Sprint Engine, valve contact

[Magenta Auto Sprint]

I am in a slightly embarrasing position, I started my sprint engine after a complete rebuild.
long story short, the two small bolts that secure the cam sprocket to the cam shaft came out. I didn't tap over the washer. at tick over the engine just stopped,
Two new bolts in and it starts but is lumpy, have I bent any valves? having said that when I turned the engine over by hand it moved easily.
I have also found a plug lead with a detached end, I did try to re-crimp this but it made no difference.

thanks
Malcolm

[Jon Tilson]

The only way to tell is a compression test....

I had a cam belt go on an astra...bent the valves...one so slight I didn't notice til I put it back together fixing the obvious ones
and it ran on 3 pots....

Grrr...

I suspect you bent one sadly...if there was any contact. Also be sure the cam bearing cap isnt cracked.

Jonners

[80Sprint]

Malcolm.

Been there,done that. The bolts fitted to mine before my rebuild we're not the correct spec and were soft. They sheared on the driveway whilst on tickover.

That resulted in 8 bent valves. Turned out the rest of the engine ended work anyway. I really do hope you are luckier and have not suffered the same fate. I would recommend a full top end inspection and also the chain and slipper again however. A set back but nothing that can't be recovered after a cup of tea and a bit of swearing....

[mahony]

This happened to me in the 80's and i manged to get away with no damage ( also soft metal bolts supplied via a mate) , fingers crossed for you

[trackerjack]

As an engineer I have always viewed the 2 tiddly 1/4 UNF bolts that hold the cam sprocket with suspicion.
I was at Thruxton to watch some Sprints racing a few years ago and two of them wrecked their engines due to these breaking.

My Sprint that is now in the USA has four bolts as does the engine I rebuilt for someone on here.
The cam can be drilled and tapped easily and bolts used must have at least 8.8 embossed on the head if metric or 5 raised lines if imperial. You can use cap head screws and these are likely to be 12.9 which is harder than 8.8. Never ever use a stainless bolt for anything that is going to take load.

[Magenta Auto Sprint]

Just an update on my situation. and thank you for your suggestions and thoughts.

I borrowed a Boroscope to check pistons for damage but I couldn't see and damge. I also checked all tappet clearances and concluded that all were ok, some were a fraction tight but not excessivley lose.

I then carried out a compression test; No 1, 115, No 2, 120, No 3, 125, No 4, 125, so all ok.
A new set of plug leads fitted but it still runs rough, then I noticed that the dash pot on the rear carb is not lifting even though it is free to do so. so more checks required.

Also S grade bolts (50 T U.T.S) on order which I hope should be good enough for future use.

Thanks again

malcolm

[matt of the vivas]

The compressions sound far too low to me.
I did a test on a standard 1850 during the week and they were all around the 180psi mark.
If its not lifting one of the carb pistons it sounds like its short of vacumn - which would tie in with the low compressions. I think my next stage would be to borrow or make up an adapter that will screw into the plug holes and allow you to do a leak down test with compressed air - i suspect you have some slightly bent valves, sorry...
Matt.

[Jon Tilson]

I always love picking up my mate Matt on points of physics.....

Atmospheric pressure is nominally 14lb per square inch.... 1 bar or 28 inches of mercury... 1000 milibar.

An engine with a 10 to 1 c r would measure 140lb per square inch...

so these numbers seem okayish...

Certainly far to high for a bent valve...which would give a reading of pretty close to zero.
When I had one slightly bent and I mean slightly cos I didnt even notice it was bent first time it showed zilch...

So 180 on your dolly means a duff gauge or some serious skimming has take place....


Jonners

[matt of the vivas]

I always love picking up my mate Matt on points of physics.....

Atmospheric pressure is nominally 14lb per square inch.... 1 bar or 28 inches of mercury... 1000 milibar.

An engine with a 10 to 1 c r would measure 140lb per square inch...

so these numbers seem okayish...

Certainly far to high for a bent valve...which would give a reading of pretty close to zero.
When I had one slightly bent and I mean slightly cos I didnt even notice it was bent first time it showed zilch...

So 180 on your dolly means a duff gauge or some serious skimming has take place....


Jonners

Put your note pad and calculator down, go outside and actually measure your compressions with a gauge. When they are all around 160 - 180 psi on a decent running engine come back here and i'll explain why..... in simple terms for you as i know you are getting on a bit
x x Matt

[Jon Tilson]

As usual Matt in these kind of hilarious debates where one of us pulls pseudo science on the other we are both
right and both wrong...

Stung by your implication that I'm an ageing old goat ( true ) I did a bit of research on that new fangled internet thing...

Its not that simple...Boyles gas law and all that, plus cranking speeds blah blah blah.

In ideal conditions - charge battery all plugs out throttle open so no suck back etc and the right type of gauge you can get about 20 x compression ratio so your 180 is about right for a healthy 9:1 engine.

But in this case its just dry, we dont know what he was doing battery wise and other plugs out how many turns and open throttle etc and the numbers are all even if a tad low.

Ergo No bent valves....

We are both right.

Snore....when I wake up I'll read your explanation with great interest...


