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Molnar BBC connecting rods in 300

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brandoncw
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Molnar BBC connecting rods in 300

Post #1 by brandoncw » Fri Sep 28, 2018 9:29 am

So i am aware that the Molnar connecting rods i bought have an off set that is incorrect for the 300. How do i alleviate this issue?
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Re: Molnar BBC connecting rods in 300

Post #2 by pmuller9 » Fri Sep 28, 2018 11:23 pm

brandoncw wrote:So i am aware that the Molnar connecting rods i bought have an off set that is incorrect for the 300. How do i alleviate this issue?

It is not an issue.
I have measured BBC rods anywhere from .030" to .060" offset. Not sure what the Molnar rods will be until you get them and measure.
They are offset to the side of the beam or to the front and back of the engine.
They operate that way in a BBC meaning there is no problem with stress on the rods.

The one concern I hear is that the piston in the BBC is also offset and somewhat centered over the small end of the rod but the piston in the 300 six is directly over the center of the rods big end and slightly offset over the small end which might allow the piston to rock sideways.

What they forget is that the width of the small end is wider than the big end for that reason and with a .990" pin there is no way the piston can rock sideways along the length of the piston pin.
Besides take a look at what .060" looks like compared to the diameter of the piston. 1.5%

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Re: Molnar BBC connecting rods in 300

Post #3 by brandoncw » Sat Sep 29, 2018 5:09 am

ok, that makes me much happier :lol:
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Re: Molnar BBC connecting rods in 300

Post #4 by pmuller9 » Sat Sep 29, 2018 9:33 am

Do you have the rods yet?
If so can you measure the offset so we know what it is for Molnar rods.

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Re: Molnar BBC connecting rods in 300

Post #5 by brandoncw » Sat Sep 29, 2018 12:23 pm

pmuller9 wrote:Do you have the rods yet?
If so can you measure the offset so we know what it is for Molnar rods.

I do, but I dont. The rods are seting back at home and I am still in Fort Benning training on the M2A3/M3A3 Bradleys. I will be here for another month or so till i can get back home for a while
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Re: Molnar BBC connecting rods in 300

Post #6 by Condor F1 » Thu Oct 04, 2018 3:05 pm

I just saw this post so I thought I would share what I did. It may not help you because you already bought the rods. When my motor builder looked for rods he came up with Molnar rods that fit a BMW 6 cylinder motor. They were the closest to the length of the stock 300 rods. I'm not sure if they were longer or shorter. I believe they had a two inch journal size on the big end and were narrower as well, so we had to weld up the crank to get the correct side play. Since I'm running a forged crank it was a little tricky doing the build up on the journals but in the end they got it right. As a result we ended up going with custom forged pistons to make sure everything worked.
So it is all put together now and is solid. The rods are also considerably lighter as well. As far as I know there was no offset issues with these rods. Together with the two inch journal size and lighter rods it should make motor scoot a little faster! If you want to switch rods I can get you the part numbers for the BMW rods.

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Re: Molnar BBC connecting rods in 300

Post #7 by pmuller9 » Thu Oct 04, 2018 3:09 pm

What do you do with a cast iron crank?

These rods are for a turbocharged application where a stout rod is needed along with a larger piston pin.
The longer rod is intended to decrease piston side loading.
The crank rod journal needs to be kept at 2.100" when looking at 600+ ft/lbs of torque.

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Re: Molnar BBC connecting rods in 300

Post #8 by brandoncw » Thu Oct 04, 2018 4:00 pm

pmuller9 wrote:What do you do with a cast iron crank?

These rods are for a turbocharged application where a stout rod is needed along with a larger piston pin.
The longer rod is intended to decrease piston side loading.
The crank rod journal needs to be kept at 2.100" when looking at 600+ ft/lbs of torque.


Sound like the rods would be good for a N/A high rpm build though. Only problem i see is just as you mentioned before is torsional vibration. Only way i see to reduce that is a balancer specifically designed to reduce torsional vibration such as fluidampr.

I have a question for you guys that drag these engines, how do you reduce torsional vibration? 240 or a reduced stroke 300 crank might help? A heavier flywheel?
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Re: Molnar BBC connecting rods in 300

Post #9 by drag-200stang » Fri Oct 05, 2018 10:22 am

Do not know V engines rod offset, but would modding the rod notch for the bearing tang on the rod, and siding the bearing over some help ??
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Re: Molnar BBC connecting rods in 300

Post #10 by CNC-Dude » Mon Oct 08, 2018 9:15 am

No, it's offset from the relationship of the width of the B/E relative to the centerline of the width of the S/E. You can just machine one side of the S/E so it doesn't rub the inside of the piston pin boss. Also, V rods only have adequate chamfering on only one side of the B/E while inline rods have them on both sides for bearing relief of the radius on the rod journal. It may be required to machine the other side of the B/E, you'll just have to mock up and verify prior to final assembly.
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Re: Molnar BBC connecting rods in 300

Post #11 by brandoncw » Mon Oct 08, 2018 10:11 am

CNC-Dude wrote:No, it's offset from the relationship of the width of the B/E relative to the centerline of the width of the S/E. You can just machine one side of the S/E so it doesn't rub the inside of the piston pin boss. Also, V rods only have adequate chamfering on only one side of the B/E while inline rods have them on both sides for bearing relief of the radius on the rod journal. It may be required to machine the other side of the B/E, you'll just have to mock up and verify prior to final assembly.

