M rod vs 13MM (pictures)

[Hank Scorpio]

This should probably be showroom, but I figured it has performance value. M rod is on the left
http://www.pbase.com/image/24930120.jpg
http://www.pbase.com/image/24930121.jpg
http://www.pbase.com/image/24930122.jpg
http://www.pbase.com/image/24930124.jpg

You can see how much wider the M rod is, and something that I didn't realize is how much bigger the rod journals are on the 21/23's.

The width for the "beam" looks to be almost identical, except at the rod journal again, the M is alot thicker.

Oh, threw a 16v piston in for good measure too lol

[grebnut]

okay... can you clear this up for me since youre on the subject? the M rod is from the 84-? and the 13mm is from the 90+? (as you can tell, im guessing!) thanks...

ben

[Hank Scorpio]

B21/23FT (well F's to probably) m rods, b230s up to late 89 are 9mm rods 89+ b230s are 13mm rods.

[grebnut]

k... cool... thats what i thought. (another random question) how much boost can the 9mm rods hold assuming that i make sure my a/f ratio is very acceptable and im not bumping up ignition timing too far... wheres the "ragged edge" where id be riding the fine line risking blowing a hole in my block? thanks for any input on that! later,

ben

[The Aspirator]

You can see how much wider the M rod is, and something that I didn't realize is how much bigger the rod journals are on the 21/23's.

The width for the "beam" looks to be almost identical, except at the rod journal again, the M is alot thicker.

Are you backwards on this or am I just confused? :???: I thought the M rod was narrower and thinner???

John

[Hank Scorpio]

M rods on the left :wink:

[texasvolvotech]

just a not m rods came in all -84 b21 b23 f and ft. Just fyi on the m rods, they too have there weaknesses..even when polished to releive stress risers and shotpeened for the same then will begin to compress at 20+psi with 9:1 compression...notice i said compress and not stretch, they get shorter.

[the poi]

just a not m rods came in all -84 b21 b23 f and ft. Just fyi on the m rods, they too have there weaknesses..even when polished to releive stress risers and shotpeened for the same then will begin to compress at 20+psi with 9:1 compression...notice i said compress and not stretch, they get shorter.hmm, intersting...do you know if that happend any worse or better on the 13mm rods?

[texasvolvotech]

to tell you the truth i dont know if the 13mm rods will compress as the m rods do. Both are about equal strength, however the metal composition is different. The m rods are a slight bit softer, however more dense. The 13mm rods are less dense to be lighter, but were made of a more rigid steel/iron to increase strength. It would be my guess they will have approx the same limit however will fail differntly.

-Jason

[Anonymous]

Thanks for the pics. Another small step in clearing up one of Volvo life's little mysteries! The rod with the larger journal diameter (B21/23?) has a wide big end - probably to give nominal end float (8thou?) on the crank journal. The smaller dia rod with narrower big end width is probably the one that "floats" 5mm on crank journal (25mm rod on 30 wide journal).
Dick Prince Ovlov.net

[The Aspirator]

M rods on the left :wink:

Ohhhhhhh, ok I get it now. I didn't realize there were three types of rods, 9mm, 13mm, and forged right? I thought by M you meant the 9mm ones :oops: .... The forged ones have that blue-ish tint to them right? Like that pic of the two cranks together, one forged one not, and it has them labeled.

Hummm, so my '82 B21ft parts car should have forged rods and pistons right? But it supposedly threw a rod, not that I really know what that means...... I guess I've got some searching to do.

John

[nothing but turbo]

Oh, threw a 16v piston in for good measure too lol

you don't need to twist that knife, ya know.... :-D

btw: are any of these the rods the ones you're using in the 16v?

[Boosted2003]

Hey as for the 13mm rods the section near crank still looks bigger...


BUt hey doug will you sell me one 16 valve piston? :x:

[Anonymous]

I didn't realize there were three types of rods, 9mm, 13mm, and forged right? I thought by M you meant the 9mm ones :oops: .... The forged ones have that blue-ish tint to them right? Like that pic of the two cranks together, one forged one not, and it has them labeled.

Hummm, so my '82 B21ft parts car should have forged rods and pistons right? But it supposedly threw a rod, not that I really know what that means...... I guess I've got some searching to do.

John - All the redblock rods are forged. Go back and look at the pictures - see the wide parting line along the beam? That means "forged". And your B21FT doesn't have forged pistons, it has reinforced cast. "Threw a Rod" means that she's broke.

[thelostartof]

what i really find interesting is how small/short the pistons are ... almost no skirts on them .. what year motor are those 13mm pistons from?? i've love to see those vs some 94+ b230ft pistons since those are said to have longer skirts to stop slap and what not

[bitjockey]

Mike, the part number for the 94+ pistons is the same... I've never been able to figure that one out.

:e-shrug:

[Hank Scorpio]

Boosted, the M rod is bigger in every comparison. I just used my tape measure (ya, I didn't feel like using a dial caliper) but it is defiently bigger.

Keep in mind, I've known people running 30psi on Mrods for a number of years with out issue (though undoubtedly they were shot peened) so they can't be bad at all.

Justins motor is using the 13mm rods, till we blow it up :wink:. Actually the minute his motor goes in, were taking his 91 block and dropping the stroker crank, forged rods and forged pistons in over the next 6-8 months.

[thelostartof]

Mike, the part number for the 94+ pistons is the same... I've never been able to figure that one out.

:e-shrug:

really? i've checked a few online parts dealers and they have the same part # listed too ... so ... yet i hear rumors about volvo doing that(longer skirts) to quiet/stop the piston slap in those motors ... only way one can know is to pull the pistons out of that motor ... if my motor was running i'd yoink them out of my 94 b230ft and find out

[linuxman51]

Boosted, the M rod is bigger in every comparison. I just used my tape measure (ya, I didn't feel like using a dial caliper) but it is defiently bigger.

