It is very important to have the correct size bolt for a tether anchor point with a pre-drilled hole and weld nut. Threading the wrong size bolt into a weld nut can ruin the tether anchor point! Bolt and nut sizes are typically either 8 mm or 5/16 inch. Bolts in many generic kits are 5/16 inch while nearly all weld nuts in vehicles are 8 mm.
First, as
Kecia has correctly made clear, to ensure safety using the correct bolt is critical. While I do not mean to complicate matters (nor write anything overly technical) I feel I should highlight some important aspects of selecting and fitting a tether anchor, particularly when modifications maybe involved. I myself only discovered such when installing the E-Z-On Heavy Duty Tether Anchors in our family vehicle.
(Please note - I can’t comment on issues regarding the approved compatibility of parts and vehicles – I will leave this to the far more knowledgeable members and CPSTs on the forum!)
To find the correct bolt to be used, the following must be addressed:
1) Diameter
This has been mentioned above. Simply put, the diameter of the bolt must match the diameter of the nut. This choice will nearly always be between imperial (inches) and metric (mm) sizes. Where car seat tether anchors are concerned, the most common sizes are 5/16 inch and 8mm sizes, respectively.
2) TPI (Threads Per Inch) / or “Pitch”
All bolts have a TPI measurement. In simple terms this informs of the fineness or coarseness of the thread. A high TPI means a finer thread, a lower a more coarse thread:
Importantly, the TPI of a bolt is
not related to the diameter of a bolt. Any bolt can have any TPI. The TPI of the bolt
must match the TPI of the nut. If it does not, and the bolt is tightened, the thread could be stripped. Without a thread, the bolt will not be secure. In the event of a crash, this could mean the bolt slips out of the nut resulting in the tether strap becoming detached from the vehicle.
3) Grade
In basic terms the grade is the quality of the bolt (and nut). The higher the grade, the better quality and thus, inherent strength of the bolt. The grade of a bolt is identified by markings on the bolt head (usually 4.6, 8.8 or 12.9 for metric bolts and a series of markings for imperial bolts –
see this chart).
This is very important and relates directly to a couple of points already mentioned in some previous messages in this thread.
First, while I do not doubt the LATCH Manual supplement, I am surprised at the following comment as written in the pdf referenced in post #12:
"A bolt marked “8.8” is compatible with an 8-mm nut."
While a bolt marked “8.8” maybe compatible with an 8mm nut, these two aspects are
not related for “8.8” relates to the grade of the bolt, not the size. Personally, I would disregard this comment – not because it is necessarily incorrect, but because it is somewhat misleading.
I'm pretty sure an anchor kit with a 5/16 bolt could be used if you swapped out the bolt with an 8mm bolt from the hardware store.
Second, for bolts which are to be used to secure tether anchors, I do not recommend purchasing bolts from hardware stores. Such bolts are often of a lower grade intended for use in applications far less demanding than a vehicle tether anchor. While they maybe strong enough, there is no way of telling unless they are marked. I would only recommend using bolts with the correct markings from a reputable supplier of vehicle parts or vehicle safety restraints.
4) Tightness
All critical bolts in vehicles, are tightened to a certain degree depending on their application. If a bolt is under-tightened it could work loose, if it is over-tightened it could shear or split. It is critical to tighten the bolt to the correct tightness to ensure performance of the bolt and thus, safety. Most manufacturers will specify this measurement and a torque wrench should be used to make sure the bolt is tightened correctly.
In short, choosing the correct bolt for the correct application is far more complicated than it may first appear! Most of the above are decided by the manufacturers (of the vehicle and/or the restraint) and the consumer rarely has a decision to make. However, in light of the original post, such factors may need to be researched and I would recommend these factors are considered with the utmost care – or the task given to a professional mechanic or company who can install the tether anchor to the correct specification.
I apologise if this has made matters too complicated, this was certainly not my intention, however I feel I should mention such aspects as they are critical to occupant safety.
If needed, I’d be happy to answer any questions regarding the above.