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Author Topic: Bump clearnace 101 for Dummies/Newbies?  (Read 7823 times)

TxBlacksmith

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Bump clearnace 101 for Dummies/Newbies?
« on: June 30, 2015, 04:47:11 PM »
I have my parts now, and will soon be reassembling the engine.  The Dursley is getting a brand new complete head, cylinder, piston, rings, bushings , gaskets, the whole nine yards.  Only the connecting rod is not being replaced on the top end.  Since it is all new, I am unsure what kind of clearance, gasket thickness etc, I need to set up in the engine.  I have never had to deal with this in my motorcycle rebuilds, so I guess I just need the 101 on Bump Clearance.  ???

dieselgman

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #1 on: June 30, 2015, 05:02:28 PM »
I believe there are already some instructional threads on this topic here on this forum... 38AC - Butch has written some pretty good step-by-step plain-English stuff.

In general terms:

The process involves putting together the cylinder and head stack and torqueing things up with a couple pieces of soft lead (usually solder works ok) installed on top of the piston crown - then rolling the machine through two strokes. You would then disassemble and measure the squished thickness of the lead to determine clearance. Be certain to use the right specs for your engine type as there is a significant difference in the clearance from 6/1 to the 8/1. You can increase clearance by adding gaskets under the cylinder and decrease it by removing gaskets from the stack. I recall 8/1 bump clearance being something around .055" to .065" and 6/1 around .035" to .045" but be sure to check the other threads for additional data. There are a lot more tricks that others should comment on regarding these settings and methods.

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38ac

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #2 on: June 30, 2015, 05:04:04 PM »
Bump is covered thoroughly in my Listeroid build thread in the WOK section among other places here. However the short is you need a clearance of .045-.050 if you retain the COV or .075-.080 with an 8/1 type plug. It is checked by placing two equal strips of solder about 3/8" long on each side of the piston right on top of the wrist pin. Assemble the engine with all shims and gaskets and torque the head, then roll it over TDC. Remove the head and check the thickness of the lead strips with a caliper or micrometer. Average the two readings and adjust number of shims under the cylinder block to correct the clearance. Two little clearance will affect engine life and could cause valves to hit piston. To much clearance will affect starting.
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TxBlacksmith

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #3 on: June 30, 2015, 05:37:48 PM »
Thank you Gents, I find that as get older, these simple explanations seem to stick with me better.  ;D
My engine rebuild experience has always been with 2 stroke motorcycles, so valve clearance is not something I have ever encountered. 
I would have thought though, that there would be much more clearance between the valves and the piston TDC in a diesel engine though.

So multiple re use and re torquing of the head will not change the critical thickness of the copper gasket?  I find that odd, given its pliability. 

BruceM

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #4 on: June 30, 2015, 07:52:41 PM »
The stock Indian gasket (copper both sides) must have the asbestos filler soaked in Permatex aviation gasket sealer, slightly diluted with alcohol, or it will weep coolant.  Jack Beck reported that here many years ago, and it works a treat.

Reuse with copper head gasket spray has been fine for me.  No worries on bump test gasket reuse, no need to coat.

The better quality head gaskets sold on ebay or gasketstogo.com are considerably thicker.  I prefer them now, but just remember to adjust your bump clearance if switching to these (remove gaskets under cylinder). These gaskets require almost no cylinder liner projection and a flat head and cylinder top. If your head is not flat, or you have a liner with some projection above the cylinder, you should stick to the copper India gaskets. 

Best Wishes on your restoration project!
Bruce

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #5 on: June 30, 2015, 09:25:00 PM »
I soaked my head gaskets in Mop&Glow then let it dry out for a few days.
Works great. No leaks so far.

Gary

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38ac

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #6 on: July 01, 2015, 02:10:53 AM »
Thank you Gents, I find that as get older, these simple explanations seem to stick with me better.  ;D
My engine rebuild experience has always been with 2 stroke motorcycles, so valve clearance is not something I have ever encountered. 
I would have thought though, that there would be much more clearance between the valves and the piston TDC in a diesel engine though.

So multiple re use and re torquing of the head will not change the critical thickness of the copper gasket?  I find that odd, given its pliability. 

