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Motor - The Analysis


mustard-z

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My father and I co-drive and mod our '78 FP autox car. We decided to build a new motor last winter, but unfortunately started too late and just finished installing everything back into the car 2 weeks ago. We decided to build the motor ourselves as our machinist didn't see any problems with it. We have done all the work on our car and are what most people would say "competent" at modifications/fabrications and whatever else. This is our first motor from the ground up.

 

 

 

Anyways to the problem; The motor:

 

-77 N42 block overbored 1mm

-stock crank balanced and polished

-stock 240 rods cleaned up and balanced

-stock main bearings

-87mm forged alum. pistons w/ deves rings

-stock balancer

-stock timing chain w/ new guides

-adjustable timing chain sprocket

-N47 head w/ liners removed and ported by us on exhaust side only, port matched. combustion chambers lightly smoothed out

-stainless valves stock sizes

-stiffer valve springs w/ new keepers/ retainers/ lashpads/ rockers

-550/300 regrind cam

-ARP head studs/main studs/flywheel bolts

-1mm steel headgasket

-13lb flywheel

-dual friction clutch

 

This motor was pre-assembled to check for valve-piston clearance and cam lobe wipe pattern. Cam wipe pattern checked out OK. Piston/valve clearance was between .065/.075 on all intake and exhaust valves.

 

Estimated Compression ratio: 10.2:1

 

 

This engine has about a total of 6hrs of run time and 13 autox runs before these pictures.

 

We need help figuring out what happened. It seems to us that the intake valves ended up touching the pistons and eventually one valve failed and broke into 6 pcs and ended up wedged in the brand new DCOE 45's we bought for this motor. It seems odd that this would be the case unless things in the combustion chamber expand and move this much. However, in the "How to Modify your OHC datsun engine" Frank says that something like .060-.070 on the intake valve is adequete clearance.

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Ouch, I hate it when that happens.

Well a couple of things. You mentioned that you have “stiffer” springs, but just how much stiffer, a little stiffer or are these the recommended springs for that much cam at the RPM’s you spin?

 

Also, a lot of the cheaper stainless valves are actually a three piece design fused into a valve, (valve stem tip, stem, and valve head). On rare occasion those three piece valves will separate/come apart at the fusion points and the result typically is what you experienced in those photos. We’ve seen this on more than one occasion with big lift cams.

 

So you are in Bend eh.. Do you do much auto crossing in Eugene through EESCC or Medford at the Jackson county sports park?

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We need help figuring out what happened. It seems to us that the intake valves ended up touching the pistons and eventually one valve failed

 

Aren,

 

Did the compression ever get fixed before you ran the motor? I had heard, perhaps incorrectly, that you had a hole that was well down compared to the others.

 

Did this just happen, or did you have a stuck throttle and this happenned. I've heard a couple different versions.

 

I had a similar problem where the cam dowel sheared and all the valves hit the pistons. In my case none broke like this but all my valves were bent and a number of guides broken.

 

Cary

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Braap,

 

The springs were part of the cam/rocker/lashpad/retainer package. It would seem that the springs should be capable of controlling it if they came all from the same place.

 

The intresting thing is that the intake valve DID have the failure that you speak of. One piece looks laminated. I will take a picture of it.

 

I do live in bend. I don't go to EESCC as I think they kinda suck. They have always been a pain to work with. I am usually in Medford or Redmond though. I was hoping to go to Bremerton or Packwood this year but. Alas, not this year.

 

 

 

Cary,

 

The failure occured at about 4500 rpm at the end of a 6 cone slolom. The throttle was wide open (excelerating). BOOM! Let off the throttle and it felt wierd, disengaged clutch and rpms started to climb, engaged clutch and shut the car down. Pulled the carb and found 6 dime size pieces of the intake valve. We never solved the compression issue. We were told that 2-3hrs of running in the garage will not seat the rings and that we needed more time on the motor. So we put time on the motor. Never again will I do that!

 

 

John,

 

I don't think that I overrev'd it. We don't have a limiter but the car wasn't revving freely above 6500. I can't be sure, but I really don't think that the valve only hit once cause every valve has a clean spot that corresponds to a clean spot on the piston. I think that they were touching much more frequently. But what do I know, this is the first motor we have built and the first one that I have trashed.

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Braap' date='

 

The springs were part of the cam/rocker/lashpad/retainer package. It would seem that the springs should be capable of controlling it if they came all from the same place.

 

The intresting thing is that the intake valve DID have the failure that you speak of. One piece looks laminated. I will take a picture of it.

 

I do live in bend. I don't go to EESCC as I think they kinda suck. They have always been a pain to work with. I am usually in Medford or Redmond though. I was hoping to go to Bremerton or Packwood this year but. Alas, not this year.

 

...We were told that 2-3hrs of running in the garage will not seat the rings and that we needed more time on the motor. So we put time on the motor. Never again will I do that![/quote']

OK then it definitely sounds like the valve came apart. I had a friend who built a 22RE and he had just gotten it running and it was idling in the driveway and we were talking about what it had in it and one of his SS valve heads fell off. I don't think the cam dowel sheared because I think you'd have a lot more carnage anyway.

 

Running a motor just with the car just sitting in the garage or on stands (I've heard both) IS NOT THE RIGHT WAY TO SEAT RINGS. Whoever told you that was absolutely correct. I know of one guy who built his engine and let it idle for 5 or 6 hours to seat the rings. Thing smoked like CRAZY. Then another friend heard about it and helped the first guy out by driving it like he stole it. As he later put it, he had to break the glaze that running at steady state for hours had put on the rings and cylinder walls. Hard acceleration and decel is what seats rings. You get good heavy wear under acceleration and the decel throws a bunch of oil onto the cylinders, repeat 1000 times and rings will seat great. Steady state running is very bad for break in...

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