3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

[Ian G.]

Hi,

Can someone in the know tell me if the #3830820 340 409 balancer will
work to replace the #3764318 270/290 HP harmonic Balancer? As I'm sure most everyone knows, there is no accurate reproduction of the #3764318. However, the center ring of the #3830820 looks correct to me. The outer ring has holes drilled around the perimeter, but overall looks very close.

From what I have read they have a similar diameter (6 1/4" vs 6 1/16") and width (5 1/2" vs 5"). (my inference from other threads)
Apparently, however 3830820 was never used on a corvette -- maybe because by then "high horsepower" was now "low horsepower" when it was used in the mid 60's. So will it fit? Will it time correctly and balance out OK?
The price at $93 on damperdudes.net is right compared to $750 for a rebuilt original (if you can find it).

Pics
3764318
IMG_0645.JPG

3830820
409balancer340hpdiameter1.jpgCH409-1.jpg

[Cecil L.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

AFAIK same hub, different ring.

[Michael H.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

The damper used on a 409 engine would probably be "tuned" differently than one designed for a small block.
I suppose it would work but it may not function exactly the way it should.

[Donald H.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Hi,

Can someone in the know tell me if the #3830820 340 409 balancer will
work to replace the #3764318 270/290 HP harmonic Balancer? As I'm sure most everyone knows, there is no accurate reproduction of the #3764318. However, the center ring of the #3830820 looks correct to me. The outer ring has holes drilled around the perimeter, but overall looks very close.

From what I have read they have a similar diameter (6 1/4" vs 6 1/16") and width (5 1/2" vs 5"). (my inference from other threads)
Apparently, however 3830820 was never used on a corvette -- maybe because by then "high horsepower" was now "low horsepower" when it was used in the mid 60's. So will it fit? Will it time correctly and balance out OK?
The price at $93 on damperdudes.net is right compared to $750 for a rebuilt original (if you can find it).

Pics
3764318
[ATTACH]30883[/ATTACH]

3830820
[ATTACH]30884[/ATTACH][ATTACH]30885[/ATTACH]

I am in the same boat as you. I am restoring a 60 270HP that had the low HP harmonic balancer on it. I am very confused and hope someone can shed some light for your thread. I had seen recommendations that current GM replacement is 3896904 which is available for about $80 from GM. Of course, as you know it looks nothing like the original correct one. Corvette Central, Paragon, and Zip all sell one and theirs all look exactly alike. Theirs has a step on the back side like the original and runs $230+.

I know they resell parts they get from other vendors, but I do not believe their balancer is the GM product and wonder what other difference there is to make it that much more expensive? I have not found that they mark stuff up that much (i.e. from $80 up to $230)

[Duke W.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Even if it physically fits, it likely doesn't have proper dynamic properties due to the differences in the torsional characterisics of the 409 and 283 crankshafts.

Whether this will create problems down the road, I can't say.

Torsional dampers are designed specifically for each application based on crankshaft torsional vibration behavior and RPM range.

Cast cranks generally have better inherent damping characteristics than steel cranks.

The high horsepower and low horsepower 327s had essentially the same crankshaft (The Tufftride surface treatment didn't have any effect on torsional behavior), so the larger damper used on SHF/FI engines must have been installed to damp out a torsional mode above 5500 RPM.

I think all 283 cranks are steel. If this is correct, the early small bearing 302 damper should be functionally correct, since they are basically the same 3" stroke, small bearing steel crankshaft as the 283.

Duke

[Michael H.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Even if it physically fits, it likely doesn't have proper dynamic properties due to the differences in the torsional characterisics of the 409 and 283 crankshafts.


Duke

That's what I said, only I used smaller words.

[Joe L.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Even if it physically fits, it likely doesn't have proper dynamic properties due to the differences in the torsional characterisics of the 409 and 283 crankshafts.

Whether this will create problems down the road, I can't say.

Torsional dampers are designed specifically for each application based on crankshaft torsional vibration behavior and RPM range.

Cast cranks generally have better inherent damping characteristics than steel cranks.

