Oh %%%$ - Sprint engine broken?

[misterp]

It was all going so well....

Changed the gearbox oil after stripping and reconditioning the overdrive and just getting ready to take it for a test drive when..

The throttle got jammed a WOT for ~10 secs.

I (as fast as my brain could work it out) shut the car down, to find the throttle arm had gotten stuck on the crank breather pipe...

Then, when I went to start it up, its now developed a knocking sound and it wont start.
The knocking noise seems to be coming from the front of the engine?. The timing chain appears to be tensioned correctly, but the noise does sound like its coming from the front of the engine.

Ice since pressure tested the engine and have various reading ranging from ~ 90 PSI for #1 and 4, and the middle pair showing around ~125 PSI.

Coolant has dropped, but more worryingly I have no oil pressure. I've confirmed the jackshaft is turning as the dizzy is rotating (can someone confirm the jackshaft also drives the oil pump?)

So...million dollar question..WTF has gone wrong?

I can understand if its a blown head gasket (loss of water makes sense)....BUT the loss of oil pressure and the knocking noise? Can you blow a gasket to such an extent that no oil gets to the top of the engine?

I have an external pressure gauge plumbed into a take-off point on the inlet manifold, and previously it had been showing a healthy ~50 PSI or more.

Any ideas what the knocking could be and the loss of oil pressure? Why would running a WOT for a short period of time cause any issues?

Any help and pointers appreciated...either way, unless I can find out more info..its head off and begin to work my way through the post mortem

[Galileo]

Ouch! I'm thinking jackshaft bearing, but best wait for the experts.

[soe8m]

The knocking will probably a bearing. Take off the head and sump and check the damage. It takes a while for the oil is fully pumped into the engine. So high revs at once is no lubrication yet. A broken bearing can also mean no oil pressure.

Jeroen

[misterp]

Cheers guys..

Thanks for the initial diagnosis...

I`ll do some reading about the jackshaft bearing...but is that an engine out scenario?

[soe8m]

Cheers guys..

Thanks for the initial diagnosis...

I`ll do some reading about the jackshaft bearing...but is that an engine out scenario?

You can do but it will be a rod bearing knocking.

Jeroen

[misterp]

Gutted that for 10 seconds of WOT its self destructed.

Jeroen, when you say rod bearing..you mean small or big end?
You think its spun a big end?

Any idea why the compressions are low as well? Double whammy..head gasket AND big end?

[tony g]

If at high rpm the engine was shut down it may well have ran backwards when nearly stopped. Mine did when i first had it. I t caused the jackshaft sprocket dowel pin to shear and was putting the timing out from the dizzy being turned at random.
Jackshaft turns the water pump then further back turns the dizzy, which in turn spins the oil pump via a hex bar shaft. If the anti rotation has happened it may well have sheared the hex shaft and then no oil pressure.
Pull the dizzy and see if the hex shaft is ok. then pull the timing cover and check the jackshaft bolt. A dry engine will always knock in very short time im afraid


Tony

[paulsprint]

if the throttle was wide open with no load the only thing that will stop it revving is valve bounce. I think your valves have hit the Pistons and are very slightly bent. Hence the low compression. Have you checked for any broken rockers. The engine won't start with compression that low.

[tony g]

Good call there Paul

Tony

[misterp]

Hi Paul, Tony

The whole engine is how I left it yesterday (in disgust)..but thankfully no apparent damage to the top of the engine (cam cover off) and no broken rockers. It was smooth when I turned over the engine manually.

So (dare I say it) is the engine fragile to an extent where it cant handle full revs without self destructing? (sincere genuine question BTW).
Would this have happened anyway (as far as I could tell, a healthy engine, no smoke, good fresh oil to the mark, and good oil pressure) or is the single root cause the WOT throttle?

So..if the oil pump had a failure, then I can understand why with no oil it would cause a catastrophic failure, why would the oil pump fail? drive shaft fail?...something internally in the oil pump?
OR..
Has if the oil pump is still working and the spun bearing is not allowing oil pressure to build and reach the upper engine (assume good oil flow and pressure just before the bearing spun)....why then did the bearing spin? Lubrication system not up to the job at high revs?

Sorry for the slight rambling. I know it wont fix the damage, but I would like to know why and get an idea if it was destined to do this at some point anyway?. ...opinions greatly appreciated.

[soe8m]

Time to take the spanners out.

Jeroen

[tony g]

pull the dizzy for reasons I mentioned above.

Tony

[misterp]

Sorry Tony..re-read your post. Get it now. Will pull the dizzy and see what I can see.

On the upside..I have been offered another sprint bottom end just in case my crank is beyond economic repair..

Just got to pull it all down now and get it onto a stand to re-build it.

Ah well..every days a learning day.

Thanks everyone once again.

[Jon Tilson]

Sprint engines are known to have some resonances that can cause crank and centre main bearing cap failure at much over 6500 so the red line is there for a reason. Some wags on race tracks with webbers like to do more so usually add some sort of block brace.

If your throttle stuck open then you would have exceeded the red line safe limit but off load...so I suspect that you did indeed have some sort of valve bounce.

I guess all you can do is take it all to bits and look at all the crucial bits like bearings, or you could see if the oil pump is working, cam timing is as expected and try another compression test?

Then try and run it carefully? Your compression readings dont exactly fill us with optimism though...

Have it out....

Jonners

[Galileo]

Time to take the spanners out.

Jeroen

Sorry Mister P, no laughing matter I know as I'd be kicking next doors cat if this happened to me, but Jeroen cracks me up!