Stripped undertray M8 bolt - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

BishamontenZC6 · 11-20-2025, 09:53 AM

One of the M8 front metal undertray bolts decided to give up after 100'000 miles on my '14. I don't know how it happened as I always torqued them properly. It's one of the 2 middle ones marked red at the bottom of that picture :

The thread in the hole apparently got stripped and the bolt is now spinning and impossible to torque down. This wouldn't be too much of a problem, but those are 2 of the bolts on wich my STI Flexible Draw Stiffener is attached, so I need to get them back in order.

I have a set of helicoils ready to be installed but I'm doubting about how much metal there is on the subframe to drill the holes up to 8.2mm in order to tap and install an M8 helicoil.

Did anyone ever try it, and do you think it will be ok ? Would there be any other solution ?

Spuds · 11-20-2025, 06:59 PM

The bolts circled in red just hold the fiberglass undertray to the steel undertray. It's just a nut clip (forgot the actual name lol) that slides over the fiberglass piece. The part where mine attached to disintegrated so I bought new clips and made a patch. I'm guessing that's what happened to yours.

The bolts in green are the only ones that are threaded into the chassis. I would be very surprised if one of those got messed up, I am sure I overtorqued them the last 30 times I've had to take that damn panel off.

Spuds · 11-20-2025, 07:04 PM

From post 154 of my build thread:

https://www.ft86club.com/forums/show...&postcount=154

I wound up having to redo it later with some fiberglass but that's actually holding up so far...

BishamontenZC6 · 11-21-2025, 04:18 AM

Yes sorry, I was in a hurry when I wrote the original post and mixed my colors.

I'm talking about the green circled M8 that holds the metal indertray, not the red small M6 holding the fiberglass. Those might hold more when they are only holding the undertray but the Flexible Draw Stiffener adds a quite a bit more load on them. The middle left one on mine is stripped. I also don't understand how it's possible as I probably removed and install them 100 times in the past 5 years.

The picture on your post gives a good view of the 2 holes that I need to helicoil.

Spuds · 11-21-2025, 10:07 PM

Quote:

Originally Posted by BishamontenZC6

Yes sorry, I was in a hurry when I wrote the original post and mixed my colors.

I'm talking about the green circled M8 that holds the metal indertray, not the red small M6 holding the fiberglass. Those might hold more when they are only holding the undertray but the Flexible Draw Stiffener adds a quite a bit more load on them. The middle left one on mine is stripped. I also don't understand how it's possible as I probably removed and install them 100 times in the past 5 years.

The picture on your post gives a good view of the 2 holes that I need to helicoil.

I was thinking that brace would be installed wrong on the red bolts, but wasn't going to mention that yet lol.

Yeah I've got no idea how that happened to your car. Those are pretty beefy.

Good luck!

NoHaveMSG · 11-22-2025, 10:27 AM

Look up an M8 Rivnut. Will probably be your best bet.

Muskoka800 · 11-22-2025, 12:48 PM

It’s been some time since I worked down there but could you not drill and tap the stripped hole for a 9 or 10mm bolt?

BishamontenZC6 · 11-22-2025, 04:57 PM

Thank you very much for all your insights. I did the repair this morning and to my surprise it was much easier than I thought.

I drilled the existing hole at 8.3mm, tapped them with my helicoil kit’s M8 tap, installed the helicoil and everything is now solid and tight again. I’ve even been able to repair the thread on the OEM bolts, until I receive a full set of new ones.

After all the problems I’ve had on the car in the past 2 months, this came as a welcome surprise. I also figured out how the damage happened : the Flexible Draw Stiffener’s pressure flattened a section of the bolt’s thread, which isn’t a problem as long as it stays on (the brace is sandwiched between the subframe and the untertray). But as I’ve been chasing some really annoying suspension clunks since the past weeks, I removed the draw stiffener to see if it’s making the noises and when I reinstalled the undertray the bolts went deeper, which is when the damaged part of the bolt stripped the thread.

11-20-2025, 09:53 AM	#1
BishamontenZC6 Member Join Date: Apr 2022 Drives: '14 BRZ Limited Location: Switzerland Posts: 35 Thanks: 6 Thanked 24 Times in 10 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	Stripped undertray M8 bolt One of the M8 front metal undertray bolts decided to give up after 100'000 miles on my '14. I don't know how it happened as I always torqued them properly. It's one of the 2 middle ones marked red at the bottom of that picture : The thread in the hole apparently got stripped and the bolt is now spinning and impossible to torque down. This wouldn't be too much of a problem, but those are 2 of the bolts on wich my STI Flexible Draw Stiffener is attached, so I need to get them back in order. I have a set of helicoils ready to be installed but I'm doubting about how much metal there is on the subframe to drill the holes up to 8.2mm in order to tap and install an M8 helicoil. Did anyone ever try it, and do you think it will be ok ? Would there be any other solution ?