When i get home ill be able to get a better look at the rods and the rod journals. Ill probably have them machined but i dont think it will be that big of a deal to have it done
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Re: Molnar BBC connecting rods in 300

Post #12 by CNC-Dude » Mon Oct 08, 2018 10:42 am

It's not a big deal or that involved. I wrote a book discussing this exact process of installing big block V8 rods into another vintage inline engine, and can be done either in a mill or lathe, whichever you or the shop you decide to use has available to them.

Another issue to point out is the possibility of the connecting rods hitting the camshaft, especially if using thicker, beefier H-beam rods. When pondering such a swap, this and even the likelihood of the rods hitting the bottom of the cylinders and pan rail areas is also something that needs to be checked and confirmed.
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Re: Molnar BBC connecting rods in 300

Post #13 by brandoncw » Mon Oct 08, 2018 1:17 pm

CNC-Dude wrote:It's not a big deal or that involved. I wrote a book discussing this exact process of installing big block V8 rods into another vintage inline engine, and can be done either in a mill or lathe, whichever you or the shop you decide to use has available to them.

Another issue to point out is the possibility of the connecting rods hitting the camshaft, especially if using thicker, beefier H-beam rods. When pondering such a swap, this and even the likelihood of the rods hitting the bottom of the cylinders and pan rail areas is also something that needs to be checked and confirmed.

I did plan on installing the pistons, rods, cam, crank, oil pump, and everything then flip it oil pan side up and turn it over by hand and check all my clearances.

And due to the lack of a lathe and my inexperience using one, I'll definitely have a machine shop do it
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Re: Molnar BBC connecting rods in 300

Post #14 by pmuller9 » Mon Oct 08, 2018 1:31 pm

The rods fall between the cam lobes but are in line with the distributor gear and the cam eccentric for the fuel pump.
The other close clearance area is the bottom of the cylinders.
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Re: Molnar BBC connecting rods in 300

Post #15 by brandoncw » Mon Oct 08, 2018 2:17 pm

pmuller9 wrote:The rods fall between the cam lobes but are in line with the distributor gear and the cam eccentric for the oil pump.
The other close clearance area is the bottom of the cylinders.

Well, when the day comes, we shall find out if any issues arise with these rods. Shooting for no issues, but it is what it is. If it's the distributor gear or oil pump the rods get modded and if it's the bottom of the cylinder the bottom of the cylinder gets notched right?
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Re: Molnar BBC connecting rods in 300

Post #16 by CNC-Dude » Mon Oct 08, 2018 2:46 pm

The rods will actually hit the cam barrel in some engines, and some cam companies even have their cores made with notches in them for factory rod clearance, but still hit the cam with beefier H-beam rods. So it good to check all possibilities and don't assume they won't hit somewhere.
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Re: Molnar BBC connecting rods in 300

Post #17 by THE FRENCHTOWN FLYER » Mon Oct 08, 2018 5:39 pm

brandoncw wrote:
pmuller9 wrote:... If it's the distributor gear or oil pump the rods get modded ...

Don't grind on the rod I-beams.

I've never used the Molnar rods. But I have used custom aluminum rods (Superods, Bill Miller, Childs and Albert, etc.) with big cross sections. The places I needed to grind was the bottom of the cylinder bores and removal of the fuel pump lobe which was non-essential since I was using electric pumps. The distributor gear was never a problem for me.
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Re: Molnar BBC connecting rods in 300

Post #18 by pmuller9 » Mon Oct 08, 2018 7:04 pm

FTF
I'm glad you chimed in. Good Info.
It would also be nice if you didn't quote me on something I didn't write.
I'm sure it was just an oops. LOL
Thanks

brandoncw wrote:
pmuller9 wrote:... If it's the distributor gear or oil pump the rods get modded ...

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Re: Molnar BBC connecting rods in 300

Post #19 by brandoncw » Mon Oct 08, 2018 8:30 pm

THE FRENCHTOWN FLYER wrote:
I've never used the Molnar rods. But I have used custom aluminum rods (Superods, Bill Miller, Childs and Albert, etc.) with big cross sections. The places I needed to grind was the bottom of the cylinder bores and removal of the fuel pump lobe which was non-essential since I was using electric pumps. The distributor gear was never a problem for me.

