[Spridgets] the engine catasrophe

John Caffrey jdc6 at Lehigh.EDU
Thu Sep 25 07:11:16 MDT 2008

Matt Hagopian wrote:
> what one of
> those pics doesnt show is on the piston, there is a 1/2" dent in the top of
> the piston. cylinder walls are a little gashed. freshen up the motor this
> winter. compression is only 160 in that cylinder and 165 in two others and
> one cyliner is 185. kind of low for 13:1 pistons, rocker arm has a crack in
> it... seems to have held up for this whole weekend. I put flat valves in, i
> know thats not as good of flow but it allows for more clearance. I dont
> the spring pressure im running tho... how do i find out?
> The high duration cam with it's increased overlap lowers the actual
compression, and that's why these cams require more CR. For example, 286
degrees cam edneeds at least 10:1, and a 300 degree cam might need 12:1. The
difference in your readings is worrysome and the cylinders may need to be
re-sized to get rid of the gashes. There are small spring testers available
that work with a bench vise, but you will need a telescopic gauge to precise
length measurements from your head with valves and retainers installed. There
are spring testers available that the stock car guys use on assembled engines
to check seat pressures at the track, but I can't say if they'll fit our
rocker arms. I do all of the prep on the bench. With a matched cam kit you'd
only need to setup the pressure on the seat for all of the valves, but mixed
components may have you also calculating the spring pressure over the nose of
the lobe using your cam lift, rocker ratio and geometry. With the wrong
springs you can have the correct seat pressure and be excessive over the nose,
and just bolting 1.5:1 rockers on a race cam made for 1.28 rockers could give
the same problem. Potential problems are excessive wear, frictional HP loss,
and component breakage. The top end details must be correct for reliability.
Just about any of the engine building books out there will show the tools and
procedures needed.
Fogelsville, PA

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