Keep in mind, I've known people running 30psi on Mrods for a number of years with out issue (though undoubtedly they were shot peened) so they can't be bad at all.

Justins motor is using the 13mm rods, till we blow it up :wink:. Actually the minute his motor goes in, were taking his 91 block and dropping the stroker crank, forged rods and forged pistons in over the next 6-8 months.


oil squirters... dont forget those

[own6volvos]

I think the stock replacement pistons are all the same on every motor, but the ones from the factory was different. I read in one of the volvo info sites that 92 year stamped motors will have the longer skirt pistons and have less of a chance to start pistons slap. You might have to go to a aftermarket piston supplier to get the longer skirt if you are doing a rebuild though. the BB FAQ mentioned that you had to ask for the longer skirt ones in rebuilds, since they normally stick in the other style.

[Boosted2003]

Boosted, the M rod is bigger in every comparison. I just used my tape measure (ya, I didn't feel like using a dial caliper) but it is defiently bigger.


maybe it the darker look from the camera flash or something.

BUt waht about you letting my get a b234 piston? :-D

[Hank Scorpio]

no.

[Boosted2003]

here sorta view i was looking at seeing it being smaller.. Maybe its just the light but dont know

http://members.cox.net/paparazzifilm/rods.JPG

[Hank Scorpio]

Your right, it is there. Now, is that because the rod journal is smaller on the 13mm?

Hmm... I wonder.

[Captain Bondo]

This has to be one of the most confusing and frustrating threads ever. :rofl: :rant:


As far as the section near the crank, the 13mm has a bit more web there due to the smaller bore, but if you'd look at all the pics you'd realize the m rod is so much "thicker" it's pretty redundant. Look at the 2nd pic from the bottom- obviously no comparison. Just like I was ranting about in the intercooler thread- look at ALL of the data. If you only look at parts you can interpret it however you want, which makes any data redundant.

Also not all b21/b23 came with m-rods. They started in 79 I believe. The compression/stretch comments are also dependent on many other factors than boost pressure/static compression ratio (ie ebr and redline).

[mikep]

You could measure it there, but the main weakness is the middle of the beam on most rods.
Also, if it is narrower along that dimension, it is wider there, so the area might be similar.

[dirtbike]

Another thing to consider with larger big end journals is that there is a lot more surface friction, which result's in more heat so be sure not to have an oiling issues or they could become super hot super quick.
However I haven't heard of this causing problems for anyone.
Laters
Nick

[Boosted2003]

me either.. But i just thought it was beefer on that section of the rod over the m rod. but all in all that section doenst mean **** as much as the main body of the rod is

[Captain Bondo]

"Another thing to consider with larger big end journals is that there is a lot more surface friction,"

Think that through... pressure on the bearing surface is in psi. With the larger surface, the forces on the big end are spread over more area, so the force per area is less, so in width these things actually canel each other out. The increase due to diameter is pretty minor relatively speaking. B21/b23 actually tend to cope BETTER with low oil pressure... any performance engine will not be happy with sub-par oiling.

[Anonymous]

Friction in this case is not based on pressure - if it was bearing sliding on crankshaft, pressure between them would be the issue. In this case it's shear friction of the oil - with a larger bearing, there is more surface area, acting like a bigger brake drum. Also, the larger bearing surface moves faster relative to the rod at a given RPM, and the shear force goes up with speed differential.

A larger bearing also needs more oil volume, for obvious reasons. Also, I'd debate the "doing better at low oil pressures", as there have been a few B21/B23 spun bearings in my little world, but I have no proof that low oil pressure caused them - probably too little flow, but it boils down to the same thing, really.

[mikep]

The reason for a smaller journal is to reduce the surface speed. It works, but you trade longevity of the bearings. The more mass any part such as a bearing shell has, the more heat it can take. That is why the older engines run longer with low oil.

Also, the wider rods with tight side clearance give higher pressure at the bearing. Loose side clearance on rods causes more oil slinging, increasing windage.

[Boosted2003]

this should almost become archived

[dirtbike]

Well done Matt and Mike, I Couldnt have said it better myself.
But as I also said I haven't heard of this being a common problem on the early engine's.
Laters
Nick

[Captain Bondo]

"Friction in this case is not based on pressure - if it was bearing sliding on crankshaft, pressure between them would be the issue."


In normal operating it would be, but in a case of significant underoiling seems like the model would more reflect the latter. :???: The more underoiled, the closer things get to being metal to metal and the more and more additional surface area and mass becomes a help I thought, thta was all I was saying. It is indeed lower friction in normal circumstances, but not necessarily when underoiled, in regards to the comment that "they could become super hot super quick" which I assumed would indicate a condition where metal-to-metal (or very nearly) is happening due to underoiling in which case the model sorta changes.

Otherwise ya, volvo themselves always touted the b230 as the " new, low friction design". Sorry for the confusion.

[Anonymous]

cool.

[iadr]

Can someone measure and describe this 30mm wide journal on the b230 crank?

What I'm curious on: is it exactly 30mm? That's ~ 1.18"- I thought it narrower.
Are the sides of the rod journals finished 'normally' from factory ie, if you found a rod that filled the full width, as in Richard's race car, does the crank need special finishing work for things to be happy?

[Anonymous]

Even tho the stock B230 rods have 5mm float on the 30mm wide crank journals, the crank journal sides are finished machined, with a nice little undercut root radius. My custom rods are set for 8 thou side clearance.
Dick Prince