Jonners

[matt of the vivas]

You are spot on as to why 160 - 180 psi is correct Jon, but that still makes 125psi worryingly low. Couple that to the lack of vacumn (rear carb piston not lifting) and that does point to valve damage. Like i said, my next move would be to do a leak down test - that would prove it one way or another. Other possibillity, is the valve timing correct? If it slipped a tooth or two it could be running (badly) but low on vacumn / compression.
But i would be double checking the valves first.

[Carledo]

Gotta weigh in here, a bent valve, no matter how tiny the error, would undoubtedly result in zero compression on that cylinder. the low comp is no real surprise as the engine is freshly built (see OP) and probably hasn't had time to build up coke lips yet and it's not that low anyway, I don't panic until ideal condition comp drops below 100.
The first thing I would check if one piston not lifting would be the throttle linkage between carbs, Is it tight? is it slipping? is the throttle butterfly turning on the affected carb? Don't ignore the obvious in favour of the obscure and unlikely (if the cam timing was out, BOTH carbs would be affected!) Shame on you both!

Steve

[Jon Tilson]

All I've commented on is the compression values prove no bent valves....

so I chuck your shame right back at you.....

but I'm glad we all agree....

Jonners

[matt of the vivas]

Gotta weigh in here, a bent valve, no matter how tiny the error, would undoubtedly result in zero compression on that cylinder. the low comp is no real surprise as the engine is freshly built (see OP) and probably hasn't had time to build up coke lips yet and it's not that low anyway, I don't panic until ideal condition comp drops below 100.
The first thing I would check if one piston not lifting would be the throttle linkage between carbs, Is it tight? is it slipping? is the throttle butterfly turning on the affected carb? Don't ignore the obvious in favour of the obscure and unlikely (if the cam timing was out, BOTH carbs would be affected!) Shame on you both!

Steve

I also reject your shame...
As a professional mechanic, if someone brought a car to you and asked you to diagnose the problem, with a description of "the cam sprocket came loose, ive fitted it back and now the engine runs rough / low compression etc etc, your first move would not be to leakdown test it as ive suggested twice? If the valves are ok it will show up on this, if they are not... theres the problem. And a bent valve does not always mean zero compression, quite often a slightly bent valve will sit slightly off-centre on its seat and allow some compression. Ive seen this recently on a Ford Zetec that someone else rebuilt after a cambelt failure - i stripped it again and replaced the slightly bent vlaves that the last person missed.
Also - its a freshly built engine, but if its been built properly it wont be low on compression! I accept the rings need to bed in but it wont make a massive difference to static compression figures - and they should be far higher than 125 psi...
I repeat again - leakdown test it. Eliminate any mechanical issue first. It wont take long.

[Carledo]

Gotta weigh in here, a bent valve, no matter how tiny the error, would undoubtedly result in zero compression on that cylinder. the low comp is no real surprise as the engine is freshly built (see OP) and probably hasn't had time to build up coke lips yet and it's not that low anyway, I don't panic until ideal condition comp drops below 100.
The first thing I would check if one piston not lifting would be the throttle linkage between carbs, Is it tight? is it slipping? is the throttle butterfly turning on the affected carb? Don't ignore the obvious in favour of the obscure and unlikely (if the cam timing was out, BOTH carbs would be affected!) Shame on you both!

Steve

I also reject your shame...
As a professional mechanic, if someone brought a car to you and asked you to diagnose the problem, with a description of "the cam sprocket came loose, ive fitted it back and now the engine runs rough / low compression etc etc, your first move would not be to leakdown test it as ive suggested twice? If the valves are ok it will show up on this, if they are not... theres the problem. And a bent valve does not always mean zero compression, quite often a slightly bent valve will sit slightly off-centre on its seat and allow some compression. Ive seen this recently on a Ford Zetec that someone else rebuilt after a cambelt failure - i stripped it again and replaced the slightly bent vlaves that the last person missed.
Also - its a freshly built engine, but if its been built properly it wont be low on compression! I accept the rings need to bed in but it wont make a massive difference to static compression figures - and they should be far higher than 125 psi...
I repeat again - leakdown test it. Eliminate any mechanical issue first. It wont take long.

Yes, if someone brought me a car where the cam sprocket had come loose (or twisted off its tiny dowl as Vauxhall ones are prone to doing) the first thing I would do is check compression. But my compression tester screws into the engine and has a rubber seal, which means it stays firmly in place while I sit in the car with my foot on the gas, crank the engine and read the guage through the screen. Its a VERY efficient bit of kit and I've had cars record over 220 psi on occaision No insult intended to Malcolm, but I doubt many amateurs have such a toy at their disposal. Next, the comp readings themselves, assume for the sake of argument, that I got similar readings to Malcolms, I would not be unduly concerned as they are all within 10 psi of each other and even one like the Zetec you tell of probably had a far bigger drop than that, I would guess 40 psi max, even after extended cranking. So no, I would not have immediately proceeded to a leakdown test, but at this point would have started looking for other causes, 40 odd (sometimes VERY odd) years in this business have seen me fall prey to tunnel vision too often to allow myself to fall for it often now! Plus the engine has just been rebuilt, therefore even a competent amateur will tune and balance the carbs as soon as its warm enough to idle and of necessity will slacken the linkages. From here its a short step to them not being fully tightened and one carb dropping out of the equation. With the clue of the piston not lifting, this is an obvious next step to check and a problem I have seen (and even done once or twice in my youth) too many times to NOT think of!

Still, maybe I was a bit harsh and I withdraw the shame!

Steve