Piston fit and clearances

[Greystone]

Guys,

Any input of the following would be appreciated.

I recently fitted a Turbo technics hybrid turbo (T25/T3) to my Rover Tomcat. On Dave Walker's rolling road it did not produce the expected power increase, only 13bhp more than it did with the T25. After some investigation Dave tracked the problem to poor performance on the leak down test (although a previous compression test had suggested that all was well).

I stripped the engine over the weekend and found that I could rock the pistons in the bores at TDC. Pistons are the Accralite T16 pistons 85mm bore 7.5:1 cr (not much skirt on them). By pushing the piston as far to one side as possible there is a gap of approx 0.6mm (24 thou) which indicates a clearance around the top of the piston of 0.3mm (12 thou) between the piston crown and the bore. I removed a piston and checked the bore. The bores have no wear ridge at all. A piston ring inserted down the bore showed a ring gap of 0.4mm (16thou) consistently all of the way down the bore.

I took the head off my other engine (standard Tomcat race series spec - Rover pistons) and found it was almost the same. The piston rocked in the same way (about 0.5mm total gap and 0.25mm clearance all the way around the top of the piston). This engine had the barest of discernable wear ridges at the top of the bore i.e. you could feel a ripple with your finger but it was not enough to catch it with your finger nail. This engine is well used, until I cut them off in order to remove the head it still had the seals from the Tomcat series on it.

It is possible that the leak down problem my be due to some bent valves. The engine came from a Tomcat that was written off at Lydden and the cam belt jumped a few teeth in the crash. I checked the head at the time and a compression test indicated that all was well so I didn't pursue this any further. I did also notice during the mapping that it was fuming slightly from the catch tank when running at max rpm, at although prior to stripping it oil consumption has not been a big issue - it leaks more than it burns.

I have put in a call to Accralite for advice and I waiting for them to come back to me but any thoughts from the assembled brains here would be welcome.

Andrew

[Robyn Slater]

Pistons are always machined oval and tapered to allow for heat expansion.
Pistons are smaller at the top than at the bottom(skirt).
Rocking the crown of the piston bore is normal and the amount will vary depending on the piston type and wear.
A quick check is to use feeler gauges to determine the clearance while the piston is in the bore. Remember to check clearance at the skirt end(bottom)of the piston.

The best way to check is to use a bore gauge and micrometer.

Accralite should provide you with the running clearance for their pistons.

And yes your problem could be bent valves or burnt valve seats.

Hope this helps.

Robyn

[Greystone]

Robyn,

Thanks for the info.

I have just finished talking to the man from Accralite and he said much the same as you have in your note. He also gave me the dimensions and clearances to measure.

His view was, providing that the measurements turn out to be within tolerances, the pistons should be serviceable and the problem would probably turn out to be elsewhere. If all is ok I will turn my attention to the valves and seats.

Thanks for your time.

Andrew

[R59]

It sounds like your power is disappearing down into the sump.

It comes down to the silicon content in the alloy.

The old pistons in my V8 were quite high silicon, and were not getting hot enough to expand fully, and it was losing power down the tubes! On the dyno it was fine.

My new pistons are a different alloy, and more suited for road course racing (according to the manufacturer), hence they expand to their "max" at a lower temperature, and were more stable (according to the manufacturer).

Best of luck with the Accralites...

Rob.

[johnny yuma]

The bores could probably benefit from a hone.Are they glazed?
Was a compression test done before and after squirting oil into the plug holes? an increase indicates poor sealing.No increase indicates valve related leakage.
The fuming is a bad sign-are the plugs sooty?

[trikesrule]

Hold On! Have a read www.mototuneusa/break-in-secrets.htm Done this to a few engines - works a treat........trikes

[dtype38]

Can you check that link trikes, I just get a "not found" error when I try it. ta.

[GORDON STREETER]

Seen Ajax put in the bellmouths on the dyno a few times , and yes it does seem to work !

[esper]

if you did a leaktest , why not having a little more pressure and try to hear where the air goes. exhaust /airfilter/dipsticktube. A slightly bend valva cant be detected with a compression test. but a leaktest does.

[Chris Y]

Originally Posted by trikesrule

Hold On! Have a read www.mototuneusa/break-in-secrets.htm Done this to a few engines - works a treat........trikes

Try http://www.mototuneusa.com/break_in_secrets.htm instead..

[Greystone]

All of the measurements suggest that there is absolutely nothing wrong in the piston/rings/bore department so I am turning my attention to the head and valves.

