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Ron, ASE Certified Technician
Category: Ford
Satisfied Customers: 23821
Experience:  23 years with Ford specializing in drivability and electrical and AC. Ford certs and ASE Certs
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I have a mustang 68 with 302 stock block and trickflow

Customer Question

I have a mustang 68 with 302 stock block and trickflow twisted wedge heads
Mutha thumps comp cam
However i get a compression of 110 on all four cylinders at drivers side and 90 on passengers
I ran the test coz oil keep leaking from the breather, not grommet
Submitted: 1 year ago.
Category: Ford
Expert:  Gary replied 1 year ago.

Hi there,

I'd say you have high blow-by, with your uneven compression readings.

The readings are low to excessively low for a camshaft like that.

Does it still go ok?

Maybe you should look at a better crankcase breather system.


Customer: replied 1 year ago.
It drives ok but won't rev over 4000
Expert:  Gary replied 1 year ago.

I'd say a 302 with heads/cam like you have should hp peak at 6500 rpm easy.

So is it now time for an engine refresh?

What happens at 4000 rpm, misfiring? Valve bounce?


Customer: replied 1 year ago.
I was trying to eliminate everything else before i do that.
It doesn't misfire but it feels restricted like the bolts are going to fly off if I don't shift
Expert:  Gary replied 1 year ago.

Yes, I know the feeling.

I've had a 308 that was gutless to 4500 rpm, I've had another 308 that went a scalded cat to 7200 rpm.

One was near stock, but high mileage AND high blowby.

Compression pressure also stops the pistons wanting to "fly out the top", if that makes sense.

I would sit back and decide on a plan of refresh.


Customer: replied 1 year ago.
Does oil type play a role here.
Im running valvoline 20 50
Expert:  Gary replied 1 year ago.

No, not generally.

Pennzoil makes a 20w50 marketed as "street machine" oil.

Do you have the cam specs handy?

What others parts went into the last build? Rockers, pushrods, lifters, cam drive, head studs, bearings etc?

You may find the springs are getting soft/lazy.


Customer: replied 1 year ago.
lifters, rockers, pushrods and all head components are new
below please find the link to cam specs
Expert:  Gary replied 1 year ago.

Hi again,

I see the cam is a fairly wild hydraulic flat tappet for a street car.

How many miles on the bottom end?

I would be performing a leakdown test to determine where the losses are occurring.

My bet is past the rings.

How often, did you need to adjust the lifter preload? Did the lifters ever become noisy and need readjusting?

I have one of my cars in the same predicament as you. I have to bite the bullet for a short motor, because of low compression on one cylinder and high blow by. No amount of tuning will solve my problem or your problem.

Can we do a leakdown test on your engine?


Customer: replied 1 year ago.
I will do a leak down test tomorrow but there is another issue since my oil dipstick is after market, would too much oil be the cause my dilemma or contribute somehow.
How do i know in my case the required oil level, should i just count the quarts as i fill it up the adjust the dipstick, a 302 would drink 5qts if im not mistaken
Expert:  Gary replied 1 year ago.

The oil level with an aftermarket dipstick will have a SIGNIFICANT impact on engine performance if incorrect either way.

How much did you use at last oil change?

If the stock sump is still used, drain the oil, and add the correct volume (say 5 quarts), run the engine, wait 10 minutes, then see where it is on the dipstick.

If the crankshaft was swimming in oil, there is your crankcase oil breather issue or part there of, AND your rpm/horsepower loss.

And then we'll see where we go next.


Expert:  Gary replied 1 year ago.

Hi again,

Have you had any further success towards an answer to your problem?


Customer: replied 1 year ago.
Hi Gary,I pulled the engine apart and it looks like my piston rings are wore down. on about half of the cylinders i found that oil have escaped to the top. Pistons are solid and cylinder sleeves are clean. Right now I'm waiting on summit racing for a refresh kit.However, some pistons had no oil leak yet they still read low compression! any thoughts on that?
Expert:  Gary replied 1 year ago.

Hi again,

Possible reasons for compression loss past the rings include if the ring gaps are larger than max spec, the gaps are lined up, one or more rings are stuck in the piston from flattened top ring land, worn ring lands from ring flutter.

Generally any combination of excessive clearance or no ring tension on the bore gives compression loss.

Now, it sounds like you'll do what I did.

A very quick pass on a honing machine, careful cleanup of pistons, etc.

If a ring doesn't rotate easily, it is also binding in the ring groove.

Hopefully the short motor rebuild will make it a rocket again.

I would still the heads inspected while they are off.


Customer: replied 1 year ago.
will keep you posted, thanks tons
Expert:  Gary replied 1 year ago.

Will be a good time to get the compression up, to into the mid 10s if decent fuel is available.

The cam you have is fairly wild for a 302, so 10.4-10.7 or higher if you dare.

And a decent ignition, preferably not MSD.

I like ICE. (Ignition)


Customer: replied 1 year ago.
I'm currently using petronix ignition, you still recommend changing it?
what will be the easiest way to increase compression?
i went with that cam for the fun at idle :)
Expert:  Gary replied 1 year ago.

Pertronix is good.

You need to do the calculation from all the "volumes".

Swept volume: bore and stroke

Head gasket volume, Cylinder head volume, piston dish or dome volume.

I'll work it for you if you can supply your piston and head volume.

While the engine is apart, there are many ways to increase compression, generally mill cylinder heads.


Customer: replied 1 year ago.
Hello GarySo heads are 58cc and piston is a dish 5ccRegards
Expert:  Gary replied 1 year ago.

Ok, with a crushed head gasket of .030, your current compression ratio is 9.96 (near enough 10:1).

With a flat top piston, you would end up at 10.66:1 which might be a little high for your fuel.

If you mill the heads down to 56cc, you'll end up at 10.23 (say 10.25:1).

I need to know what crushed thickness head gasket you are using, I have assumed .030 in the above maths.


Customer: replied 1 year ago.
Gasket is 0.047, however comp cams recommend 9.5:1 ratio on this cam. Any idea why so low?
Expert:  Gary replied 1 year ago.

Hi again,

Comp recommends "needs 9.5:1" meaning at least 9.5.

Their wording should be minimum.

Your .047 works out to 9.51:1.

I would run as high as your current pump fuel will allow safely.

Early 10s should be ok.

Recheck piston to valve clearance as a mockup assembly, lightweight checking springs etc.


Customer: replied 1 year ago.
Hi gary
So i put everything back together and the engine fire up perfect, except for a leak at the back of the intake manifold. I had to pull the intake out and fix the gasket properly. However, when i put it back I can't get the distributor to align to it's timing. It fitted perfectly after assembling the engine but now it won't go back in!
Any thoughts on what happened?
Customer: replied 1 year ago.
Piston is at BTDC but it won't align
Expert:  Gary replied 1 year ago.

Hi Moe,

I would put the engine at say 15 BTDC, drop the distributor in until it picks up the cam gear, then rock the engine forward, and partly back until the oil pump drive realigns, at that point the distributor should just drop in.

Get the engine back to your timing point, check your static position, fit timing light, fire it up, adjust timing at idle etc.


Customer: replied 1 year ago.
Perfect, got it to fit but now there is an issue with the timing i guess, carb spits the fuel out or flame when starting,
Expert:  Gary replied 1 year ago.

Hi Moe,

Now you know how to do it, it's a case of having the engine in the right place when you pick up the cam gear.

Noting where the rotor button and the distributor reluctor wheel are in relation to the balancer is the trick.