32 replies to this topic

[Cooperman]

If the head has been seriously gas-flowed, the resultant amount of skimming may well cause the valves to have interference with the top of the pistons, even with a standard cam. However, it MUST be measured before fitting to establish the clearance/interference.

So, yes, I think the valves should clear the pistons in the event of cam drive failure, but measure it to be certain.

[CityEPete]

It's often tge mechanical parts on a timing belt wbere the failures occur. Tensioners, idler whhels, even the water pump. Sometimes well inside service intervals too.

My moms Alfa Romeo GT ruined its engine 2 weeks out of manufacturers warranty due to the water pump failing which ripped the belt off

[timmy850]

I was looking into this the other day to check two of my heads...

Worst case scenario would be bottoming out the valve springs with the valve guides contacting with the spring top cap (unless the springs have bound by then?)

In that situation the exhaust value sits down approximately 5mm below the bottom of my 12G295 head, and the intake is down about 3.5mm. The head has been skimmed about 2mm, and it has big valves.


And for a stock 12G202 head:

The valves on the 202 head barely come down past the bottom of the head.


You'd still need to measure the compressed head gasket thickness, and figure out how high the pistons come up the block.

Edited by timmy850, 29 June 2014 - 10:03 PM.

[Ethel]

I once changed a chain because the crank sprocket snapped! I reckon it was the chain that broke it, as it had rollers missing. That did no other damage, the engine was a standard 998 A+.

 

I've learned through TMF that a 1275 crank will hit the cam, but I've yet to confirm it for myself. I think Minispares specify something like 20,000 miles as the belt change interval, the belt probably is a bit lighter than most production car's, and a lot shorter. Though it also has less flexion per lap, and shorter will be better for stretch. It's quieter and more accurate than a chain (2 things that will be less noticeable on an A series  ) but I reckon the main advantage is the ease it offers for tinkering around with cam timing, if you're a racer.

[69k1100]

http://www.cdxetextb...checkclear.html

Not a big job to check if your going to the trouble to fit a belt. Also std a-series pistons don't have valve reliefs so it's unlikely to be interference, stock of course. But If you drop a valve it's toast anyway.

[The Matt]

 

BMW & Mercedes now use chain drive again and others are going back to it.

I think that with a standard cam the valves won't hit the pistons (nor is block pocketing necessary when putting a 940 head onto a 998 with a standard depth head & a standard cam).

If the depth from head deck level to valve face level is more than the valve lift, there is no risk. As with most engineering things, it just needs accurate measurement, which many on here always seem reluctant to do.

which many on here always seem reluctant to do?  well you clearly haven`t done this yourself as you wouldn`t be stateing  I THINK

 

 

I'm pretty sure he will have.  His point is that it may well be different for each engine build (well, it probably will be).

in completely standard form, with an unskimmed head, unskimmed block, they won't touch as far as I know on neither the 1275 nor the 998.  If you've got a higher lift cam, higher rocker ratios, skimmed head, skimmed block then YOU need to check it when you build the engine.  As all of these Mini engines are getting old now, there's a chance that some of the engine components will have been modified or reconditioned.

 

It's an easy enough check to make.  All you've got to do is know your valve lift and then measure the depth of the valve face to the head face.

 

I'd imagine that the belt drive kit is normally reserved for performance engine builds, which will have had some or all of the components modified, so each engine build needs checking on its own merits.

[jime17]

BMW & Mercedes now use chain drive again and others are going back to it.
I think that with a standard cam the valves won't hit the pistons (nor is block pocketing necessary when putting a 940 head onto a 998 with a standard depth head & a standard cam).
If the depth from head deck level to valve face level is more than the valve lift, there is no risk. As with most engineering things, it just needs accurate measurement, which many on here always seem reluctant to do.

which many on here always seem reluctant to do?  well you clearly haven`t done this yourself as you wouldn`t be stateing  I THINK

More to the point, it wasn't Cooperman that was asking the original question.

If he were considering converting to belt, I dare say he would measure it.

[HarrysMini]

Thanks for all the input guys. It seems there isn't a definitive answer due to the fact that all A series configurations are different. It was just me wondering really, I think it isn't all that likely that it would snap unless another component failed. It does follow a very simple route compared to some I've changed, so I can't imagine it's under a huge amount of stress, although it is shorter so there's less of it to wear, meaning it'll wear more quickly.

