my V8 vibrates

[Deleted]

I am a new member hoping to learn lots about driving and owning my P5B coupe. I bought it last October and so far so good except for a vibration which I think is slowly getting worse. My non specialist garage have checked it and are pretty sure it is not drive/prop shaft related. In fact he is fairly certain it is the engine, which, incidentally, is possibly from a P6.
The vibration is worst between 1500 and 2000 revs. While slowing on a down hill gradient it becomes even more noticeable as the engine slows.
Above and below these rev limits the vibration could go un-noticed but at its worst it is bad enough to shake the steering column.
The last bit of info I can add is that the vibration has 'beats' as if there are 2 vibrations of slightly different frequency.
I am not ready to start working on the car myself but will be taking it to a 'classic car mechanic'. I would prefer to give him somewhere to start.
Any suggestions of what to investigate next would be gratefully received.

[Phil Nottingham]

Prop shaft joints often do cause vibration and the only way to test them properly is to disconnect each end and test for the slightest of stifness and/or play. It could also be out balance.

Is the engine in tune - this can cause uneven running as can head gasket trouble

Another check should be with the engine mounts and exhaust

The torque converter can be out of balnace or even loose or perhaps the flex plate is cracked

[Deleted]

Thanks for the suggestions on both.

[jlalexander]

I have been servicing & rebuilding V8 for nearly 20 years. After eliminating all the basics (i.e.engine mounts, flexible drive plate, etc), your problem reminds me of a vibrating problem I came across several years ago, the vibrations were caused by the crank mains bearings that were down to red metal. The oil pressure was quite admisible. Checking the condition of the bearings can be done quite easily b y removing the sump.

[Smallfry]

Another one to look for is the matching piston/conrod combinations. We had one a few years ago that was rebuilt by a so called v8 expert, and it turned out that the engine had six 10.5 compression pistons and two 9.35 ones from an SD1 with their SD1 conrods, which are a slightly different weight. Little wonder it it didn't run smoothly.

Do you feel its engine related, or more to do with the running gear ?

[Deleted]

Thanks for the suggestions. I'll action each in order of expense....cheapest 1st.
Is the forum the best way to find the spares I need?