I'm hoping I can get some help identifying the size of these threads on this engine. I picked up a replacement engine for my garden tractor and all is good except for the threads in the exhaust ports. This is a Briggs opposed twin flathead, model 422447. The previous owner tried to thread 3/4" NPT pipe fittings in and mangled the threads. I'm hoping I can clean these up with a tap but I need to identify what they are first. If all else fails, I can use the tapped holes on either side of the port to mount a flange but I'd rather not deal with gaskets. One side isn't too bad but the other is looking pretty taxed. Are these normally 3/4" NPT and the previous owner just overtightened or could these be 3/4" NPS? Any advice is much appreciated.
DJ54
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Looking up the model number engine, then finding an online parts manual showing exhaust parts, it appears that an adapter was used picture below. L&R have different P/N's and different prices.
Right, that is a last resort option that is not original to the tractor. I'll have to take a pic of the exhaust setup and post it later today. The exhaust setup appears to be original and is threaded directly into the ports on the original engine then straight into muffler mounted high and to the front. These engines were used in a lot of different applications so I'm sure there are different setups. If I can keep something close to the original setup I can keep the original heat shielding in place as well.
mrmikey
Veteran Member
I believe it 3/4" NPT
I've read many threads on this trying to figure out what exactly these threads are. I can confirm mrmikey, the threads of at least the stock nipple on the briggs opposed twin engine I have are in fact 3/4" NPT. I removed the original threaded fittings today and they are definitely tapered 3/4"-14 NPT threads. I can also say that the depth of the threaded hole in the block is well past the max engagement. Once the fitting/pipe is as tight as it should get, there is still a good 1/2" of threaded hole left. If you try and tighten any further you will start stripping out the block. Notice in the attached pics that you can get only about 4-5 turns. On a standard pipe fitting that still leaves a lot of exposed thread left. The previous owner of this block kept going and damaged the threads. I'm not a pro at NPT fittings and not familiar with what would be considered normal. Also in my ignorance, I can't confirm what the threads in the block actually are, only what the original fitting was.
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RandyT
Elite Member
You do realize that the ring around the fitting with the spikes sticking out is a separate piece and is actually a lock ring that you tighten to hold the fitting in place after installing the threaded portion by hand.
jigs_n_fixtures
Veteran Member
The first photo of his cylinder head has threaded holes adjacent to the exhaust port. Which might be for the adapter in post #2.
RandyT
Elite Member
I am sure it is. And I have seen both styles used on engines over the years. A lot of that choice is whatever the mower manufacturer wanted to use. So either the threaded insert or the bolt on could of been used depending on the exact engine type code.
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If it was mine, I'd use the 2 bolt external flange and a suitable exhaust gasket. Reason being is the exhaust port will flow better minus the restriction of the threaded pipe inserted in the port. and..
I'd clean up the buggered threads already in the port as they could cause hot spots while the motor is running. Have a similar engine sitting in the shop and mine is threaded 3/4" NPT. Iy has no exhaust system on it presently and it appears in the application it came out of, the internal threads were never used. Instead an external 2 bolt flange was.
Keep in mind that a 3/4" NPT tap will be an expensive date. I could loan you one if you want to go that route however.
Threaded engagement on any NPT threaded hole will entirely depend on how deep the tap was inserted initially. Usually, engagement of 2 or 3 complete revolutions of whatever you install is enough. It's a tapered engagement so sealing is always dependent on the taper, not the actual physical insertion. Works the same way when threading pipe.
I'd clean up the buggered threads already in the port as they could cause hot spots while the motor is running. Have a similar engine sitting in the shop and mine is threaded 3/4" NPT. Iy has no exhaust system on it presently and it appears in the application it came out of, the internal threads were never used. Instead an external 2 bolt flange was.
Keep in mind that a 3/4" NPT tap will be an expensive date. I could loan you one if you want to go that route however.
Threaded engagement on any NPT threaded hole will entirely depend on how deep the tap was inserted initially. Usually, engagement of 2 or 3 complete revolutions of whatever you install is enough. It's a tapered engagement so sealing is always dependent on the taper, not the actual physical insertion. Works the same way when threading pipe.