The high horsepower and low horsepower 327s had essentially the same crankshaft (The Tufftride surface treatment didn't have any effect on torsional behavior), so the larger damper used on SHF/FI engines must have been installed to damp out a torsional mode above 5500 RPM.

I think all 283 cranks are steel. If this is correct, the early small bearing 302 damper should be functionally correct, since they are basically the same 3" stroke, small bearing steel crankshaft as the 283.

Duke

Duke------


Most, if not all, 283 small journal cranks are steel. However, large journal 283 cranks are nodular cast iron.

[Duke W.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

...large journal 283???

I thought the 283's last year was 1967, and all SBs except the 350 were small journal.

Then, the 1968 SB lineup was all large journal in displacements of 302, 307, 327, and 350 CID. (The 307 was a 283 bore block with a 327 crank - 3.875" x 3.250".)

The above leaves no room for a large journal 283.

Duke

[Joe L.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

...large journal 283???

I thought the 283's last year was 1967, and all SBs except the 350 were small journal.

Then, the 1968 SB lineup was all large journal in displacements of 302, 307, 327, and 350 CID. (The 307 was a 283 bore block with a 327 crank - 3.875" x 3.250".)

The above leaves no room for a large journal 283.

Duke

Duke-----


Yes, that's correct; I mis-spoke. Only the 327 was manufactured in both small (1962-67) and large (1968-69) journal configurations.

[Ian G.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Donald, I believe it is manufactured by Goat hill classics. Everything I have read indicates you should not buy from him except in person however.

Duke, so based on what you are saying it should work OK correct?
Damperdudes.net advised it is not interference fit however, so I'd need to drill the crank for a harmonic balancer bolt, which is recommended anyway right?

Just as an FYI they told me there would be an additional $150 core charge.

So I'm looking at:

$750 for a 3764318
$343 for a 3830820 (not quite correct looking)
$250 for an incorrect goat hill reproduction
$45 for a new current GM replacement 3896904
hmmm

[Loren L.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

#3764318 is also shown in the 1961 parts book for 59-61 348s with 3 x 2 bbl carbs.

[Duke W.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Duke, so based on what you are saying it should work OK correct?
Damperdudes.net advised it is not interference fit however, so I'd need to drill the crank for a harmonic balancer bolt, which is recommended anyway right?

Just as an FYI they told me there would be an additional $150 core charge.

So I'm looking at:

$750 for a 3764318
$343 for a 3830820 (not quite correct looking)
$250 for an incorrect goat hill reproduction
$45 for a new current GM replacement 3896904
hmmm

No, the fact of the matter is that I don't know and have no way to determine.

I notice that my 1977 P&A catalog shows the 3896904 as the replacement for all 283s and 250/300 HP 327s through '65. Then the 3896903 is listed for '66-67 300 HP. (I wonder what the difference is! Joe?)

I assume you're looking for minimum Flight judging deduction, but given my concern for engineering issues, I would probably go with the ...904 and take the points hit for an OE replacement.

Duke

[Ian G.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Thanks for your advice Duke. I don't want to ruin my engine for one point.

I wonder if I could take the hub from a 3830820 and the ring from another balancer that looks similar from the side and MAKE a replica 3764318 with the same rotational properties? I'll ask damperdudes.net and damperdoctor.com.

Also, does anyone have a scan of the blueprint for the 3764318?

[Joe L.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Donald, I believe it is manufactured by Goat hill classics. Everything I have read indicates you should not buy from him except in person however.

Duke, so based on what you are saying it should work OK correct?
Damperdudes.net advised it is not interference fit however, so I'd need to drill the crank for a harmonic balancer bolt, which is recommended anyway right?

Just as an FYI they told me there would be an additional $150 core charge.

So I'm looking at:

$750 for a 3764318
$343 for a 3830820 (not quite correct looking)
$250 for an incorrect goat hill reproduction
$45 for a new current GM replacement 3896904
hmmm

Ian-----


You waited too long. The GM #3896904 is discontinued. There may still be some around, though.

[Ian G.]

Re: 3830820 340 409 vs 3764318 270/290 HP harmonic Balancer

Looks to me like the one on there is a 904 right now. I'll leave it and see if I can find a holy grail 318.

Thanks for the heads up Joe!