Don Terrill’s Speed-Talk

I have a 406 cui sbc
this season i had about 35-40% on leakdowntest !!on all cylinders
i found out that i have got wrong rings in the pack i bougth, the number on the box was rigth but rings inside was to a smaler cylinder bore , my cylinder is 4.125 ive got for 4.120 ( file fit rings)

the engine had alot of crancase smoke so i did a leakdown test and found 35-40% leakdown on all cylinders

the car went 5.68 and 122 mph

i have new rings in the engine now and on the engine stand i have 3-4% leakdown with the new rings

i will have the engine dyno tested and breaking in the rings on the dyno

i now that with full trotle we cant now how mutch leakdown there is but do you think i will se some improvement with the new rings

thanks

The new rings will help with the blow by. Don't know if it will run any faster.

.005 is not going to make a difference. and I doubt it was your leakage problem.

I think if you have a 4.130 to 4.125 its dosnt matter but if you have 4,120 with file fit rings in a 4.125 bore it matters
the rings are file fit for 4.120 thats going to be wrong tension in the rings and with file fit its almost for a 4.115 bore


the block is new dart block with the right honing and bore

new brodix 18C heads with new titanium valves no leek in the valves or heads when i took the leakdown test i heard the air in the crankase

with the new rings installd i dont have any leakdown in the crankase and and only leaking 3-4%

maby im wrong here

engine have about 25 1/8 passes

cylinders all looking fine so no scratches or scuffing . pistons looks good with good ringlands and no scuffing

is there sumthing more a can look at ?

thanks
(from sweden so sorry for my writing ,spelling)

hjallle123 wrote: ↑Fri Oct 20, 2017 7:11 pm I think if you have a 4.130 to 4.125 its dosnt matter but if you have 4,120 with file fit rings in a 4.125 bore it matters
the rings are file fit for 4.120 thats going to be wrong tension in the rings and with file fit its almost for a 4.115 bore

Nope.

A few years ago I had a real eye opener with regards to ring seal tested with a leakdown tester & it's relevance to the performance of a running engine.

I had a dirt late model speedway customer with a beat to hell rodeck 350+ block, he had another 350+ block that was near new but had a cracked main, didn't have the $ to do a proper build on that one.
To get him on the track for a season I slapped the rough short motor together, that thing was bad, real bad - .007-.010" piton to bore clearance, up to .003" out of round, heaps of taper, scores, low spots etc. It was so bad I couldn't even think about honing it 'properly' - piston to bore would have been .010-.013" by the time I got the bores round & straight. The pistons were nearly as bad, varied .003" in size, scuffed & scored etc.
So I shut my eyes & ran the flex hone through it, knocked the worst of the raised scores off the piston skirts & slapped it together.
There were no guarantees given that it'd even last a race meet & no expectations from the customer - it was a last ditch effort to get the car on the track till he could afford the good short motor.

The damn thing ran hard.
After 2 race meets it was showing over 50% leakdown, don't know exactly, only checked 2x cylinder, broke out in a sweat & decided we didn't want to know.
At idle it breathed real bad, pushed oil out the breathers & smoked.
Get on the gas & it cleared right up.

In the off season we built the new short, used the crank & rods out the old one, new pistons, fresh bores etc - top end remained the same with a freshen & new springs, only things changed in the combo was the block & pistons.
It was much nicer at idle, leaked down at 8% after a couple of meetings (alloy block - never leak down great cold).
It made around 10-15hp more (700+hp engine).

Or in other words the junk block with beat to hell bores & pistons only made 10-15hp less.

Obviously the fresh motor was much nicer, didn't foul plugs, didn't require rags tied over the breathers etc, but if the only thing you were looking at was on track performance there was very little in it.

Once you go WOT combustion pressure shoves those rings against the bores & they seal up much better than a static leakdown test would have you believe.

So yeah, after that little ramble - you're not gonna see much of a gain in et sorting the rings, so don't bank on it, but it will be a nicer motor!

File-fit ring set should contain compression rings that are .005" larger than the actual bore diameter. If your cylinder diameter is 4.125", you should be using a 4.130" FILE FIT set. If you have installed 4.120" rings in a 4.125" bore, they will not seal correctly; tangential tension will too low and there may be too little back clearance between the ring ID and the ring groove in the piston.