11-20-2025, 06:59 PM	#2
Spuds The Dictater Join Date: Apr 2017 Drives: '13 Red Scion FRS Location: MD, USA Posts: 9,646 Thanks: 26,684 Thanked 12,705 Times in 6,293 Posts Mentioned: 88 Post(s) Tagged: 0 Thread(s)	The bolts circled in red just hold the fiberglass undertray to the steel undertray. It's just a nut clip (forgot the actual name lol) that slides over the fiberglass piece. The part where mine attached to disintegrated so I bought new clips and made a patch. I'm guessing that's what happened to yours. The bolts in green are the only ones that are threaded into the chassis. I would be very surprised if one of those got messed up, I am sure I overtorqued them the last 30 times I've had to take that damn panel off. __________________ If a picture is worth a thousand words, a model is worth ten thousand pictures. Also: "Build Thread"

11-20-2025, 07:04 PM	#3
Spuds The Dictater Join Date: Apr 2017 Drives: '13 Red Scion FRS Location: MD, USA Posts: 9,646 Thanks: 26,684 Thanked 12,705 Times in 6,293 Posts Mentioned: 88 Post(s) Tagged: 0 Thread(s)	From post 154 of my build thread: https://www.ft86club.com/forums/show...&postcount=154 I wound up having to redo it later with some fiberglass but that's actually holding up so far... __________________ If a picture is worth a thousand words, a model is worth ten thousand pictures. Also: "Build Thread"

11-22-2025, 12:48 PM	#7
Muskoka800 Senior Member Join Date: Jul 2018 Drives: 2016 Scion FRS Release Series 2#439 Location: Chatham, Ontario, Canada Posts: 777 Thanks: 4 Thanked 169 Times in 139 Posts Mentioned: 4 Post(s) Tagged: 0 Thread(s)	It’s been some time since I worked down there but could you not drill and tap the stripped hole for a 9 or 10mm bolt? __________________ Edelbrock S/C with CSG 94 & 91 maps TRD Short Shifter, CTS-V/EvoX Brembos Ohlin’s R&T Dampers, Hotchkis F/R Bars, Blox LCA & Toe Arms 235F/245R Conti ECS on TRD/Enkei wheels (street), 245/40 sq. ECF on 17x8 720Form GTF2 wheels (track)

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11-21-2025, 04:18 AM	#4
BishamontenZC6 Member Join Date: Apr 2022 Drives: '14 BRZ Limited Location: Switzerland Posts: 35 Thanks: 6 Thanked 24 Times in 10 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	Yes sorry, I was in a hurry when I wrote the original post and mixed my colors. I'm talking about the green circled M8 that holds the metal indertray, not the red small M6 holding the fiberglass. Those might hold more when they are only holding the undertray but the Flexible Draw Stiffener adds a quite a bit more load on them. The middle left one on mine is stripped. I also don't understand how it's possible as I probably removed and install them 100 times in the past 5 years. The picture on your post gives a good view of the 2 holes that I need to helicoil.

11-22-2025, 04:57 PM	#8
BishamontenZC6 Member Join Date: Apr 2022 Drives: '14 BRZ Limited Location: Switzerland Posts: 35 Thanks: 6 Thanked 24 Times in 10 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	Thank you very much for all your insights. I did the repair this morning and to my surprise it was much easier than I thought. I drilled the existing hole at 8.3mm, tapped them with my helicoil kit’s M8 tap, installed the helicoil and everything is now solid and tight again. I’ve even been able to repair the thread on the OEM bolts, until I receive a full set of new ones. After all the problems I’ve had on the car in the past 2 months, this came as a welcome surprise. I also figured out how the damage happened : the Flexible Draw Stiffener’s pressure flattened a section of the bolt’s thread, which isn’t a problem as long as it stays on (the brace is sandwiched between the subframe and the untertray). But as I’ve been chasing some really annoying suspension clunks since the past weeks, I removed the draw stiffener to see if it’s making the noises and when I reinstalled the undertray the bolts went deeper, which is when the damaged part of the bolt stripped the thread.

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