Rod shortening as an alternative to a thicker headgasket

[JimR]

First off, I'm in the middle of a rebuild from the bottom up.

I just got my head and block back from the machine shop. They needed to take off 8 thou from the block (fire rings, corrosion) and 8 thou from the head (warpage) so I'm looking at a total of 16 thou (0.4064mm) to make up.

I'm replacing my rod bolts with ARP hardware, so as part of the rod resizing work my machinist recommended that we shorten the rods by the exact amount required to run the thinnest MHG available. I'm aiming for stock compression.

This sounds reasonable to me.

When I talk about "rod resizing", I'm referring to IJ's description here: http://www.supramania.com/forums/showpost.php?p=666150&postcount=8

When I talk about "rod shortening", I mean cutting a little from the big-end mating surface in addition to the cap, which will move the center of the hole upwards after the rod is bored/honed.

I'd like your opinions / experiences with the rod shortening method. It sounds like a great way to increase reliability through using a thinner (and less expensive!) headgasket.

Thanks,
Jim

 

[IwantMKIII]

edit....hmmm might need customized bearings...they'd have to be rather thick. personally i wouldn't do it....just get a thicker headgasket

 

[jdub]

Cometic will make a MHG at any thickness/bore size you require...a 1.8mm thick would be about right.

I would resize the rods, but only to the extent to get them round. Bearing size is determined by what is done to the crank journals. FYI, taking off more than 10 thou will remore the induction hardening Toyota uses...nitrate harding would be a good idea in this case.

 

[IJ.]

Back when I played with Nissan L Series motors I used pistons with a different pin height to drop them down the bores to decompress the motor for the turbo's I was running.

There are +'s and -'s doing it this way as it destroys the Squish built into the motor but does allow stock components to be used.

With the selection in sizes of MHG's available for the 7M I wouldn't bother.

 

[JimR]

edit....hmmm might need customized bearings...they'd have to be rather thick. personally i wouldn't do it....just get a thicker headgasket

I believe the bearing thickness should not be affected by a rod resize. The diameter of the rebored/lined hole can be made to the exact size as it was before. I think that's the whole idea of a rod resize... to make the hole the right size and perfectly round again, after the rod bolt replacement has warped them.

 

[JimR]

Cometic will make a MHG at any thickness/bore size you require...a 1.8mm thick would be about right.

I would resize the rods, but only to the extent to get them round. Bearing size is determined by what is done to the crank journals. FYI, taking off more than 10 thou will remore the induction hardening Toyota uses...nitrate harding would be a good idea in this case.

My crank appears to be perfect. This engine has never seen rodknock, and all bearings were within spec when I took them out, with no spinning. I checked all the journals with a micrometer and they are all perfectly round and in spec. So I wasn't planning to do anything to the crank.

When you talk about taking off material and the induction hardening, is this on the crank?

 

[IJ.]

Heckler: You understand the process correctly "I want" doesn't..... :nono:

The pistons in a 7M protrude 20 thou from the deck at TDC you could offset the big end holes this amount safely but it's an extreme way of doing it compared to the correct thickness MHG.

 

[JimR]

Heckler: You understand the process correctly "I want" doesn't..... :nono:

The pistons in a 7M protrude 20 thou from the deck at TDC you could offset the big end holes this amount safely but it's an extreme way of doing it compared to the correct thickness MHG.

You mentioned Squish earlier... I'd be very interested in understanding how I can optimize for that at this point. Variables like cylinder head protrusion from the block, (FYI I'm using the stock GTE pistons) headgasket bore size, etc.

As for rod resizing/shortening, there is zero extra cost to have the rods shortened as part of the resizing work. This means that I have the luxury of precisely controlling my final compression ratio. For example... take 8 thou off the rods so the pistons protrude 20 thou as opposed to the current 28thou if I was to slap it together right now, and get a gasket in the 1.6mm range (stock hg of 1.37mm + head decked amount of 0.2032mm).

Jim

 

[IJ.]

Jim: If it's a no cost option I'd do it!

The motor is designed with the correct squish when everything is at stock spec, I doubt there is much in the way of power hiding in the squish band but there should be improved economy/efficiency.
(I've ran 3mm MHG's on basically stock spec head and block and still made great power)

 

[jdub]

Jim: If it's a no cost option I'd do it!

Ditto! I was thinking extra $$$ was involved

 

[IJ.]

Would be a bitch to do the work though from a machinist's PoV

 

[IwantMKIII]

Heckler: You understand the process correctly "I want" doesn't..... :nono:

meh, it happens...live and learn

 

[IJ.]

meh, it happens...live and learn

Each and every day and I probably will be still learning till the day I die

 

[Jeff Lange]

Yes and perhaps learning the same things again, provided you live long enough .

 

[jdub]

^^^ I call it "being reminded"

 

[IJ.]

Who were you again?

Where am I ????

Maaaatlockkkkkkkkkkkk

 

[JimR]

Well I've just remembered to say: Thank you everyone for your input!

I ran that 8 thou from rods + 1.6mm HG by my machinist and he said that's exactly what he would have recommended himself. That's a GOOD SIGN to me! ))

 

[HommerSimpson]

why not just take 8k off top of the piston ?

 

[jdub]

And thin the top of the piston? Change it's shape? What about the indents for the valves?

You're opening up a can of worms Hommer.
Using a thicker MHG is easiest...changing the rod is more difficult, but if it's no extra cost, it's the way to do it.

 

[JimR]

.......
The pistons in a 7M protrude 20 thou from the deck at TDC
.......

IJ, do you mean that the pistons in a completely stock 7MGTE protrude from the block deck (what I originally thought you meant), or that the "deck" refers to the head deck, meaning 20 thou of space in-between the top sides of the piston and the head deck surface?

When I try to account for a piston protrusion from the block, it really messes up my compression ratio calculations!

Cheers,
Jim