1st gen 6 bolt vs 7 bolt-Actual differences??

[Streetwize]

This is the HARDCORE TECH section, ok, I'm about as Hardcore as it comes with motors (24 years of building just about anything with pistons) so Impress me.

What are the actual differences in the block casting and crank, I know the rods are a little lighter in the 7 bolt but aside from that.

I'm hearing/reading all the "urban legend" and Heresay stuff, but whenever specifics are asked, nobody seems to be able to nail it down. Has anybody sonic checked the blocks to measure thicknesses in key areas? It's not unusual to have thick and thin blocks in the same casting run depending how many times the casting core boxes were used and the grade and curing of the iron pour at the foundry. I think with normal production variance (yes, even the Japanese have variances in core quality), it wouldn't be unusual to find thin relatively 'Weak' 6 bolt blocks and Thick 'Strong' 7 bolts.

I have a friend who will gladly and objectively sonic check and compare all blocks at any and all critical dimensions for a reasonable fee, might be interesting to gather this info for future reference.


I am assuming the second gen (95-later) 7 bolt is a different casting? Does anybosdy know for sure?

The block casting particularly casting thickness in the main webbing is critical for crank stability and the motors ability to withstand heavy loading, particularly in turbos and nitrous applications where the power curve is steeper and more instantaneous.

In fact, I wouldn't be suprised if Crankwalk is not so much an issue with the crank as much as it is the block flexing around it. The block is the foundation of a motors ability to make power. The crank is the Hammer, the block is what "get's hammered". If you look at aftermarket race blocks, the main differences are they are cast stronger in the main bearing webs and also usually on the thrust side of the cylinder bores. This is to minimize deflection which reduces ring seal and can induce cracking and catastrophic failure. I've seen IHRA Top sportsman 2 stage N2O 1300hp 672" BB Chevies split in three pieces.

Anyway, not trying to sound like I know it all, I don't.... but I know a pretty respectable amount. Somebody who knows the facts about these motors please share with me what youy know. I have a 91 6 bolt and a 93 7 bolt, tell me WHY I need to build the 6 bolt.

Thanks,

Wize

 

[BISHILVR]

one of the main downfalls of the 7 bolt block is that the oil supply for the oil squirters is tapped off of the main bearing journals possibly starving the mains at certain times, this is the popular theory for crankwalk (the thrust bearing is starved for oil too often), on the 6 bolt block the oil supply has its own channel in the block that has nothing to do with the main bearing oil supply..........

 

[herostar]

one of the main downfalls of the 7 bolt block is that the oil supply for the oil squirters is tapped off of the main bearing journals possibly starving the mains at certain times, this is the popular theory for crankwalk (the thrust bearing is starved for oil too often), on the 6 bolt block the oil supply has its own channel in the block that has nothing to do with the main bearing oil supply..........

Yes for 2nd gen oil squirters, but first gen 7 bolts are tapped in the same spot as the 6 bolts.

The main girdle for the crank is stronger in a 7 bolt. The oil passages on the front cover are different. There are taps for a crank sensor on a 1g 7 bolt, but not on a 6 bolt. The head studs are 12mm in a 6, but 11mm in a 7.

 

[char]

odd though because some super high performance dragracing motors take for instance the donnavan 650 ci all aluminum roller motor is a split block where as the block spilts in 2 halfs i guess its all engineering

 

[Streetwize]

THAT's what i'm talking about!! Real info!!

Thanks guys!

 

[pneumo]

Since you have both a 6 and 7 bolt motor, check the rod bearing width at the big end. IIRC you'll find the 7 bolt rod is narrower, with a narrower bearing = less load carrying capacity as the power goes up.

BTW, I have a head that has core shift of about 1.5mm at one end, and only .5mm at the other, so yes there can be significant differences in quality through a manufacturing run.

 

[crash89]

i know this thread is old guys so Im bringing it back for hopefully more info. The OP also asked about crank differences, as am I.. I was wondering what the real differences are between the 6 bolt and 7 bolt cranks besides the obvious 6 and 7 bolt holes? Both cranks have an 88mm stroke and 85 mm bore, so what else is different? Main or rod journal size? Length? Come on guys lets see if we can get some more info. Thanks