Con-Rod bolts don't fit! whatever next?

[steve briance]

hello all,

Frustratingly I have hit a ridiculous snag which has put the brakes on getting my engine rebuild under way. I finished checking all my bearing running clearances and gapped the new piston rings but came to press in my new con-rod bolts and found them to be really very tight.

I understand there should be a certain amount of interference to hold them in place in the rod but there shouldn't be any interference with the cap, I've got so much it struggles past the threaded section and doesn't sit down on the rod without doing the nuts up!

I pressed the new bolts out again and found that the original bolts were 12.9mm on the shoulder which locates the bearing cap and the new bolt was 13.0mm.

I got the Febi bolt 02126 as this is widely available i thought i can't possibly go wrong - they wouldn't be so widespread if they didn't fit would they? well...

OEM number appears to be 026 105 425A / 048 105 425A and both point back to Febi 02126.



Other than fitting ARP, anyone any thoughts or fixes having been there done that? I don't really want to splash out on ARP as the rebuild job is a standard 7A and though I get the ARP benefit for high performance engines, mine has gone for well over 200,000miles with standard bolts and is fine so i don't see i will gain anything

cheers!

[Error404]

No experience with Febi bolts, although all of their parts have been of questionable quality.
The cap needs to use the bolt shank to locate it, so I would expect to need to tap the cap on with a copper hammer and it should be tight, even without the nuts on.

[90/90qs]

Anyone correct me if I'm wrong, but if you don't have a shop install the bolts and then re-hone the rod big ends I'm pretty sure they are going to be out of round; then cause wear problems with the rods and/or journals. This info came from the last time I installed rings and rod bolts etc.

[steve briance]

I have heard this mentioned also but wasn't really sure why the end of the con rod would go out of round when the bolt torque is actually quite low - 22ft/lb plus 90deg.

I actually read it on the ARP website fitting instructions but as their selling point is the ability to torque up high and resist huge boosted forces, you could expect there to be some deformation... ?

[Error404]

The pressing process can distort the big end, you will probably find they are fine but your machine shop should check them for straightness and parallelism as well as roundness after fitting the new bolts

[90/90qs]

I have heard this mentioned also but wasn't really sure why the end of the con rod would go out of round when the bolt torque is actually quite low - 22ft/lb plus 90deg.

I actually read it on the ARP website fitting instructions but as their selling point is the ability to torque up high and resist huge boosted forces, you could expect there to be some deformation... ?

don't mean to be offensive but you might be glossing over some important details and rating their relevance without understanding everything involved. Not saying I know, but you'll probably hear the same from anyone who knows well.
btw it's not the torquing everything together at the assembly stage that causes the deformation, it's the force required to install the bolts IIRC.

[steve briance]

https://www.youtube.com/watch?v=75MW8Gvn13o


Not the greatest quality video as i think it pre-dates HD cameras by a few years but, it does explain quite nicely the problem of con rod bolts and distortion.

The fact that my pants quality 0.1mm too big Febi bolts were so tight to fit makes me think that the re-build will now have to wait until my machine shop has checked for any OOR.

Now off to the dealer to have my pants pulled down on the cost of some factory bolts

[90/90qs]

Great info! Good luck with the rest of your rebuild, and avoiding snags!