Stripped thread in mower head

[Stevedore]

I just did a set of recommended maintenance tasks on my Honda lawnmower. For the first time since I bought it, maintenance included a valve adjustment.

The rockers are under a small valve cover held in place by 4 bolts into the aluminum head. One of the bolts was noticeably more difficult than the other 3 to back out. Not hard to break loose, just a lot of drag when turning. When it was removed I saw a lot of aluminum on the bolt threads, and as I expected, when I reinstalled everything, that bolt won't tighten properly. I'm assuming that the bolt was overtorqued during original factory assembly.

It seems to be holding for now, i.e., no oil leakage, after running the mower for a while, but I fully expect to need to address the stripped thread at some point in the future.

My first thought is a helicoil insert. I've never used them, but they seem relatively straightforward to install. A kit with the size I need (M6x1) costs around $25-30.

My other thought is to replace the bolt with the next Metric size up, which would be M7x1. With the thread pitch being the same, and with so little difference in the size, I'm thinking that I could just run an M7 tap right into the stripped hole. Worst case, I might have to clean out the hole with the correct drill bit for the M7x1 tap, which would be easy enough.

Am I overlooking anything here? Does this seem like an OK idea? Or would I be better off with a helicoil?

Thanks!

 

[eric_koski]

Locktite thread repair

 

[raddksn]

Locktite thread repair

Never heard of It! Elaborate please!

 

[Boneill230]

Worst comes to worst just do what you said. Cant tell you how many times at work we have had to do stuff like that. I think the worst was having to drill and tap a new bolt hole for a hydraulic manifold, wayyy more pressure than a lawnmower engine lol. Works fine though, so if it turns to be a problem just do what you said drill and tap

 

[racingtadpole]

M7 is not a common size. If there is room I would go up to M8 (1.25mm pitch if you're using ISO coarse). Or helicoil as you discussed.

 

[Bigplum]

If you've got a m7 tap ,use it , if not pop the lot down to the local engineering place , shouldn't take a few minutes to sort out , offer beers as payment

 

[EdT]

Since this is a valve cover bolt and is not mission critical I'd just leave it be unless it leaks. If you need the bolt in place, then you might try running a bottoming tap of the existing size down the hole and see if a slightly longer bolt would work. I wouldn't be surprised if the bolts are thread forming bolts and the holes will be deeper that they have to be and the end of the bolt is (probably) a little tapered which might also give some purchase to a new slightly longer bolt.

 

[Lx460]

The three intact bolts and whatever clamping force the fourth is offering is likely enough to seal it. It's not under pressure at all, just sealing from splash lubrication. I'd leave it alone until it leaks then go with either of the methods you described. No sense making more work or $$ if you don't have to.

 

[CJM8515]

If it doesnt leak leave it alone, it does m7 is tough to find but 1/4" is EVER so slightly larger than 6mm so it should work fine.

 

[70Chevy]

Slightly longer is a good idea for this low pressure screw.

 

[Stevedore]

Thanks for all of the replies.

The cover isn't leaking for now, and it may never leak, but i's not my nature to leave something like that, and would rather fix it now. It's a curse I have to live with. I picked up an M7x1 bolt at Home Depot, and I can get the right tap through a local hardware store for about $5.

EdT - You're correct in your suspicion that they are some kind of thread forming bolts. I hadn't noticed, but looked at one this morning, and the end is in fact somewhat tapered, and has several lengthwise grooves cut into the tapered area. I probably have a regular, somewhat longer, M6 bolt somewhere, and possibly a tap, so I may try that first.

Thanks again!

 

[rslaback]

Personally it would bug the **** out of me knowing that was there, even if it wasn't leaking. I'd stick a helicoil in it. It seems to me that is the "right" way to fix it but that may be because I detest different size fasteners on projects.

 

[gte718p]

Personally it would bug the **** out of me knowing that was there, even if it wasn't leaking. I'd stick a helicoil in it. It seems to me that is the "right" way to fix it but that may be because I detest different size fasteners on projects.

I'll second that. No rational reason for it, but it would piss me off every time I looked at it.

 

[eric_koski]

I have used this to repair threads in alum housings and it works provided you follow instructions.

 

[eric_koski]

This video is for a Permatex product. Locktite makes a similar product. Both work great.

 

[Doug Arthurs]

For any non metric guys if you need to determine the correct drill for a m7 x 1.0 tap just subtract the pitch for the diameter M7 - 1.0 = 6 mm drill.

 

[harvey29]

On a lawn mower I wouldn't be afraid to try JB weld?

 

[mbret2004]

For any non metric guys if you need to determine the correct drill for a m7 x 1.0 tap just subtract the pitch for the diameter M7 - 1.0 = 6 mm drill.

Pretty slick, I'll remember that. Thanks Doug!

 

[Doug Arthurs]

Pretty slick, I'll remember that. Thanks Doug!

That's the best thing about metric. The math is easy.