Metric 6.9 Studs

gerlbaum · Nov 22, 2017

Just early morning sleepless reading and I saw a thread about re-tapping a 6.9 block to use 9/16 studs and the debate if there was enough meat to do so. It was suggested to not re-tapping a 7/16 hole to 1/2 because there would only be 95% thread engagement. Why not go metric and tap a 7/16 to 13mm (maybe 14mm)? Sure, would require custom studs but I would think 13mm studs would be relatively "uncustom" with all the metric engines today. I also saw it someone suggest re-tapping a 7/16 to 1/2 fine thread but looking at my Machinist Handbook I remember seeing coarse thread is stronger than fine thread.

Anyway, early morning thinking. Thoughts from any real machinists?

gerlbaum · Nov 22, 2017

Just did a quick look at ARP's website and they stock Toyota head studs some of which I believe use m13x1.75. 13mm is 0.3 larger than 1/2" so I'm not sure if that 0.3" would give 100% thread engagement.

That being said, m13 is kind of oddball so 14mm would be perfect but at that point a 9/16 may just be easier. I saw a thread where cummins dropped their head bolt size down from 1/2 to m11 and those guys re-tap for 9/16.

Anyway, I can't sleep.

Macrobb · Nov 22, 2017

I know Justin of R&D did a topic about up-sizing a 6.9 over on FtE forums; might want to look it up.
If I recall, upsizing to 1/2" coarse worked, and the threads in the block were still stronger than a 1/2" ARP stud, so the thread engagement isn't an issue.
The only issue was in keeping the bores straight when drilling them out. He figured you'd need a milling machine; I'm hoping it can be done with a piloted bit and drill bushing to keep it straight - would make the process a ton easier and cheaper.

I'd still like to find a way to use 9/16 socket head cap screws(grade 10), instead of $$$ of arp 1/2 studs.

gerlbaum · Nov 22, 2017

Interesting thanks for the heads (no pun intended) up. I have a set of NOS 6.9 new castings head I am going to put in once I blow up my gaskets with a new turbo and my omega moose and was thinking ahead. I don't think I'd attempted it not squared on a big jig bore machine just to be sure.

icanfixall · Nov 22, 2017

Might work but if it did not. Then you have a junk block. We have a member that drilled out a 6.9 head to the 1/2 inch bolt size with a simple hand drill. He used the 6.9 heads on his 7.3 block.Its been several years back but it worked. Only way I would do this is on a CNC Bridgeport mill or CNC radial arm drill press.Same with the block.

gerlbaum · Nov 22, 2017

Yeah that makes sense. The few hundred for the right machine work is worth it when spending the kind of money it takes to build these engines.

rwk · Nov 23, 2017

Fine threads are stronger then coarse, minor dia.( dia at bottom of thread) of fine thread is larger in dia then that of a coarse. (why fine tread tap drill dia. are larger then coarse for a given size). Strength (tensile) is base on dia. and material. Thread engagement is a ratio between hole dia. and tap dia. example, 7/16 -14- drill it letter drill U .368, its 75% thread engagement, drill it V .377 its 65%, All hand drilled holes will be oversized of the bit used, PERIOD. Almost all holes drilled on a machine will be oversized .001-.003 or more if drill is dull or wrong feeds and speeds are used. If you want an accurate hole( hi percent of threads) it must be reamed to size

rwk · Nov 23, 2017

oops, premature post,lol, yes if you plan to DYI a block drilling I would hand drill under size a little if a drill of that size is available then ream it to size, custom reamers can be had ground to any dia. for a small fee, Mc Master and others, Taps for different grades of thread fit are available also, like 2B and 3B being closer fit, studs are usually a closer fit so they don't unscrew. Not to sure about 100% thread never seen it, don't think you could assemble it, 70-80% is the norm.
This ends the boring information share I have acquired over the years. LOL

! Happy Thanksgiving to all

gerlbaum · Nov 24, 2017

Hahahaha happy belated turkey day as well! I don't think I'd ever attempt to hand drill it, way too many holes for way to many chances of something going wrong. I hate tapping as it is, too stressful. That being said, if I were to advise someone doing it by hand I would suggest making a fixture out of one of those honing plates they bolt to the top of the block when boring and honing cylinder walls. I see them on eBay every now and then for these motors.

Metric 6.9 Studs

gerlbaum

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