The Ford Barn

293131 · 10-13-2014, 03:07 PM

Hello,

I am posting this for my dad. He is not a mechanic nor am I. He has a '31 Cabriolet 68c and his engine has what he believes is a harmonic vibration between a narrow RPM range. At idle and other ranges, it sounds perfect.

While at Hershey, he was talking to someone who asked him if the crank hole is perfectly lined up with the engine or does one have to fish for the hole. My dad does not believe that it is lined up. The person he spoke to said that that was most likely his problem and the cause of the vibration. I don't think he has stock mounts on the engine...the float a motor mounts.? Are there any big secrets to get the engine lined up or is it pretty self explanatory? Anything he should be looking for or having his mechanic look for?

Thank you for your help.

tbirdtbird · 10-13-2014, 04:32 PM

It is very unlikely that the misalignment you describe is the source of the problem. Many As have this problem.

Almost every A has a harmonic at some speed range, often somewhere around 45 mph.

I would disregard what the Hershey guy said (he may have been in the hot sun too long) and have an experienced Model A guy drive your car and advise you. You may not have to do anything

Gary Gregory · 10-13-2014, 06:42 PM

I have float-a-motors on the back and orig style mount in front...if they are to tight it will vibrate a lot more than normal. Just went through this problem with my new engine....id check the mounts..you nothing to lose.

tbirdtbird · 10-13-2014, 06:52 PM

Interesting observation. Tightness is diff. than alignment of course.

What is the correct tightness for the float-a-motors

Mitch//pa · 10-13-2014, 07:02 PM

if the vertical alignment of the crank hole is off i have seen the FAM rear mounting brackets installed upside down. it did not cause a vibration on the car we fixed.

Gary Gregory · 10-13-2014, 07:31 PM

Sponsored Links (Register now to hide all advertisements)

The tightness of the front mount can affect the hole ..at least with a few of those older after-market front mounts like the one that was on my Coupe when i got it. I have a orig mount now. I have no idea what the correct tightness should be....i backed mine off to where the lock nut just starts to get snug. The front just snug enough to where i could get the cotter key in. I do know these adjustments help a great deal.

293131 · 10-21-2014, 07:04 PM

Thanks for all of the responses, everyone.

10-13-2014, 03:07 PM	#1
293131 Junior Member Join Date: Oct 2012 Posts: 24	Harmonic vibration - engine placement? Hello, I am posting this for my dad. He is not a mechanic nor am I. He has a '31 Cabriolet 68c and his engine has what he believes is a harmonic vibration between a narrow RPM range. At idle and other ranges, it sounds perfect. While at Hershey, he was talking to someone who asked him if the crank hole is perfectly lined up with the engine or does one have to fish for the hole. My dad does not believe that it is lined up. The person he spoke to said that that was most likely his problem and the cause of the vibration. I don't think he has stock mounts on the engine...the float a motor mounts.? Are there any big secrets to get the engine lined up or is it pretty self explanatory? Anything he should be looking for or having his mechanic look for? Thank you for your help.

10-13-2014, 04:32 PM	#2
tbirdtbird BANNED Join Date: May 2012 Location: inside your RAM Posts: 3,134	Re: Harmonic vibration - engine placement? It is very unlikely that the misalignment you describe is the source of the problem. Many As have this problem. Almost every A has a harmonic at some speed range, often somewhere around 45 mph. I would disregard what the Hershey guy said (he may have been in the hot sun too long) and have an experienced Model A guy drive your car and advise you. You may not have to do anything __________________ '31 180A

10-13-2014, 06:42 PM	#3
Gary Gregory Senior Member Join Date: Jun 2010 Location: Maryville,Tn Posts: 302	Re: Harmonic vibration - engine placement? I have float-a-motors on the back and orig style mount in front...if they are to tight it will vibrate a lot more than normal. Just went through this problem with my new engine....id check the mounts..you nothing to lose.

10-13-2014, 06:52 PM	#4
tbirdtbird BANNED Join Date: May 2012 Location: inside your RAM Posts: 3,134	Re: Harmonic vibration - engine placement? Interesting observation. Tightness is diff. than alignment of course. What is the correct tightness for the float-a-motors __________________ '31 180A

10-13-2014, 07:02 PM	#5
Mitch//pa BANNED Join Date: May 2010 Location: Bucks County, PA Posts: 11,454	Re: Harmonic vibration - engine placement? if the vertical alignment of the crank hole is off i have seen the FAM rear mounting brackets installed upside down. it did not cause a vibration on the car we fixed.

10-13-2014, 07:31 PM	#6
Gary Gregory Senior Member Join Date: Jun 2010 Location: Maryville,Tn Posts: 302	Re: Harmonic vibration - engine placement? Sponsored Links (Register now to hide all advertisements) The tightness of the front mount can affect the hole ..at least with a few of those older after-market front mounts like the one that was on my Coupe when i got it. I have a orig mount now. I have no idea what the correct tightness should be....i backed mine off to where the lock nut just starts to get snug. The front just snug enough to where i could get the cotter key in. I do know these adjustments help a great deal.

10-21-2014, 07:04 PM	#7
293131 Junior Member Join Date: Oct 2012 Posts: 24	Re: Harmonic vibration - engine placement? Thanks for all of the responses, everyone.