As there has been slight piston/valve contact in the past it is likely that there may be slightly bent valves.

I have another head so that may be my best solution.

Thanks for all the help, info and suggestions guys.

Andrew

[Al Weyman]

Good engine builders clamp a honing plate made from inch thick plate down on to the head to emulate the forces imposed on the block by the clamping of the cylinder head, then then finish with a fine grit hone. Just maybe your bores may look fine now with the head off but may distort when the head is torqued down. Also check the glaze on the bore as suggested by the other poster as I have experienced a problem like this caused by glaze. When honed like this the engines require virtually no break in.

[ian.stewart]

13hp increase, was the car set up on Daves rollers previously??
and, another thought, if you are running synthetic oil, this can hinder the bedding in of rings due to its better lubrication abilities, I have allways run any new engine that I have built on a good mineral oil before going over to syn.
Ian

[zefarelly]

I read that link about running in, and its part believable, although I suspect its partly a load of American ballcocks as well !

I bed my engine in with straight mineral oil, then switch to 20/50 have been using the ZX1 Valvoline stuff lately which seems good.

after 2 years my pistons look new and a slight brown colouring between the top of the piston and the first ring, nothing below, and a slight glaze to the bores, which honed off with a light stoning. I ran the engine for 20 minutes on axle stands at 2k rpm in 4th gear, so a light load, then a few easy laps before winding it up a bit !

[dtype38]

So the black crustyness on the top of my pistons, and the burn marks between the top and second rings isn't right then? Doh!

[zefarelly]

I think its fairly normal, I guess it depends how often you rebuild engine and what sort of fuel or 'supernmarket liquid' you feed it

what I don't believe isan aleged race engine thats run for 55 millenia being taken apart and someone posting a photo of a shiny silver piston !

[COLIN STUBBS]

IMHO if you remove the manifolds,put plenty of air pressure down your leak down tester and try to keep the piston near the top you will soon hear where the leakage is and ill bet its the valves that need a look at.Those engines often need a valve job and if you can see theyve caught the pistons a bit change em as you can bet your life it will snap a head off one of em.then do another leakdown check with the piston 1/3 of the way down and check if you can hear air through the dipstick tube or if you have the sump off its easier. if it is then its time to eithr change the pistons or the rings,but have a close look to see if theres any cracks behind where the rings fit..

[dtype38]

Originally Posted by zefarelly

I think its fairly normal, I guess it depends how often you rebuild engine and what sort of fuel or 'supernmarket liquid' you feed it

Only had the pistons out twice in five seasons of racing, both times to sort out crank bearing problems. Use Tesco 99 mostly. Anyway, this is the first time I've actually paid them any attention and I found my ring gaps were enormous, as was their vertical flap space. So I've treated the old girl to a nice new set of TotalSeal jobbies. As for valves... I would never strip the head without re-seating the valves with grinding paste... which may account for good valve sealing but poor crankcase pressure!

[trikesrule]

I wonder of the merits of compressing the engine with air pressure. Hmm. I actually go the other way and test an engine with a suction guage. It eliminates the bent valve scenario. It needs to hold negative suction numbers for at least 15 minutes. Or is it possible none of the valves are bent but u may have a couple of suspect seats. The bore probably needs a plateau hone job. The average stone sometimes dosen't quite seat the ring(s)....trikes

[I Cooke]

Suction test?!

That doesn't make sense to me. If a bent valve is causing a leak, then it will leak whether it's positive pressure or depression. In fact I would say it's more likely to leak as the depression will try to open the valve.

Positive pressure testing, in my opinion, is the only way of simulating the sealing capabilities of the engine as would happen during combustion.

Or have I missed something?!

[zefarelly]

re pressurising engines, I've got a gutted spark plug welded to a compressed air fitting which enables one to change valve springs without removing the head. works a treat . . . . if your pistons and rings fit/work !

[GORDON STREETER]

Zef I though I was the only one with that idea ! I copied it from a race mechanic in 1962 he even had an 18ml one and he had copied it in the 50s !
I know that if a valve is only very slighly bent the spring will pull it onto the seat and it wont leak under pressure . Seen it a few times on these new fangled 16v things with 6 inch nails for valves !!

[trikesrule]

I agree that compressed air in the cylinders is the way to go. But for the rest of the engine a suction test works well and dosen't stress the seals like compressing the crankcase does. Guess I'm just a 2 stroke man at heart and I've found by paying detailed attention to the crackcase seals / gaskets etc pays dividends.......trikes