gx390 Exhaust Stud Fix???

B

Badzuk

New member
Does anyone have a good fix for the GX390 exhaust studs? Or can you let me know what I'm doing wrong?

I have 3 GX390 based engines currently in service, 2 genuine Hondas and 1 clone. All these motors seem to have issues with the exhaust studs coming loose, or snapping off completely (that could be from employees over tightening a loose stud). I have tried new hardware, different flavors of thread lock, double nutting, physically distorting the threads, etc.

Has there been any success in heli-coiling the heads, and even upgrading to larger hardware? What about JB Weld as a thread locker? Tap the exhaust port for NPT and run the exhaust system remote mounted?

Right now I have a pile of old heads with broken exhaust studs or bad threads in the stud holes, and I'm sick of mufflers falling off.

Overall the Honda's serve me well, most of the motors are 7-10 years old before the rod locks to the crank, and most of those failures were probably the result of years of neglect, coupled with someone forgetting to check the oil.
 
Sorry about the missing Signature, should be below.
 
I have had great luck retapping the holes to 3/8" and using grade 5 bolts with lock washers. The exhaust pipe must also be reamed to 3/8".
 
Russ Johnson said:
I have had great luck retapping the holes to 3/8" and using grade 5 bolts with lock washers. The exhaust pipe must also be reamed to 3/8".
Click to expand...

Thanks, that's the direction I was planning on going next. I already have some 3/8's heli-coils I was going to use for some additional durability in the head, but haven't started drilling yet.

Just checked my Power Washer Graveyard, and I have 5 old heads, 4 with broken studs, and 1 with stripped threads.
 
I had the same problem on a Koehler engine.... Lost 2 studs. My auto mechanic helicoiled them.... Then he added a brace that bolted the muffler to the aluminum belt cover housing to support it and reduce vibration.... That solved the problem..... I will try to remember to take a pic of it tomorrow, and post again here....
 
Bob Keenan said:
I had the same problem on a Kohler 30hp engine.... Lost 2 studs. My auto mechanic helicoiled them.... Then he added a brace that bolted the muffler to the aluminum belt cover housing to support it and reduce vibration.... That solved the problem..... I will try to remember to take a pic of it tomorrow, and post again here....
Click to expand...
IMG_1480.jpg
 
So I tore into it last Friday, with the intention of taping the holes for 3/8" bolts like Russ suggested. Turns out I had already helicoiled one of the holes for the stock size stud, so I had to drill it bigger and install 3/8 helicoils too. So far they have held. Time will tell. The nice thing was I was able to do it in the back of the van and didn't have to remove the machine or motor. That saved a bit of time.

When I get some spare time I'll work on some of my extra heads and drill out the old broken stud and then tap them as well. Hopefully this fixes the issue.
 
Get a good torque wrench for tightening the head bolts - low range.
 
Remote mounting the muffler is great, takes all the weight off the studs and keeps the area from being an oven.
 
Exhaust Stud failures

back in the early 2000's Honda GX390's had a major rash of failures at the exhaust studs..
They started failing at 6 to 18month old.
..2 little 8mm bolts holding that heavy muffler on there, was a design flaw to begin with, but 10.9's worked,
until leaner fuel mixtures increased manifold temperatures. (A complication of clean-air requirements.)

Use High-temp thread locker, with grade 10.9 studs,
and torching out the bad studs, to save the original threads is best.
On the clones..
it is highly likely that the stud chosen were NOT "the best"
..nor installed with the best thread locker.

get grade 10.9 studs at Fastenal.. I think it was part #11118304
use LockTite thread locker #294

If you have to upsize to 3/8 SAE studs, use grade 8
 
You must log in or register to reply here.
Back
Top