Plugging holes in a Spridget block

C

craig bryant

Well-known member
Okay, here's another question for the British brain trust. On a Spridget block, there is a threaded hole on the side just under the oil pressure relief valve, and another on the bottom sump pan rail between two of the mounting holes near the rear main cap. I do believe that I know what they are for, and also that they are supposed to be plugged. My question is, what is the best way to plug them so they won't leak? They are between a 1/4 and a 5/16 thread. What is the thread size of those holes, and is it better to tap them to a 5/16, or just use a small brass plug to seal them? Thanks in advance for any and all suggestions.

Craig
 
These are part of the oil pressure relief valve porting. The holes are not threaded from the factory in a stock block. Just plugged with a tappered brass plug.

As part of block prep activities, I drill the plug and tap for 10x24 and use a long screw and some nuts and flat washers to pull the plugs. Same for two of the oil galley brass plugs except I use a 5/16 x 24 tap, bolt and nut setup. Then I use a long steel rod to knock the other two out. Then drill the two relief holes 11/32" and tap for 1/8" npt. Watch the drilling. The bit likes to grab if you drill into a cross passage. I use some Permatex #1 hardening sealer to help seal the threads. Don't over tighten. The oil galley holes get a 1/4" npt plug. Don't tap too deep or the front and rear bearing port can get covered with the end of the plug.

All these holes get opened before the block goes into the hot tank. That way, the galleys all get cleaned by fluid passing down and through them. Opening the pushrod hole side allows the cleaning brushes to get in there after I've drilled out the pushrod holes to 1/2" for better oil drain back to the block. Caution here. The pushrod holes are at an angle. Angles vary between 2-4 degrees. The block can be angled or the drill head on the mill can be angled.

When drilling and tapping the pan rail hole, sometime the lower back plate bolt hole gets opened. No problem, just use a little low strength thread sealant on the bolt.

HTH, Mike
 
Drill out both brass plugs and replace them with 1/8" NPTF steel allen head plugs. This way you can truly clean the block of machining swarf waiting to destroy your engine. Be sure the pan rail plug that is covered by the pan rail gasket does not stand proud of the flange surface. The other bottom hole MUST remain open in order for the relief valve to work.

For years I wondered about these passages as well so I finally sectioned a junk block right to left through the center of the pressure relief valve plug on a GIANT bandsaw. Now I have a trick piece of wall art AND a complete understanding of the pressure relief system on an A series block. Alarmingly simple once you see all of the passages.

Thanks, Jay
 
Holes made when cross drilling oil passages in block. Originally plugged with small taper brass plugs not intended to be removed. Still have some plugs; wouldn't use.

If memory serves, drill and tap for 1/16 NPT, use BRASS pipe plug, and teflon sealer (not tape)

RJS
 
As I mentioned in my first post, both of those holes are already threaded. There are no plugs in there right now. A 1/4 inch bolt just slid right through, and a 5/16 bolt didn't even start, so it was a thread size between those two. I went and got some 1/16 npt brass plugs and they are too big as well. Just to satisfy my own curiosity on the odd size thread, I also got a 7 mm bolt to try, and lo and behold it fits. It's a bit sloppy, but it's the only thing that fits. Now I'm a bit confused. Why would it be tapped at such a strange size...is it Wentworth, did it come that way, or was there a reason someone sized it that way?

Craig
 
Holes never tapped at factory that I've ever experienced.

Is it a straight thread or is it tapered? Are the threads deep enough for a plug, or they only witness marks from tapping and pulling the original plug?

Plug in hole on pan rail has to seat below flange surface, so not a headed plug of any kind.

Just drill and tap for new plug of your choice, and leave it a mystery.

RJS
 
Can you check pitch of threads? Screw 5/16" dowel into hole, or place round toothpick in hole and wedge into threads. Measure indents in wood.

I just discovered that 1/16-27 NPT is NOT listed on most tap drill charts, Machinery Handbook, McMaster-Carr pipe chart, etc., but I suspect this is what you have.

1/16-27 NPT; .313" O.D.; "D" (.246") tap drill (from Metrics Unlimited tap drill chart). Believe holes in block to be 1/4", so tap drill unnecessary.

1/16-27 NPT brass hex socket solid plug McMaster-Carr #50785K267, tap #2553A11. (note tap specifies "C" drill [.242"]) ? ? ? Google McMaster-Carr

Why brass? I learned that both brass and steel plugs seized. Brass easier to drill out next time.

Let us know outcome.

RJS
 
The threads in the two holes go all the way in to the oil channels, and yes, I do believe that they are tapered. There is about 10 to 15 thousandths difference between the front and back of the thread length. When I put the 7 mm bolt in, it is sloppy and then really grabs the middle portion of the thread. The 1/16 npt brass plugs that I got are 27 thread pitch. I'm not sure what the pitch is on the holes, but I appreciate the suggestion to check them with a wood dowel. I am going to try that tonight. The problem with this whole situation, is that the motor was delivered to me put back together this way. The builder had some health issues in the family that I fear took his attention away from details in rebuilding the motor properly. I'm a bit hesitant in drilling and tapping and I was hoping not to have to take the motor apart to solve this problem.

Craig
 
Appreciate your not wanting to tap into an already assembled engine.

Another possibility: British Pipe Thread. But curious differences. British Standard Pipe (BSP) threads available in both straight (parallel-BSPP) and tapered (BSPT) threads. Tapered thread seals on the increasing interference of the taper like NPT whereas straight (parallel) thread uses a flange seat. 1/16-28 BSP is .304" diameter, 28 TPI.

BUT; I don't believe they make a tapered 1/16-28 (BSPT) thread, only the parallel (BSPP), which requires a head to seat. And as mentioned above, hole on pan rail can't use a headed plug.

Hope this doesn't add to the confusion.

RJS
 
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