5.0MX5

Question for the Ford guys, my 1992 Ford Thunderbird 5.0 with an Ebay King Cobra clutch and original harmonic balancer seems to be out of balance. It idles smooth and feels fine cruising, but when accelerating up to speed in normal driving, it almost feels like a miss. But that's not the case.

I originally wrote it off to the x-pipe, then to a misfire (not the case) now I'm looking elsewhere. One possibly important detail is this is an engine I put together with parts from a 90 Mustang GT and a 1992 Thunderbird 5.0. My understanding at the time is they were the same engine with the front trim being the only differance.

The 92 Thunderbird block is bored 60 (sonic testing showed this to be OK), has hypereutectic pistons and I did "not" mix the rods, I don't recall which crank I used, just the one with the best looking journals. And I didn't have it balanced since this is a street engine, my belief was factory balance would be fine.

Fact is I'm not Ford knowledgeable being a Mopar guy, having cut my teeth on LA engines and driving/racing 340's back in the day. This gap in instinctively knowing Windsor engine details causes me no end of doubt when problem solving.

I'm considering a fluid or jell damped harmonic balancer, but get very mixed opinions with net searches. Is it possible the 22 year old balancer is at fault? It looks fine.

Any and all input welcome.

Thanks, RR

.boB

The 302 changed from 28 to 50 oz in the early 80's. All Ford 5.0's had a 50oz balance after that. Those parts should interchange w/o any problems.

If you have an out of balance engine - by that much - the vibration will be mostly rpm dependent. It will also be worse at some rpm's.

If it's under a load, I would look at drive shaft, axles, etc.

If the vibration is speed dependent, then consider things like tires, wheel bearings, etc.

tbone heller

A worn out inner c/v joint will cause that type of vibration/shimmy on acceleration.

My dyno-tuner guys found a bad harmonic balancer on mine during its first dyno session. I replaced the stock unit with one from Ford racing.

5.0MX5

tbone, what exactly did your tuner notice on your damper.

It may just be a little automotive hypochondria. It's the kind of thing a passenger doesn't notice and as mentioned it's not apparent on steady cruise. The harmonic damper is what I'm hoping it is, the other most likely possibility is the new clutch assembly.

I will likely just drive it a while and see what happens. I may do a couple oil analysis and see if accelerated bearing wear is taking place.

tbone heller

These guys are really good at what they do:Injected Engineering: There's a Reason Our Customers Will Drive Eight Hours to See Us.. After a couple of pulls, Aric gave me a list of 3 things to address. Re-gap spark plugs, install an MSD box, & replace the balancer. I assume that he collected some type of data that indicated a concern.

.boB

Did you put in the three locating pins for the pressure plate?

5.0MX5

Now that's a heck of a question a year and half after I installed the clutch. But a good one. Fact is I don't recall, however I'm sure I didn't remove them and I didn't have the flywheel surfaced, just cleaned and scuffed with 80 grit. So, that said my belief is the three pins are in place.

.boB

If you're not sure, it's easy to check. You can look through the clutch arm hole and see the edge of the pressure plate. Just rotate the engine until you see that each required hole has a pin in it.

5.0MX5

.boB, that is one excellent idea. I will check..

Thank You!

MRM331

Your balance is most likely fine and both it and the flywheel should have a 50oz imbalance factor with the stock parts you are running. Your engine is out of balance because you changed out a major component of the rotating assembly (the pistons) and did not rebalance it. I did the same thing on my first build and had the same exact issue. Since that point I've ether purchased prebalanced rotating assemblies or have taken the crank (with Bob weights) to a TRUSTED machine shop for balancing. Even factory Ford balancing is inaccurate enough that it will be felt with the much stiffer mounts our builds use when compared to stock mustang mounts.

Good luck,
-Jason

tbone heller

Having the engine balanced makes a huge difference. When I replaced the factory 5.0 with a DSS short block, it was totally different.

5.0MX5

Thanks for the reply's. I have had small block Mopars balanced when building a new engine, as in new block, crank etc, or when making changes such as knife edging the crank. It does make for a nicer engine that'll live at higher rpm.

I've also rebuilt several low rpm street engines without a balance job with no issues. However, Guess if I build another for the Miata I'll have it balanced.