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Old 30-03-2013   #91
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Re: 695 Engine Project

To the best of my knowledge and with reference to the attached image, I think this is what is happening -

The conrod length in both the 500 and 650 is 118mm and the piston heights are also the same at 40mm giving a combined height of 158mm. If you add this to one half of the stroke (35mm) you get a grand total of 188mm which when you add the deck clearance (2mm) gives the deck height of 190mm. The deck height is made up of the crankcase height (100mm in the 500 and 110mm in the 650) plus the cylinder height (90mm in the 500 and 80mm in the 650).

When I rebuilt my engine I mated 30mm pistons with 130mm conrods giving a height of 160mm which left me with a deck clearance of ~0.20mm.

Compression ratio is determined by a combination of deck clearance, head gasket thickness, piston crown shape and of course cylinder head shape and volume. Because I used standard flat topped pistons and didn't want to shave the head, the only way I could increase my CR was to decrease the deck clearance and use a thin gasket (copper).

I probably should have explained a little better what I did at the time, but I was so chuffed at actually getting a response from the sellers in Europe and having the stuff arrive intact, that I got a bit carried away and proceeded to assembly after all of it was checked by a local engine rebuilder. I should have taken more photos, but didn't - maybe next time

I thought about all of this the other night after Roger had problems with his rebuild and unless he has bought 32mm pistons with 130mm conrods thus leaving him a 2mm excess above the deck, I have no other explanation. Once he fits 2mm spacers, all will be well. Maybe I just got lucky.

In the end I think my CR up from the standard 7.5:1 to around 8.5:1 though I have no real way of measuring it at present other than doing some basic maths on the other standard engines.

Chris
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Old 30-03-2013   #92
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Re: 695 Engine Project

I assumed these numbers might be compression height but the large discrepancy can only be achieved through different-length con rods, which I (mistakenly) assumed was unlikely. Longer con-rod has the advantage of lower side thrust and friction on piston, lighter piston probably more than cancelled out by heavier con rod, I'd just make sure that its reduced angularity doesn't put it too close to the bottom of the cylinder barrel.
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Old 04-05-2013   #93
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Re: 695 Engine Project

Hi Chris, I agree with the basics of your calculation, but the math is a little out. 118mm rods, plus 40mm pistons, plus 35mm of stroke equals 193mm, not 188. No biggie of course, but I was scratching my head reading your post over and over thinking "there's something not right"!

While I waited for my 2.5mm barrel gasket to be made I measured my rods and pistons. They are the same as yours, so the only thing that can be different is the crankcase height, ie mine must be about 2mm lower than yours. The exact height is not easy to measure on a half built up engine so I haven't bothered, but it is interesting. I gave away my old barrels so have not been able to check for them for marks which would show how far up the pistons came on the original rods and barrels.

I had a bit of extra fun and games with the engine once the base gasket arrived, I'll post an update on my thread later this week with some extra details.

Meanwhile I have solved the conrod torque setting question. I emailed the supplier and like you got no reply. But then I googled the code ARP 2000 that was on the bolts and it turns out ARP are massive global suppliers of conrod bolts. The ARP 2000 bolts are tempered to a strength of between 200,000 and 220,000 psi. You can see more on this at http://arp-bolts.com/pages/technical_metallurgy.shtml

I then checked their torque spec sheet at http://arp-bolts.com/pages/technical_torque_us.shtml and it seems the torque should be between 28 and 32. I'll be using 28 when I torque the bolts tomorrow - note that value includes using a proper lube.

Cheers Roger
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Old 05-05-2013   #94
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Re: 695 Engine Project

Sorry all - 193mm is right. Don't know how I got 188mm ?? I blame my Grade 3 teacher for making me use those useless cuisenaire blocks

Lucky I didn't need to add up much to rebuild the engine. Looks like I'll have to remove the sump and torque my con rods big ends a bit more as well. Funny that the crankcases should be out by that much when the rest of these engines are built to quite close tolerance.

Thanks for pointing out the errors and corrections Roger. As one of your previous PMs once commented about the net migration of Kiwis to Oz - 'at least it's raised the IQ of both countries'

Chris
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Old 04-07-2013   #95
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Re: 695 Engine Project

Hi Chris, a very small point, but I wondered what your experience may have been with different spark plug heat ranges for your tune? I am dreaming of the day when I get to test such things myself....

Cheers Roger
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Old 04-07-2013   #96
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Re: 695 Engine Project

Roger,

I started with NGK BP6HS plugs but they seemed too cold so I replaced them with 5HSs and have had no trouble. The engine cold and hot starts easily, idles well and doesn't foul the plugs.

I must admit that I asked around a bit and got a lot of confusing information, so I went with a reliable general purpose plug in the first instance.

What have you heard and what will you use?? I'd be interested in your opinion.

Chris
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Old 05-07-2013   #97
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Re: 695 Engine Project

Thanks Chris, I planned to start with a 6 like you and take it from there. I'll let you know how I get on.
Cheers Roger
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Old 07-06-2015   #98
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Re: 695 Engine Project

Him
Wow you did a great job,, I like one day soon to start with mine,, but you might help me how could I know that my engine is 500 cc,,, or 595 cc,, because on my log book of my 1967fiat it says 594 ,, is there any way that I could find out which engine I have without take the cylinder head out
If some could help that would be great like always
By the wat did you reinforce the block to put 650 cylinders when you took it to the engineering
Well thanks for now
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Old 08-06-2015   #99
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Re: 695 Engine Project

The engine block should be stamped on top of the cylinder head (between carb inlet and rear of engine) with the displacement in CC.


Quote Originally Posted by Timothy500 View Post
Him
Wow you did a great job,, I like one day soon to start with mine,, but you might help me how could I know that my engine is 500 cc,,, or 595 cc,, because on my log book of my 1967fiat it says 594 ,, is there any way that I could find out which engine I have without take the cylinder head out
If some could help that would be great like always
By the wat did you reinforce the block to put 650 cylinders when you took it to the engineering
Well thanks for now
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