Common sense tells me not to grind away the rods support anyways, kinda defeats the purpose of the "H" beam design lol. Only place I'd ever grind the rods is the very end of the small end and big end for balancing. And I'd assume it's ok to shave a little off the sides of the big end if it presents a clearance issue and that's the only good way to fix it? If the rods hit the cylinder walls I'd just grind out a smooth rounded notch that is just enough for the rods to clear by say an 1/8"?
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Re: Molnar BBC connecting rods in 300

Post #20 by THE FRENCHTOWN FLYER » Mon Oct 08, 2018 8:44 pm

pmuller,
You're right - it was an OOPS and I'm sure I couldn't replicate it if I had to. I'm sorry.

This post has me thinking, however. I wonder what the cost is of a Molnar rod if one has not got in-house capabilities to center up the rod and chamfer the big ends vs just ordering a custom rod the way one wants it?
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Re: Molnar BBC connecting rods in 300

Post #21 by pmuller9 » Mon Oct 08, 2018 9:01 pm

It will be the shoulder of the big end that would have clearance issues with the camshaft.
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We have had to grind a small amount off the shoulder of aluminum rods to clear big cam lobes on V8 engines.

I don't see you having an issue since the Molar rods have a small block size big end and isn't any wider at the shoulder than the stock rods.
However you said you will be checking clearances all around the crankcase and posting the results anyway which is good.

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Re: Molnar BBC connecting rods in 300

Post #22 by brandoncw » Mon Oct 08, 2018 9:23 pm

THE FRENCHTOWN FLYER wrote:pmuller,
You're right - it was an OOPS and I'm sure I couldn't replicate it if I had to. I'm sorry.

This post has me thinking, however. I wonder what the cost is of a Molnar rod if one has not got in-house capabilities to center up the rod and chamfer the big ends vs just ordering a custom rod the way one wants it?

True, well I already got em so oh well lol
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Re: Molnar BBC connecting rods in 300

Post #23 by pmuller9 » Mon Oct 08, 2018 10:38 pm

THE FRENCHTOWN FLYER wrote:pmuller,
You're right - it was an OOPS and I'm sure I couldn't replicate it if I had to. I'm sorry.

Not a problem. I've done some fancy editing myself and after signing off and realizing there was a problem, I hoped I could get back on in time to correct my mistake before it was noticed.

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Re: Molnar BBC connecting rods in 300

Post #24 by pmuller9 » Mon Oct 08, 2018 10:52 pm

brandoncw wrote:
THE FRENCHTOWN FLYER wrote:This post has me thinking, however. I wonder what the cost is of a Molnar rod if one has not got in-house capabilities to center up the rod and chamfer the big ends vs just ordering a custom rod the way one wants it?

True, well I already got em so oh well lol


Not even close!
I have spent much time on the phone with many rod makers and days at the PRI trade show and I have not found a custom rod less than $225 each.
The Molnar and Callies Compstar small journal BBC rods are by far the least expensive (at less than $80 each for eight) aftermarket rods available for the 300 six even with machine work if needed.

If you tell the custom piston supplier what the small end width is along with any small end offset, the small end of the rod will not need any work.
You may also find that the small bevel on the big end and the rod bearing thickness provides sufficient clearance for the small fillet on the crank rod journal.

FTF do you recall how much you paid for your last set of Oliver rods if you don't mind sharing that info?

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Re: Molnar BBC connecting rods in 300

Post #25 by THE FRENCHTOWN FLYER » Tue Oct 09, 2018 1:07 pm

I'll check my records to see what I paid for my rods and matching Diamond pistons, if I can find them - its been years. Once I purchase them its like "water under the bridge". I know I have at least seven racing seasons on the rods and got six seasons out of the pistons before boring the block another .005" to tighten things up again.
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Re: Molnar BBC connecting rods in 300

Post #26 by THE FRENCHTOWN FLYER » Tue Oct 09, 2018 4:58 pm

The rods were $1625 for eight @ 6.79" C-C and the pistons, pins, rings and pin clips were $851 for seven.
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Re: Molnar BBC connecting rods in 300

Post #27 by brandoncw » Wed Oct 10, 2018 8:58 am

THE FRENCHTOWN FLYER wrote:The rods were $1625 for eight @ 6.79" C-C and the pistons, pins, rings and pin clips were $851 for seven.


I suddenly feel better about how much i spent on my rods :lol:
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Re: Molnar BBC connecting rods in 300

Post #28 by CNC-Dude » Wed Oct 10, 2018 5:35 pm

Molnar makes a set of Chevy 292 6 cylinder H-Beam rods that are near the same specs as your rods Greg, they are only $620 for a set of 6 for the Chevy engine, so they may not be that terrible to have them make you a set for the Ford 6. They are correctly double chamfered on each side and not offset like a V8 rods is. They also make an H-beam rod for the Slant 6 guys for about the same price. They have been doing if for both of those brands for many years now, and I haven't heard any complaints from any racers that really push the envelope with them, 6-700 HP and beyond.
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Re: Molnar BBC connecting rods in 300

Post #29 by THE FRENCHTOWN FLYER » Wed Oct 10, 2018 5:59 pm

Thanks. I'll look into Molnar on my next big engine build. I vaguely recollect that my big end width needed to be 1.050" and Oliver could accommodate me which was one reason I tried them.
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