The clearance problem does not occur at TDC. It occurs just before TDC during the overlap event when the piston is chasing the exhaust valve closed  and the intake valve is coming open towards the piston.
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contaucreek

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #7 on: July 01, 2015, 02:21:18 AM »
Butch, you say 6/8 hp bump differs because of the cov or plug as the case may be. I was under the impression that 8hp engines require the higher clearance due to aluminum pistons? Does expansion depend more on compression ratio.
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38ac

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #8 on: July 01, 2015, 12:20:34 PM »
 The Aluminum piston does indeed grow more than iron but just a few thousands, not enough to affect compression pressures over a few pounds. The reason for bump height changes is an 8/1 type plug gives same combustion chamber area as COV when screwed in. (Indian bastardization of the plug aside) Lister must have thought that running under load at the higher C/R was damaging so they lowered it by increasing bump. Here are the original specs.
6/1 starting C/R is 19-1 and yields  about 600PSI
6/1 running C/R is 15-1  and yields about 450 PSI
8/1 C/R is 17.5-1 and yields about 560 PSI

It is apparent that switching back and forth between a plug and a COV involves more than screwing one out and the other in, that is if you wish to remain with original engineering specs.  That being said in conversations with some old timers in GB they indicate that not many people messed with the COV, they started them on H/C and ran them on H/C.  The 6/1 I have on my buzz saw is set to spec and ignition knock is much less prevalent when operating on  L/C. If I forget to switch it back to H/C it will not start at 70 F
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mike90045

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #9 on: July 02, 2015, 08:09:43 AM »
And the other clearance issue - De-compressed exhaust valve.  The lever is holding the valve open and you don't want the piston to smack into it while getting up to starting speed.

38ac

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #10 on: July 02, 2015, 01:50:55 PM »
And the other clearance issue - De-compressed exhaust valve.  The lever is holding the valve open and you don't want the piston to smack into it while getting up to starting speed.

Oops forgot that one! 
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TxBlacksmith

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #11 on: July 02, 2015, 04:10:38 PM »
I am curious about the whole soaking the gasket thing.  Given the age of this engine design, and the way things were
 usually over-engineered in those days, why is this necessary?  I would have thought a bug like this would have been fixed 50+ years ago.
Is it change in the quality/materials of the gasket? Normal wear and tear?
Does it occur more on the Indian engines than the originals?
« Last Edit: July 02, 2015, 05:48:07 PM by TxBlacksmith »

38ac

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #12 on: July 02, 2015, 04:58:01 PM »
The leak problems noted are not a design problem with either the engine or the gasket, it comes from two sources neither one of which originated in Dursley.

Excess sleeve protrusion is one cause and one that you do not have to worry about.  The sleeve sticks up so far that the rest of the gasket is not clamped hard between the deck and head.

Garbage head gaskets  are the other reason. Luckily  its not hard to sort the wheat from the chaff.  Get a modern composition  from John at Gaskets to Go and  you will have no issues. If you are using a copper sandwich gasket of Indian origin first look it over  closely under strong light. Pay attention to the edges where it was cut, are they silver? If so chunk it in the trash because it is made of copper washed aluminum. Next look to see if it has a copper fire ring around the cylinder so the filler material is not exposed to combustion. If no fire ring chunk it in the trash. Next check the copper to see if it was properly annealed. At a corner outside of one of the big bolt holes  stick a fingernail under the copper and over the fiber material and lift up as if you are trying to bend it at the middle of the bolt hole.  Lift it up about 1/2 inch and let go,if it stays bent you are good to go.  The very best of Indian gaskets will have the water holes ringed with copper just like the cylinder but good sealing can be had without that  feature if all is flat and the gasket is made of the correct metal.

The sealers are used by enterprising people to get around these problems. Its not bad, but neither is it needed with non-sleeved engines and well constructed  gaskets.
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Thob

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Re: Bump clearnace 101 for Dummies/Newbies?
« Reply #13 on: July 03, 2015, 11:04:27 PM »
In case it's not obvious, you want to assemble the engine for the bump test either with no valves or no pushrods.  You don't want to bend something if the clearance starts too close!  If you have the valves in but no pushrods, leave out the injector or the COV so you don't have to fight compression.
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