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Road Race Enduro L28 Engine Build


z-ya

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This is the lash pad that broke:

 

P4300006.jpg

 

There are no grooves like this OEM one:

 

cyl5_prob2.JPG

 

This is what Dave Rebello was referring to. With this groove, it give the lash pad a weak spot where it can break. He has never seen one snap like this when there wasn't a groove.

 

This is a full competition head with spring pressures to match. I'm also running slightly less lift than what it ran with in the day. If there were valve float we would have known it way before this happened (going on 1.5 days of tuning).

 

The rocker that came off took out the one next to it. The valves look straight, and the springs are in tact. I will pressurize the cylinder to see if the valves are leaking.

 

Thanks,

 

Pete

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Valve float? What's that?

 

Seriously, we haven't floated the Bonneville engine yet...

 

Someone would have to use a very unstable cam profile to make an L-Valvetrain float!

 

Or worn stock springs on a ITS 2.4L engine. At Cal Speedway and a couple Buttonwillow configurations you have to run that engine over 7,300 rpm for a time. After a race weekend you see a 5% drop in valve spring pressure on the stock springs from doing that. The next race weekend you will only see 7,150 rpm before the engine flattens out. Keep pushing it and you won't be able to go over 7,000 by the end of the weekend. If you don't replace the valve springs you'll bend a valve the following race weekend on one of your heroic pass efforts at the end of the longest straight. Ask me how I know that.

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Or worn stock springs on a ITS 2.4L engine. At Cal Speedway and a couple Buttonwillow configurations you have to run that engine over 7,300 rpm for a time. After a race weekend you see a 5% drop in valve spring pressure on the stock springs from doing that. The next race weekend you will only see 7,150 rpm before the engine flattens out. Keep pushing it and you won't be able to go over 7,000 by the end of the weekend. If you don't replace the valve springs you'll bend a valve the following race weekend on one of your heroic pass efforts at the end of the longest straight. Ask me how I know that.

 

Ditto... Been there and done that... not too fun :(

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I've been excitedly following your build and don't remember if you mentioned what you're doing for the exhaust. Out of curiosity, what are your exhaust specs if you don't mind sharing?

 

We are running a Datsun competition style 6-3-2 header with dual 2-1/4 pipes to either Magnaflow glass packs and straight pipes exiting out the center, or megaphones. We will be able to swap the glass packs for Megaphones at tracks that allow those sound levels.

 

Kind of like this:

 

691_.jpg

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We are running a Datsun competition style 6-3-2 header with dual 2-1/4 pipes to either Magnaflow glass packs and straight pipes exiting out the center, or megaphones. We will be able to swap the glass packs for Megaphones at tracks that allow those sound levels.

 

Kind of like this:

 

691_.jpg

 

Nice, that is going to sound amazing! I love the sound of twice pipes on these engines.

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Valve float? What's that?

 

Seriously, we haven't floated the Bonneville engine yet...

 

Someone would have to use a very unstable cam profile to make an L-Valvetrain float!

 

Understood - I was more pointing out that in my case this was caused by the rocker coming off the lash pad far enough to allow the pad to move or rotate far enough to get the ears in a bad place. Throwing out ideas as to things that might case this to happen, assuming that z-ya didn't drop a valve seat.

 

On the theory about the high shoulders on the retainers, if this were an issue you would be seeing pretty obvious witness marks on both the retainer and the underside of the rocker, which I haven't noticed in the pics so far.

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  • 2 weeks later...

OK, I'm pretty certain we found the problem. You know what happens when you ASS-U-ME.

 

All looks good with guides and valves as expected. Except, exhaust valve length is 119mm. Mmmm, 3mm longer than stock. Looking closer at the geometry when the valve is closed the surface of the rocker tip that is touching the lash pad is almost on the casting. There is barely any clearance between the retainer and the rocker. In fact some of the rockers have a little groove worn in that area.

 

When I got the head, the cam towers were not installed, but there was a bunch of cam tower spacers in the "bad day at the track" box. I measured the stack of front shims (the only ones that are different), and it measured 0.072". The head is shaved about 0.055", and they used a stock head gasket. So I calculate that the shims should be 0.055" for stock valves. The cam towers were shimmed 0.017" more to account for the longer valves (if my assumptions are correct).

 

I'm running a 2mm gasket, and had shimmed the cam towers 0.017" to account for the thicker gasket and head shaving. This would have been OK for stock valves. 0.025" would have been exactly correct but I didn't have the right thickness shim, so I went with a set of 0.017" shims from the stack I had. So to keep the geometry the same I should shim the towers 0.025" + 0.017" = 0.042".

 

This make sense?

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This make sense?

 

Building a race engine and noone checking the wipe pattern on the rockers?

 

No. That doesn't make sense.

 

Your assumption only addresses the change in distance between the crank and the cam sprocket. Your assumption doesn't account for a smaller base circle on a lumpy aftermarket cam. You are probably looking at a new cam, rockers and custom lash pads.

 

I recently cut .050" off a N42 to go with a stage 3 cam. After shimming the cam towers, I ended up using lash pads between .175" and .254" in order to get a consistent wipe pattern close to the edge of the rocker stud side. A page back in this thread you mentioned using .170" lash pads, which I assume you meant on all cylnders. There is no way that you can shave a head, use an aftermarket cam, and be able to use .170" lash pads on every cylinder.

 

I hope you can salvage your current cam and rockers. You have an awesome build going and I absolutely love your car. If you do end up buying new lash pads, instead of trying to buy a bunch of different sizes, consider buying them all at the tallest height you need and then having a machinist trim them down to the exact size you need. Either Xnke or 1FastZ recommended that to me and it was a really good idea.

Edited by tennesseejed
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Building a race engine and noone checking the wipe pattern on the rockers?

 

No. That doesn't make sense.

 

Your assumption only addresses the change in distance between the crank and the cam sprocket. Your assumption doesn't account for a smaller base circle on a lumpy aftermarket cam. You are probably looking at a new cam, rockers and custom lash pads.

 

I recently cut .050" off a N42 to go with a stage 3 cam. After shimming the cam towers, I ended up using lash pads between .175" and .254" in order to get a consistent wipe pattern close to the edge of the rocker stud side. A page back in this thread you mentioned using .170" lash pads, which I assume you meant on all cylnders. There is no way that you can shave a head, use an aftermarket cam, and be able to use .170" lash pads on every cylinder.

 

I hope you can salvage your current cam and rockers. You have an awesome build going and I absolutely love your car. If you do end up buying new lash pads, instead of trying to buy a bunch of different sizes, consider buying them all at the tallest height you need and then having a machinist trim them down to the exact size you need. Either Xnke or 1FastZ recommended that to me and it was a really good idea.

 

Maybe you didn't read the entire thread?

 

Yes, of course, I did check the wipe pattern, and a 0.170" lash pad produced a good wipe pattern on all cylinders. The cam and rockers are fine, as they survived almost 2 days on the dyno. What was wrong was the rocker arm geometry due to the fact that the head has longer than stock valves. Longer valves will allow more lift. The head was not built by me, but someone from Bob Sharp Racing. I don't have the original cam, so I had Isky grind me one.. I do have the lash pads that were run on the head.

 

So I shimmed the cam towers another 0.034", and now the original lash pads (0.210) give a good wipe pattern on at least one valve. The geometry also looks better where the rocker meets the lash pad.

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