from the centerline, for a total distance of 2.332" between them. Since these are coolant passages, there is no requirement that they match exactly between head and block, but I drilled the block to the same dimensions as the head , and they intersected properly with the coolant chambers around the cylinder sleeves, but I'm hoping there isn't some other clearance issue that will only reveal itself somewhere down the road. That's why my opening sentence says I hope I made progress.

Bob

 

[quinette7]

I forgot to mention another puzzle with my drawing set.

In the top right corner of Drawing No. SB-2, it says "Drill .250 for pressure regulator". The written instructions say "Drill and tap the pressure regulator hole 1/4"-28." The pressure regulator does indeed require a 1/4"-28 thread, and not a .250 hole. If someone was just following the prints, and not the instructions, it would be easy to drill this hole too large, since the dip stick tube on the other side of the engine does require a 1/4" hole. This wouldn't be a catastrophe, but it would be an irritation that needed a redesigned part.

Bob

 

[michael-au]

If you are referring to the water passage through the head I tapped both ends and made a small stainless bolt to fit in the back of the head and a stainless outlet for the front

The inlet on the block is on the side on my engine, this works well as you only need a short pipe to go from the pump to the block

I used all metric socket heads so I just adjusted the holes to what ever I needed to

 

[quinette7]

Thanks, michael-au, for your reply and mentioning the advantage of placing the port on the side.

I made the cylinder liners today. The puzzle solving aspect of this hobby is what offers some of the biggest rewards, but it also felt nice to make a part that offered no frustration.



Bob

 

[michael-au]

Coming along nicely

 

[quinette7]

I pressed the cylinder liners into the block of the Silver Bullet. The instructions called for heating the block and then inserting the liners, but with hopes of avoiding any water leaks into the combustion chamber or oil pan I pressed them in cold with a slight interference fit and a light coat of Loctite RTV silicone gasket material just in case the seal wasn't perfect. There is only an 1/8" wide area keeping the water out of the combustion chamber and even the smallest leak would be a problem. Then I honed the cylinders. It was really convenient that I had made a previous engine with the same diameter bore, so I didn't have to make a new hone. Then I made the tappets and tappet guides.

Bob

 

[michael-au]

Good work Bob, its coming along nicely

When i put the liners in mine they were slightly over size
I put the block in the oven for about an hour and the liners in the freezer

When I went to put the liners in they just fell in and once the block had cooled down they were stuck, the block shrunk back down on to them

I have not had any problems with water

Any way you should have a good seal around the liners and thats all that matters

Nice to the pics of your build, brings back memories

Michael

 

[quinette7]

I've begun to make the pistons for the Silver Bullet. Drawing SB-4 gives three different dimensions for the depth of the piston rings. Take your pick!

The Ring/Groove Data box in the upper right gives a groove depth of .040".
The drawing beneath it says "Cut grooves .043 W x .049 D". This would leave the piston skirt only .013" thick at this point, so this looks like a typo.
And the drawing in the upper left gives a piston diameter of .752" and a groove diameter of .680", which results in a groove depth of .036".

The ring is .030" thick, so I went with a groove depth of .040".

The groove width, on the other hand, was perfectly chosen at .043", since it turned out that by pawing thru my box of little odd bits that my Grandpa had ground a .043" HSS grooving tool! It worked great.



The drawing also gives two different depths for drilling the .312" hole, but since this is just for roughing out it is of no real consequence, just another oddity.

The plans call for a 1-72 socket head screw to retain the wrist pin, but I chose to omit that and make brass or aluminum end caps for the wrist pin to keep it from scoring the cylinder. The MLA Diesel was built this way.

I made a simple fixture to vertically hold the piston spigot and I cut the slots in the pistons.



With the cutter so deep into the part that you can't see what is happening inside, I had to rely on faith. And digital readouts. And a shop-vac to clear out the chips. We'll see if I'm right. And I made the wrist pins.



I think you can see the little brass pads on the ends, as an alternative to the tiny screw keepers. While it was fresh in my mind, I made a sketch of how I made them.



I borrowed this idea from the MLA Diesel. In fact I borrowed the drawing from that, too, and just Photo-shopped in the changes. There are no guarantees implied with this drawing. It is just the easiest representation of what worked for me. The stub end of the pad had a slight, but unplanned for, taper which allowed an easy intro into the wrist pin for about 2/3 of the distance, with the remainder pressed in with my vise. Your mileage may vary. If there are penalty points for serendipity, I claim some here. Maybe that will balance the fact that, despite cleaning the swarf bin and placing a newspaper under the lathe, that one of these tiny pads absolutely disappeared after parting it off! So I had to make 5 of them to get 4 pieces. Where could it have gone? Once I got to the point of spending more time looking for it than making another, I made another. It must have gone to "Little Part Heaven". If that is the case, I can only be envious of it. We'll see if I'm right.

Bob Shores' instructions say to lap the aluminum piston to fit the cast iron cylinder. My past trials with lead, brass, and aluminum lapping tools has been that my most effective lapping tools have used aluminum lapping forms impregnated with diamond grit. So I was leery of introducing any diamond grit to the piston, for fear that it may wear the cylinders. Since this design uses three piston rings, I turned the pistons to the earliest running fit, and called that "good". They pass the "drop test" when I remove my thumb from the end of the cylinder. We'll see if I'm right.

Before making the piston rings, I read all the articles I had about how to do it properly. Boy, what a topic of discussion! It seemed like every writer had a different technique or approach, and they commonly gave credible technical details about how somebody else's approach was wrong. What I took away from all that is that, for their size, "perfect" rings have a lot of technical details that are difficult to produce, and optimizing one detail sometimes compromises another. It boils down to how much time and money one wants to expend. Since I'm not building a competitive racing engine, like some of the authors, I took a simplified approach. My rings are certainly not perfect, but I think they will work well enough for my purposes with this engine. We'll see. None of them broke when I stretched them over the pistons. The pistons are complete and I'm pleased with the results.



Bob

 

[michael-au]

Nice work on the pistons Bob

This is how I held the wrist pin in place

View attachment ImageUploadedByModel Engines1447378896.575189.jpg

I made a circlip from piano wire and cut a small grove in the wrist pin hole right near the edge

The reason I did it this way was because there is no drag on the liner and also I didn't like the idea of a screw to hold it in because it could come loose and fall out

 

[quinette7]

mike-au -

I prefer your solution to retaining the wrist pins! That's a tiny circlip, and a tiny groove! At this point, I'd fear scrapping the part if I tried to replicate it, so I'm going to stay with what I have.

I'm leaving for NZ vacation soon, and by the time I return my garage will be below freezing and too cold to enjoy working, so I don't expect to make much progress for several months.

Bob

 

[michael-au]

mike-au -

I prefer your solution to retaining the wrist pins! That's a tiny circlip, and a tiny groove! At this point, I'd fear scrapping the part if I tried to replicate it, so I'm going to stay with what I have.

I'm leaving for NZ vacation soon, and by the time I return my garage will be below freezing and too cold to enjoy working, so I don't expect to make much progress for several months.

Bob

Have a good holiday Bob

Bummer about the shed freezing, don't like the cold