 

I think I'll do some measuring.

Edited by HarrysMini, 30 June 2014 - 04:00 PM.

[albo]

Thanks for all the input guys. It seems there isn't a definitive answer due to the fact that all A series configurations are different. It was just me wondering really, I think it isn't all that likely that it would snap unless another component failed. It does follow a very simple route compared to some I've changed, so I can't imagine it's under a huge amount of stress, although it is shorter so there's less of it to wear.

 

I think I'll do some measuring.

above your avitar you state you dont have short legs, mine are prob shorter then yours so if we both walk a mile together my legs would of have had to work nearly twice of hard,i had a midget with a hump bk working with me for years , he was always buying new boots,so point is smaller more wear,and as for coops stating ,which many on here always seem reluctant to do,well thats why we all visit this great site to learn more knoledge

Edited by Cooperman, 30 June 2014 - 06:43 PM.

[Cooperman]

The key to engine building is measure and measure again. Many on here always seem reluctant to do so. Not everyone, but it seems to crop up quite often, sometimes in respect of fitting a 12G295 head on a 998  or changing to a 12G940 on a 998.

I measure everything every time I build an engine, which is less frequently than I did years ago, but still happens. In particular the crankshaft end float should always be measured, the valve to piston clearance (in the event of timing chain failure), the valve spring compressed state (i.e. not coil bound and still with 0.020" clearance between coils at max opening), the piston ring installed gaps, the distance from piston top to block deck at TDC, the cam timing, the primary gear seating (play & end float), the big-end & main bearing running clearances when fitting new bearings without a re-grind, or checking the re-ground diameters before assembly, etc.. Also measuring and checking the compression ratio then correcting as necessary is also important.

I'm sure some engine builders don't do this, but they should. A full 'trial build' before final machining and assembly is vital.

[mini13]

I can confirm the rods can hit the cam if  a belt or chain goes on a 1275, you might get away with it with a std cam, but I had  the cam notch all the rods and rod bolts when I had a belt let go.

[HarrysMini]

The key to engine building is measure and measure again. Many on here always seem reluctant to do so. Not everyone, but it seems to crop up quite often, sometimes in respect of fitting a 12G295 head on a 998  or changing to a 12G940 on a 998.

I measure everything every time I build an engine, which is less frequently than I did years ago, but still happens. In particular the crankshaft end float should always be measured, the valve to piston clearance (in the event of timing chain failure), the valve spring compressed state (i.e. not coil bound and still with 0.020" clearance between coils at max opening), the piston ring installed gaps, the distance from piston top to block deck at TDC, the cam timing, the primary gear seating (play & end float), the big-end & main bearing running clearances when fitting new bearings without a re-grind, or checking the re-ground diameters before assembly, etc.. Also measuring and checking the compression ratio then correcting as necessary is also important.

I'm sure some engine builders don't do this, but they should. A full 'trial build' before final machining and assembly is vital.

Thank you, that is a very useful reply.

 

Edit: Just realised that sounded really sarcastic, it wasn't

Edited by HarrysMini, 30 June 2014 - 07:02 PM.

[Cooperman]

Someone asked my why some engine building companies charge a high figure for an engine build, whilst others are less than half the price.

The answer is in the quality and accuracy of the build. It takes between 35 & 40 hours to build an engine properly and assemble it onto a gearbox, so work out for yourselves the cost of the labour, then add the cost of the new parts.

You mostly get what you pay for in life. When I retired from having a 'proper job' I decided to do some engine building to earn a few bob and keep busy doing Mini things. However, I get a bit fed up with being asked how cheaply I could build a powerful engine. So I decided to just build my own engines and engines for friends at no charge. I then concentrated on some restorations for myself & my grandson, including a 1060 cc Mayfair, a 1275 Cooper MPI and an Innocenti 1300 Export. Now I'm doing a 1964 850 Mini and a 1971 MGB-GT.

[albo]

wow it is so reasuring that you are now awake mr coops, and giving good and correct advice, keep it up sir, you are a diamond,

[Cooperman]

I just like to help keep our little classic cars going and I love building engines, but simply hate gearboxes.