Front cover alignment

[seniorchristo]

I am rebuilding a twincam that came with a Dave Bean replaceable water pump kit. In trial assembly I cannot get proper alignment of the top and bottom surfaces while also getting the crankshaft seal concentric with the crankshaft. The problem may be compounded by the block deck being machined an unknown amount. With the crank seal concentric, the plate mating to the engine deck is flush but the cover is about .010" low while at the bottom of the block the cover and plate are off about .025" (not shown).

Three questions;
1} Should all the top surfaces be flush? My Cometic head gasket will crush to .043 while the cork gasket is .090" before use. Will cork compress enough to allow full pressure on the head gasket
2) Will I be able to seal the sump gasket with a step of .025" between cover and block? The goal is no oil leaks.
3) Will the front seal accommodate some misalignment?
Thanks
Chris

[rgh0]

Hi Chris
My comments on your questions below

Three questions;
1} Should all the top surfaces be flush? - Ideally yes but a small amount of misalignment like you have should not be a problem as it should be accommodated by the cork gasket

My Cometic head gasket will crush to .043 while the cork gasket is .090" before use. Will cork compress enough to allow full pressure on the head gasket -- The cork gasket will compress to nothing and extrude out before it stops compression of the head gasket. However you don't want to compress a cork gasket more than around 30% to 40% of its original thickness which you will do with the head gasket and cork gasket you are using. You can source various thicknesses in the cork gasket. Are you using a Cometic MLS gasket ? I personally believe they are to stiff for the relatively flexible and often soft Lotus head, some people get them to work and some seem to struggle to get them to seal.


2) Will I be able to seal the sump gasket with a step of .025" between cover and block? The goal is no oil leaks. Which direction is the step of 0.025 inch ? It is getting towards the limit of what will seal but should be OK with careful assembly and a good sealant on the cork to hold it in place so it does not extrude out

3) Will the front seal accommodate some misalignment? --- it tolerates very little misalignment so do your best to keep it centred during assembly I use a machined spacer that fits the crank and the cover opening to centre it while assembling and only remove it once every thing is bolted up as its very easy to move the cover up and down with the bolts into the sump and into the head

cheers
Rohan

[seniorchristo]

Rohan

Thanks for the response.

The Cometic head gasket part number is shown on the packaging as:
Lotus 4 CYL 85mm Head .043 CFM20 Head Gasket #H0464SP2043F. I
I assume .043 is the compressed thickness.

I reassembled the cover, this time installing the crankshaft seal and front pulley to position everything as close to correct as possible. The plate is now about .010" above the block while the cover is about .010" below the plate.
If I milled the plate.010" they would all be about even but it would still be a problem if I have to compress the cork gasket to .043". The cork gasket at .090" looks to be two thin cork slices sandwiching aluminum foil. This might resist squeezing out. Perhaps i should compress it and see?
The step at the bottom is from cover/plate to block and since raising the cover/plate it is now only about .018" so that shouldn't be a problem.

Should I mill the cover and plate to match the block or possibly even lower? Should I fabricate a gasket of thinner material? Any suggestions appreciated!

Thanks
Chris

[RichardHawkins]

Chris,

I also had problems with this fit. I did what Rohan advised and made a centring ring that locates in the position where the seal fits. I did this after checking the SKF catalogue for acceptable run out, which I think was a maximum of 0.004” (best to check as my old man memory is not good) having checked I realised I needed som kind of alignment tool to achieve this.

There is a rubber covered piece of steel flat bar inside the cover, held in place with two countersunk screws. In my case this item was proud of the casing by about 0.010” and the casing joint could never have sealed. Worth checking.

Hope this helps,

Richard Hawkins

[seniorchristo]

Richard

I don't believe the Dave Bean replacement kit has that piece. What is it's purpose?

Thanks
Chris

[rgh0]

Richard

I don't believe the Dave Bean replacement kit has that piece. What is it's purpose?

Thanks
Chris

Its the chain guide and essential

cheers
Rohan

[RichardHawkins]

Chris,

This piece of rubber covered flat bar is on the opposite side of the casing to the tensioner. I think it is to stop the chain slapping against the aluminium and the rubber may be to muffle the noise.

Richard Hawkins

[seniorchristo]

Ok thanks
I thought there might be something else besides the chain guide attached to the original cover.

Chris

[gjz30075]

Rohan, and Richard, does your machined centering pieces take the place the front pulley, for assembly?
What am I missing, ie, why can't the pulley and its machined 'snout' do the job?

[Billmack]

Those guys align the cover without a seal and using a tom made spacer tfor greater precision. That alignment is the primary thing to be careful about. Ive had decent luck just centering it with the seal in place and the pulley but you need to be extremely careful to not let it shift when tightening it up. On the pan side if you are a machinistbtype of guy it would be nice to even that out a little better.

[2cams70]

1. +1 have a mandrel made up to ensure the seal aperture in the front cover stays concentric with the crankshaft. Although you may think you can get it right by using a telescopic gauge things start to move around once you start tightening bolts plus it makes things much, much easier. The mandrel will lock things in place to ensure the seal opening stays in alignment whilst still giving you freedom to rotate the cover slightly for best fit. I had my mandrel turned from a piece of plastic. There's no need for it to be steel. Easier and cheaper to have it made out of a block of hard plastic. I had mine made by a guy on Ebay for $A60 after I sketched up a drawing with dimensions. See picture.

2. The Lotus specification for maximum timing cover surface mismatch in height is .010". See extract from manual attached.

3. I highly recommend doing a trial build first and loosely fitting the head with all bolts together with the timing cover and it's backplate (no timing chain, no sprockets and no cork gasket but with backplate to block paper gasket fitted). By doing this you and easily determine by eye whether or not it is possible to assemble these components with the top face of the back plate and timing cover parallel with the deck surface of the head and also when in this position the sump face on the underside of the timing cover and backplate is parallel with the sump face of the block. Make sure you loosely fit all of the timing cover bolts too as it is possible that you may need to file some of the bolt holes in the cover and/or backplate in order to be able to rotate things enough so these surfaces are indeed parallel. Nothing is guaranteed when it comes to aftermarket cover assemblies! Yes you can use a straight edge on the block to check alignment but it's easier and nothing beats doing a trial build to fully determine whether everything is as it should be

When you are satisfied with the alignment nip up the single backplate to block bolt sufficiently to prevent the backplate from moving whilst you disassemble prior to final assembly.

4. I recommend you use a cork gasket that is pure cork (like the original one) rather than the commonly available ones that are a cork/synthetic rubber granule mix. Pure cork has much better compressibility than cork/rubber mix. The pure cork gaskets are a laminated construction and have aluminium foil faced with cork either side. I know QED supply them - unfortunately these still aren't quite as good as the original ones because the adhesive used to bond the foil to the cork isn't as good and can separate unlike the OEM cork gasket.

5. I recommend that before you fit the head for the final time that you give the cork gasket a thin coating of Threebond 1215 and fit the head bolts and three timing cover to head bolts nipping them up finger tight only and allow the Threebond to cure overnight before final tightening. This will allow the Threebond time to firmly adhere the cork gasket to the surfaces and will prevent the cork from any further extrusion both during final tightening and over time whilst the engine is in regular use. Tighten the 3 timing cover to head bolts to the lower end of the specified torque range - i.e 10ft/lb rather than 15 ft/lb of the (10-15ft/lb range). This will help prevent excessive stress on the cork gasket and the timing cover itself as the cork gasket being very soft offers no resistance to bolt loads so by tightening these bolts you are effectively trying to bend the front of the head over the block!

Threebond 1215 is great stuff. Do not substitute for other RTV silicones. Unlike other silicones it's sag grade so very thin and you can spread it easily with a finger over surfaces. It has a long tack-free time and being sag grade excess tends to spill over and flow onto interior surface and adhere rather than hang suspended in mid air ready to break off (still no excuse for using too much however!) Japanese OEMs use it.

There's probably a few other hints I could offer but these are the main ones

[rgh0]

Generally I don't like using silicone sealants - even the Threebond 1215 or similar stuff on cork or paper gaskets. As far as I am aware this sort of sealant has been adopted by OEMs for metal to metal joins that are specifically designed for it in terms of sealing face width and flatness and bolt spacing without using a gasket and with the bead applied by a computer controlled applicator during automated assembly to ensure just the right amount is applied.

I prefer high tac sealants designed for sealing paper or cork gaskets as they help prevent the cork gaskets especially from extruding out of the join as they compress.

I know people successfully use Threebond on all sort of joins in all sorts of combinations of gasket or no gasket, and I am not saying it does not work, it just not my preferred option for a twin cam due to the risk of putting to much on and ending up with lumps in your oil systems. If you wait for the Threebond to set then I guess it can prevent the cork extruding out but I prefer something that sets up quicker especially on the head and sump as you need to get a significant compression on the cork seal on top of the chain cover or semi circular sump seals when first setting the head or sump into position.

just my 2 cents worth YMMV

cheers
Rohan

[seniorchristo]

Very detailed and informative post 2cams!

My dilemma is still the fact I will have to compress the cork gasket to about .035" from it's original .090" thickness. What is the typical compressed thickness of the standard head gasket? It would have to match the compressed cork gasket.

Thanks
Chris

[2cams70]

You'll get far greater adhesion (and hence gasket extrusion resistance) with Threebond 1215 on the cork gasket than Aviation (shellac based) cement. I've found the problem with Hi-Tack is that the grab is instant so positioning of the head on the block for correct alignment is difficult. Any minor movement of the head required to line up bolt holes etc. results in a torn cork timing cover to head gasket. No such problems when using the 1215.

I had similar compression of the cork gasket during my build. No problems with excessive extrusion if using a pure cork and not cork/synthetic rubber gasket together with Threebond 1215. You can see the granulated rubber particles in the matrix of gaskets that aren't pure cork.

[RichardHawkins]

Chris,

Bill Mack, 2 cams and Rohan have described better than I could what I have done with the timing cover and the cork gasket between the cover and the head.

Like you I was not happy with the amount of compression the cork gasket would experience. My solution was to search for an alternative gasket material that was resistant to oil but was soft and easily compressible.

Automotive engineering is not my speciality, I have spent my post grad life in the chemical industry, and am fortunate to have an industrial friend who owns a gasket cutting company. I ran some simple accelerated trials of rubber in hot oil. That is oil of the grade I had decided to use. My trials were interesting giving results I did not expect. Nitrile rubber (generally thought to be good with oil) was disappointing as it expanded by about 25%. I found that fluoroelastomer (FKM) (Viton by DuPont) was not affected by my hot oil tests. My gasket cutting friend was able to find some FKM of the right thickness to give me the amount of compression that I guessed I needed. I stuck this gasket to top of the timing chest with one of the Loctite silicones that had also survived my hot oil test while bonding aluminium to FKM. I did not used any gasket dressing between the head and the FKM rubber gasket.

I am an old man and getting slower by the day. Although my engine is assembled, it has not yet run.

Richard Hawkins